Issue #2342: Remove MOZ_MUST_USE macro

Also a few misc low-volume places.

CLOBBER

@@ -22,4 +22,4 @@
 # changes to stick? As of bug 928195, this shouldn't be necessary! Please
 # don't change CLOBBER for WebIDL changes any more.
 
-Removal of SIMD.js and associated scalar types requires CLOBBER. (Issue #2307)
+Switching to C++17 requires CLOBBER to pick up compiler conf. (Issue #2281)

accessible/ipc/DocAccessibleParent.h

@@ -176,7 +176,7 @@ private:
   uint32_t AddSubtree(ProxyAccessible* aParent,
                       const nsTArray<AccessibleData>& aNewTree, uint32_t aIdx,
                       uint32_t aIdxInParent);
-  MOZ_MUST_USE bool CheckDocTree() const;
+  [[nodiscard]] bool CheckDocTree() const;
   xpcAccessibleGeneric* GetXPCAccessible(ProxyAccessible* aProxy);
 
   nsTArray<DocAccessibleParent*> mChildDocs;

accessible/windows/msaa/AccessibleWrap.h

@@ -173,7 +173,7 @@ public: // construction, destruction
   /**
    * Find an accessible by the given child ID in cached documents.
    */
-  MOZ_MUST_USE already_AddRefed<IAccessible>
+  [[nodiscard]] already_AddRefed<IAccessible>
   GetIAccessibleFor(const VARIANT& aVarChild, bool* aIsDefunct);
 
   virtual void GetNativeInterface(void **aOutAccessible) override;
@@ -198,7 +198,7 @@ protected:
   /**
    * Find a remote accessible by the given child ID.
    */
-  MOZ_MUST_USE already_AddRefed<IAccessible>
+  [[nodiscard]] already_AddRefed<IAccessible>
   GetRemoteIAccessibleFor(const VARIANT& aVarChild);
 
   /**

build/moz.configure/toolchain.configure

@@ -340,17 +340,16 @@ def check_compiler(compiler, language, target):
         if info.type in ('clang-cl', 'clang', 'gcc'):
             append_flag('-std=gnu99')
 
-    # Note: this is a strict version check because we used to always add
-    # -std=gnu++14.
-    cxx14_version = 201402
+    cxx17_version = 201703
     if info.language == 'C++':
-        if info.type in ('clang', 'gcc') and info.language_version != cxx14_version:
-            append_flag('-std=gnu++14')
-        # MSVC 2015 headers include C++14 features, but don't guard them
-        # with appropriate checks.
-        if info.type == 'clang-cl' and info.language_version != cxx14_version:
-            append_flag('-std=c++14')
-        # MSVC from 2015 on defaults to C++14.
+        if info.language_version != cxx17_version:
+            # Compiler doesn't default to C++17, so add the appropriate flag.
+            if info.type in ('clang', 'gcc'):
+                # We're on Clang or GCC, enable C++17 and add GNU extensions.
+                append_flag('-std=gnu++17')
+            else:
+                # We're on MSVC; enable C++17 language mode.
+                append_flag('-std:c++17')
 
     # We force clang-cl to emulate Visual C++ 2015 Update 3 with fallback to
     # cl.exe.

build/moz.configure/warnings.configure

@@ -52,7 +52,7 @@ check_and_add_gcc_warning('-Wclass-varargs')
 check_and_add_gcc_warning('-Wloop-analysis')
 
 # catches C++ version forward-compat issues
-check_and_add_gcc_warning('-Wc++1z-compat', cxx_compiler)
+# check_and_add_gcc_warning('-Wc++2a-compat', cxx_compiler)
 
 # catches unintentional switch case fallthroughs
 check_and_add_gcc_warning('-Wimplicit-fallthrough', cxx_compiler)

caps/BasePrincipal.h

@@ -51,12 +51,12 @@ public:
   // Don't use this method for anything else than debugging!
   void CreateAnonymizedSuffix(nsACString& aStr) const;
 
-  MOZ_MUST_USE bool PopulateFromSuffix(const nsACString& aStr);
+  [[nodiscard]] bool PopulateFromSuffix(const nsACString& aStr);
 
   // Populates the attributes from a string like
   // |uri!key1=value1&key2=value2| and returns the uri without the suffix.
-  MOZ_MUST_USE bool PopulateFromOrigin(const nsACString& aOrigin,
-                                       nsACString& aOriginNoSuffix);
+  [[nodiscard]] bool PopulateFromOrigin(const nsACString& aOrigin,
+                                        nsACString& aOriginNoSuffix);
 
   // Helper function to match mIsPrivateBrowsing to existing private browsing
   // flags. Once all other flags are removed, this can be removed too.

dom/base/FragmentOrElement.h

@@ -147,8 +147,7 @@ public:
   virtual bool TextIsOnlyWhitespace() override;
   virtual bool HasTextForTranslation() override;
   virtual void AppendTextTo(nsAString& aResult) override;
-  MOZ_MUST_USE
-  virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) override;
+  [[nodiscard]] virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) override;
   virtual nsIContent *GetBindingParent() const override;
   virtual nsXBLBinding *GetXBLBinding() const override;
   virtual void SetXBLBinding(nsXBLBinding* aBinding,

dom/base/nsContentAreaDragDrop.cpp

@@ -84,7 +84,7 @@ private:
                                             nsIContent **outImageOrLinkNode,
                                             bool* outDragSelectedText);
   static already_AddRefed<nsIContent> FindParentLinkNode(nsIContent* inNode);
-  static MOZ_MUST_USE nsresult
+  static [[nodiscard]] nsresult
   GetAnchorURL(nsIContent* inNode, nsAString& outURL);
   static void GetNodeString(nsIContent* inNode, nsAString & outNodeString);
   static void CreateLinkText(const nsAString& inURL, const nsAString & inText,

dom/base/nsContentUtils.h

@@ -1508,9 +1508,10 @@ public:
    * @param aResult the result. Out param.
    * @return false on out of memory errors, true otherwise.
    */
-  MOZ_MUST_USE
-  static bool GetNodeTextContent(nsINode* aNode, bool aDeep,
-                                 nsAString& aResult, const mozilla::fallible_t&);
+  [[nodiscard]] static bool GetNodeTextContent(nsINode* aNode,
+                                               bool aDeep,
+                                               nsAString& aResult,
+                                               const mozilla::fallible_t&);
 
   static void GetNodeTextContent(nsINode* aNode, bool aDeep,
                                  nsAString& aResult);
@@ -1869,28 +1870,29 @@ public:
    */
   static bool CanAccessNativeAnon();
 
-  MOZ_MUST_USE
-  static nsresult WrapNative(JSContext *cx, nsISupports *native,
-                             const nsIID* aIID, JS::MutableHandle<JS::Value> vp,
-                             bool aAllowWrapping = true)
+  [[nodiscard]] static nsresult WrapNative(JSContext* cx,
+                                           nsISupports* native,
+                                           const nsIID* aIID,
+                                           JS::MutableHandle<JS::Value> vp,
+                                           bool aAllowWrapping = true)
   {
     return WrapNative(cx, native, nullptr, aIID, vp, aAllowWrapping);
   }
 
   // Same as the WrapNative above, but use this one if aIID is nsISupports' IID.
-  MOZ_MUST_USE
-  static nsresult WrapNative(JSContext *cx, nsISupports *native,
-                             JS::MutableHandle<JS::Value> vp,
-                             bool aAllowWrapping = true)
+  [[nodiscard]] static nsresult WrapNative(JSContext* cx,
+                                           nsISupports* native,
+                                           JS::MutableHandle<JS::Value> vp,
+                                           bool aAllowWrapping = true)
   {
     return WrapNative(cx, native, nullptr, nullptr, vp, aAllowWrapping);
   }
 
-  MOZ_MUST_USE
-  static nsresult WrapNative(JSContext *cx, nsISupports *native,
-                             nsWrapperCache *cache,
-                             JS::MutableHandle<JS::Value> vp,
-                             bool aAllowWrapping = true)
+  [[nodiscard]] static nsresult WrapNative(JSContext* cx,
+                                           nsISupports* native,
+                                           nsWrapperCache* cache,
+                                           JS::MutableHandle<JS::Value> vp,
+                                           bool aAllowWrapping = true)
   {
     return WrapNative(cx, native, cache, nullptr, vp, aAllowWrapping);
   }
@@ -1914,9 +1916,8 @@ public:
    * @param aString the string to convert the newlines inside [in/out]
    */
   static void PlatformToDOMLineBreaks(nsString &aString);
-  MOZ_MUST_USE
-  static bool PlatformToDOMLineBreaks(nsString &aString,
-                                      const mozilla::fallible_t&);
+  [[nodiscard]] static bool PlatformToDOMLineBreaks(nsString &aString,
+                                                    const mozilla::fallible_t&);
 
   /**
    * Populates aResultString with the contents of the string-buffer aBuf, up

dom/base/nsGenericDOMDataNode.h

@@ -144,9 +144,8 @@ public:
   virtual bool TextIsOnlyWhitespace() override;
   virtual bool HasTextForTranslation() override;
   virtual void AppendTextTo(nsAString& aResult) override;
-  MOZ_MUST_USE
-  virtual bool AppendTextTo(nsAString& aResult,
-                            const mozilla::fallible_t&) override;
+  [[nodiscard]] virtual bool AppendTextTo(nsAString& aResult,
+                                          const mozilla::fallible_t&) override;
   virtual void SaveSubtreeState() override;
 
 #ifdef DEBUG

dom/base/nsHTMLContentSerializer.h

@@ -37,18 +37,16 @@ class nsHTMLContentSerializer final : public nsXHTMLContentSerializer {
                                  nsAString& aStr) override;
  protected:
 
-  MOZ_MUST_USE
-  virtual bool SerializeHTMLAttributes(nsIContent* aContent,
-                                       nsIContent *aOriginalElement,
-                                       nsAString& aTagPrefix,
-                                       const nsAString& aTagNamespaceURI,
-                                       nsIAtom* aTagName,
-                                       int32_t aNamespace,
-                                       nsAString& aStr);
+  [[nodiscard]] virtual bool SerializeHTMLAttributes(nsIContent* aContent,
+                                                     nsIContent* aOriginalElement,
+                                                     nsAString& aTagPrefix,
+                                                     const nsAString& aTagNamespaceURI,
+                                                     nsIAtom* aTagName,
+                                                     int32_t aNamespace,
+                                                     nsAString& aStr);
 
-  MOZ_MUST_USE
-  virtual bool AppendAndTranslateEntities(const nsAString& aStr,
-                                          nsAString& aOutputStr) override;
+  [[nodiscard]] virtual bool AppendAndTranslateEntities(const nsAString& aStr,
+                                                        nsAString& aOutputStr) override;
 
 };
 

dom/base/nsIContent.h

@@ -576,8 +576,7 @@ public:
    * Append the text content to aResult.
    * NOTE: This asserts and returns for elements
    */
-  MOZ_MUST_USE
-  virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) = 0;
+  [[nodiscard]] virtual bool AppendTextTo(nsAString& aResult, const mozilla::fallible_t&) = 0;
 
   /**
    * Check if this content is focusable and in the current tab order.

dom/base/nsIDocument.h

@@ -2513,8 +2513,8 @@ public:
                 mozilla::ErrorResult& rv);
   virtual mozilla::dom::DOMImplementation*
     GetImplementation(mozilla::ErrorResult& rv) = 0;
-  MOZ_MUST_USE nsresult GetURL(nsString& retval) const;
-  MOZ_MUST_USE nsresult GetDocumentURI(nsString& retval) const;
+  [[nodiscard]] nsresult GetURL(nsString& retval) const;
+  [[nodiscard]] nsresult GetDocumentURI(nsString& retval) const;
   // Return the URI for the document.
   // The returned value may differ if the document is loaded via XHR, and
   // when accessed from chrome privileged script and

dom/base/nsINode.h

@@ -1873,7 +1873,7 @@ public:
     aNodeName.SetStringBuffer(nsStringBuffer::FromString(nodeName),
                               nodeName.Length());
   }
-  MOZ_MUST_USE nsresult GetBaseURI(nsAString& aBaseURI) const;
+  [[nodiscard]] nsresult GetBaseURI(nsAString& aBaseURI) const;
   // Return the base URI for the document.
   // The returned value may differ if the document is loaded via XHR, and
   // when accessed from chrome privileged script and

dom/base/nsJSUtils.h

@@ -143,7 +143,7 @@ public:
     // After getting a notification that an off-thread compilation terminated,
     // this function will take the result of the parser and move it to the main
     // thread.
-    MOZ_MUST_USE nsresult JoinCompile(void** aOffThreadToken);
+    [[nodiscard]] nsresult JoinCompile(void** aOffThreadToken);
 
     // Compile a script contained in a SourceText.
     nsresult Compile(JS::CompileOptions& aCompileOptions,
@@ -167,7 +167,7 @@ public:
     JSScript* GetScript();
 
     // Execute the compiled script and ignore the return value.
-    MOZ_MUST_USE nsresult ExecScript();
+    [[nodiscard]] nsresult ExecScript();
 
     // Execute the compiled script a get the return value.
     //
@@ -180,10 +180,10 @@ public:
     // compartment given as argument to the ExecutionContext constructor. If the
     // caller is in a different compartment, then the out-param value should be
     // wrapped by calling |JS_WrapValue|.
-    MOZ_MUST_USE nsresult
+    [[nodiscard]] nsresult
     ExtractReturnValue(JS::MutableHandle<JS::Value> aRetValue);
 
-    MOZ_MUST_USE nsresult ExecScript(JS::MutableHandle<JS::Value> aRetValue);
+    [[nodiscard]] nsresult ExecScript(JS::MutableHandle<JS::Value> aRetValue);
   };
 
   static nsresult CompileModule(JSContext* aCx,

dom/base/nsObjectLoadingContent.h

@@ -382,7 +382,7 @@ class nsObjectLoadingContent : public nsImageLoadingContent
     void GetNestedParams(nsTArray<mozilla::dom::MozPluginParameter>& aParameters,
                          bool aIgnoreCodebase);
 
-    MOZ_MUST_USE nsresult BuildParametersArray();
+    [[nodiscard]] nsresult BuildParametersArray();
 
     /**
      * Loads fallback content with the specified FallbackType

dom/base/nsTextFragment.h

@@ -136,7 +136,7 @@ public:
    * Append the contents of this string fragment to aString
    * @return false if an out of memory condition is detected, true otherwise
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendTo(nsAString& aString,
                 const mozilla::fallible_t& aFallible) const {
     if (mState.mIs2b) {
@@ -170,7 +170,7 @@ public:
    * @param aLength the length of the substring
    * @return false if an out of memory condition is detected, true otherwise
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendTo(nsAString& aString, int32_t aOffset, int32_t aLength,
                 const mozilla::fallible_t& aFallible) const
   {

dom/base/nsXHTMLContentSerializer.h

@@ -46,7 +46,7 @@ class nsXHTMLContentSerializer : public nsXMLContentSerializer {
                           nsAString& aStr,
                           nsresult& aResult) override;
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AfterElementStart(nsIContent* aContent,
                                  nsIContent* aOriginalElement,
                                  nsAString& aStr) override;
@@ -70,7 +70,7 @@ class nsXHTMLContentSerializer : public nsXMLContentSerializer {
   virtual void MaybeEnterInPreContent(nsIContent* aNode) override;
   virtual void MaybeLeaveFromPreContent(nsIContent* aNode) override;
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool SerializeAttributes(nsIContent* aContent,
                            nsIContent *aOriginalElement,
                            nsAString& aTagPrefix,
@@ -82,13 +82,13 @@ class nsXHTMLContentSerializer : public nsXMLContentSerializer {
 
   bool IsFirstChildOfOL(nsIContent* aElement);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SerializeLIValueAttribute(nsIContent* aElement,
                                  nsAString& aStr);
   bool IsShorthandAttr(const nsIAtom* aAttrName,
                          const nsIAtom* aElementName);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AppendAndTranslateEntities(const nsAString& aStr,
                                           nsAString& aOutputStr) override;
 

dom/base/nsXMLContentSerializer.h

@@ -72,23 +72,21 @@ class nsXMLContentSerializer : public nsIContentSerializer {
   /**
    * Appends a char16_t character and increments the column position
    */
-  MOZ_MUST_USE
-  bool AppendToString(const char16_t aChar,
-                      nsAString& aOutputStr);
+  [[nodiscard]] bool AppendToString(const char16_t aChar,
+                                    nsAString& aOutputStr);
 
   /**
    * Appends a nsAString string and increments the column position
    */
-  MOZ_MUST_USE
-  bool AppendToString(const nsAString& aStr,
-                      nsAString& aOutputStr);
+  [[nodiscard]] bool AppendToString(const nsAString& aStr,
+                                    nsAString& aOutputStr);
 
   /**
    * Appends a string by replacing all line-endings
    * by mLineBreak, except in the case of raw output.
    * It increments the column position.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendToStringConvertLF(const nsAString& aStr,
                                nsAString& aOutputStr);
 
@@ -96,7 +94,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * Appends a string by wrapping it when necessary.
    * It updates the column position.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendToStringWrapped(const nsASingleFragmentString& aStr,
                              nsAString& aOutputStr);
 
@@ -104,12 +102,12 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * Appends a string by formating and wrapping it when necessary
    * It updates the column position.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendToStringFormatedWrapped(const nsASingleFragmentString& aStr,
                                      nsAString& aOutputStr);
 
   // used by AppendToStringWrapped
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendWrapped_WhitespaceSequence(
           nsASingleFragmentString::const_char_iterator &aPos,
           const nsASingleFragmentString::const_char_iterator aEnd,
@@ -117,7 +115,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
           nsAString &aOutputStr);
 
   // used by AppendToStringFormatedWrapped
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendFormatedWrapped_WhitespaceSequence(
           nsASingleFragmentString::const_char_iterator &aPos,
           const nsASingleFragmentString::const_char_iterator aEnd,
@@ -126,7 +124,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
           nsAString &aOutputStr);
 
   // used by AppendToStringWrapped and AppendToStringFormatedWrapped
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendWrapped_NonWhitespaceSequence(
           nsASingleFragmentString::const_char_iterator &aPos,
           const nsASingleFragmentString::const_char_iterator aEnd,
@@ -139,7 +137,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * add mLineBreak to the string
    * It updates the column position and other flags.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendNewLineToString(nsAString& aOutputStr);
 
 
@@ -147,7 +145,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * Appends a string by translating entities
    * It doesn't increment the column position
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AppendAndTranslateEntities(const nsAString& aStr,
                                           nsAString& aOutputStr);
 
@@ -197,7 +195,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
                                      nsIContent *aOriginalElement,
                                      const nsAString& aTagNamespaceURI);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool SerializeAttributes(nsIContent* aContent,
                                    nsIContent *aOriginalElement,
                                    nsAString& aTagPrefix,
@@ -207,7 +205,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
                                    uint32_t aSkipAttr,
                                    bool aAddNSAttr);
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SerializeAttr(const nsAString& aPrefix,
                      const nsAString& aName,
                      const nsAString& aValue,
@@ -239,7 +237,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * aElement and aOriginalElement are the same as the corresponding arguments
    * to AppendElementStart.
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool AppendEndOfElementStart(mozilla::dom::Element* aEleemnt,
                                mozilla::dom::Element* aOriginalElement,
                                nsAString& aStr);
@@ -249,7 +247,7 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * after the serialization ot the start tag.
    * (called at the end of AppendElementStart)
    */
-  MOZ_MUST_USE
+  [[nodiscard]]
   virtual bool AfterElementStart(nsIContent* aContent,
                                  nsIContent* aOriginalElement,
                                  nsAString& aStr) { return true; };
@@ -299,17 +297,14 @@ class nsXMLContentSerializer : public nsIContentSerializer {
    * add intendation. Call only in the case of formating and if the current
    * position is at 0. It updates the column position.
    */
-  MOZ_MUST_USE
-  bool AppendIndentation(nsAString& aStr);
+  [[nodiscard]] bool AppendIndentation(nsAString& aStr);
 
-  MOZ_MUST_USE
-  bool IncrIndentation(nsIAtom* aName);
+  [[nodiscard]] bool IncrIndentation(nsIAtom* aName);
   void DecrIndentation(nsIAtom* aName);
 
   // Functions to check for newlines that needs to be added between nodes in
   // the root of a document. See mAddNewlineForRootNode
-  MOZ_MUST_USE
-  bool MaybeAddNewlineForRootNode(nsAString& aStr);
+  [[nodiscard]] bool MaybeAddNewlineForRootNode(nsAString& aStr);
   void MaybeFlagNewlineForRootNode(nsINode* aNode);
 
   // Functions to check if we enter in or leave from a preformated content

dom/bindings/ToJSValue.h

@@ -24,7 +24,7 @@ class Promise;
 // JSContext.
 
 // Accept strings.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const nsAString& aArgument,
           JS::MutableHandle<JS::Value> aValue);
@@ -35,7 +35,7 @@ ToJSValue(JSContext* aCx,
 // desirable.  So make this a template that only gets used if the argument type
 // is actually boolean
 template<typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsSame<T, bool>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           T aArgument,
@@ -123,7 +123,7 @@ ToJSValue(JSContext* aCx,
 }
 
 // Accept CallbackObjects
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx,
           CallbackObject& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -139,7 +139,7 @@ ToJSValue(JSContext* aCx,
 // Accept objects that inherit from nsWrapperCache (e.g. most
 // DOM objects).
 template <class T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<nsWrapperCache, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           T& aArgument,
@@ -155,7 +155,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept typed arrays built from appropriate nsTArray values
 template<typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<AllTypedArraysBase, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           const TypedArrayCreator<T>& aArgument,
@@ -175,7 +175,7 @@ ToJSValue(JSContext* aCx,
 // Accept objects that inherit from nsISupports but not nsWrapperCache (e.g.
 // DOM File).
 template <class T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<!IsBaseOf<nsWrapperCache, T>::value &&
                   !IsBaseOf<CallbackObject, T>::value &&
                   IsBaseOf<nsISupports, T>::value, bool>::Type
@@ -193,7 +193,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept nsRefPtr/nsCOMPtr
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const nsCOMPtr<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -202,7 +202,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const RefPtr<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -211,7 +211,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const NonNull<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -221,7 +221,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept WebIDL dictionaries
 template <class T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<DictionaryBase, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           const T& aArgument,
@@ -231,7 +231,7 @@ ToJSValue(JSContext* aCx,
 }
 
 // Accept existing JS values (which may not be same-compartment with us
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, JS::Handle<JS::Value> aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -240,7 +240,7 @@ ToJSValue(JSContext* aCx, JS::Handle<JS::Value> aArgument,
 }
 
 // Accept existing JS values on the Heap (which may not be same-compartment with us
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, const JS::Heap<JS::Value>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -249,7 +249,7 @@ ToJSValue(JSContext* aCx, const JS::Heap<JS::Value>& aArgument,
 }
 
 // Accept existing rooted JS values (which may not be same-compartment with us
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, const JS::Rooted<JS::Value>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -259,7 +259,7 @@ ToJSValue(JSContext* aCx, const JS::Rooted<JS::Value>& aArgument,
 
 // Accept existing rooted JS objects (which may not be same-compartment with
 // us).
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 ToJSValue(JSContext* aCx, const JS::Rooted<JSObject*>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
 {
@@ -269,7 +269,7 @@ ToJSValue(JSContext* aCx, const JS::Rooted<JSObject*>& aArgument,
 
 // Accept nsresult, for use in rejections, and create an XPCOM
 // exception object representing that nsresult.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           nsresult aArgument,
           JS::MutableHandle<JS::Value> aValue);
@@ -277,14 +277,14 @@ ToJSValue(JSContext* aCx,
 // Accept ErrorResult, for use in rejections, and create an exception
 // representing the failure.  Note, the ErrorResult must indicate a failure
 // with aArgument.Failure() returning true.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           ErrorResult& aArgument,
           JS::MutableHandle<JS::Value> aValue);
 
 // Accept owning WebIDL unions.
 template <typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsBaseOf<AllOwningUnionBase, T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           const T& aArgument,
@@ -296,7 +296,7 @@ ToJSValue(JSContext* aCx,
 
 // Accept pointers to other things we accept
 template <typename T>
-MOZ_MUST_USE
+[[nodiscard]]
 typename EnableIf<IsPointer<T>::value, bool>::Type
 ToJSValue(JSContext* aCx,
           T aArgument,
@@ -306,14 +306,14 @@ ToJSValue(JSContext* aCx,
 }
 
 // Accept Promise objects, which need special handling.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           Promise& aArgument,
           JS::MutableHandle<JS::Value> aValue);
 
 // Accept arrays of other things we accept
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           T* aArguments,
           size_t aLength,
@@ -340,7 +340,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const nsTArray<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -350,7 +350,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const FallibleTArray<T>& aArgument,
           JS::MutableHandle<JS::Value> aValue)
@@ -360,7 +360,7 @@ ToJSValue(JSContext* aCx,
 }
 
 template <typename T, int N>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ToJSValue(JSContext* aCx,
           const T(&aArgument)[N],
           JS::MutableHandle<JS::Value> aValue)

dom/html/nsTextEditorState.h

@@ -154,7 +154,7 @@ public:
     // Whether the value change should be notified to the frame/contet nor not.
     eSetValue_Notify                = 1 << 2
   };
-  MOZ_MUST_USE bool SetValue(const nsAString& aValue, uint32_t aFlags);
+  [[nodiscard]] bool SetValue(const nsAString& aValue, uint32_t aFlags);
   void GetValue(nsAString& aValue, bool aIgnoreWrap) const;
   bool HasNonEmptyValue();
   // The following methods are for textarea element to use whether default

dom/ipc/nsIContentParent.h

@@ -63,11 +63,11 @@ public:
   virtual bool IsForApp() const = 0;
   virtual bool IsForBrowser() const = 0;
 
-  MOZ_MUST_USE virtual PBlobParent*
+  [[nodiscard]] virtual PBlobParent*
   SendPBlobConstructor(PBlobParent* aActor,
                        const BlobConstructorParams& aParams) = 0;
 
-  MOZ_MUST_USE virtual PBrowserParent*
+  [[nodiscard]] virtual PBrowserParent*
   SendPBrowserConstructor(PBrowserParent* actor,
                           const TabId& aTabId,
                           const IPCTabContext& context,

dom/media/webaudio/AlignedTArray.h

@@ -36,7 +36,7 @@ public:
     base_type::SetLength(newLen + sExtra);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SetLength(size_type newLen, const mozilla::fallible_t&)
   {
     return base_type::SetLength(newLen + sExtra, mozilla::fallible);
@@ -89,7 +89,7 @@ public:
     base_type::SetLength(newLen + sExtra);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   bool SetLength(size_type newLen, const mozilla::fallible_t&)
   {
     return base_type::SetLength(newLen + sExtra, mozilla::fallible);

dom/plugins/ipc/PluginModuleParent.cpp

@@ -2210,7 +2210,10 @@ PluginModuleParent::NPP_NewInternal(NPMIMEType pluginType, NPP instance,
         if (supportsAsyncRender) {
           // Prefs indicates we want async plugin rendering, make sure
           // the flash module has support.
-          CallModuleSupportsAsyncRender(&supportsAsyncRender);
+          if (!CallModuleSupportsAsyncRender(&supportsAsyncRender)) {
+            // The actual support call failed. Should not happen; abort.
+            return NS_ERROR_FAILURE;
+          }
         }
 #ifdef _WIN64
         // For 64-bit builds force windowless if the flash library doesn't support

dom/plugins/ipc/PluginUtilsWin.cpp

@@ -34,7 +34,9 @@ public:
 
     for (auto iter = mAudioNotificationSet->ConstIter(); !iter.Done(); iter.Next()) {
       PluginModuleParent* pluginModule = iter.Get()->GetKey();
-      pluginModule->SendNPP_SetValue_NPNVaudioDeviceChangeDetails(mChangeDetails);
+      if (!pluginModule->SendNPP_SetValue_NPNVaudioDeviceChangeDetails(mChangeDetails)) {
+        // Sending of notification failed. Ignore for now.
+      }
     }
     return NS_OK;
   }

dom/script/ScriptLoader.h

@@ -287,14 +287,14 @@ public:
     insertBack(aElem);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   already_AddRefed<ScriptLoadRequest> Steal(ScriptLoadRequest* aElem)
   {
     aElem->removeFrom(*this);
     return dont_AddRef(aElem);
   }
 
-  MOZ_MUST_USE
+  [[nodiscard]]
   already_AddRefed<ScriptLoadRequest> StealFirst()
   {
     MOZ_ASSERT(!isEmpty());

dom/script/ScriptSettings.h

@@ -224,11 +224,11 @@ public:
   // If aGlobalObject represents a web-visible global, errors reported by this
   // AutoJSAPI as it comes off the stack will fire the relevant error events and
   // show up in the corresponding web console.
-  MOZ_MUST_USE bool Init(nsIGlobalObject* aGlobalObject);
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject);
 
   // This is a helper that grabs the native global associated with aObject and
   // invokes the above Init() with that.
-  MOZ_MUST_USE bool Init(JSObject* aObject);
+  [[nodiscard]] bool Init(JSObject* aObject);
 
   // Unsurprisingly, this uses aCx and enters the compartment of aGlobalObject.
   // If aGlobalObject or its associated JS global are null then it returns
@@ -238,15 +238,15 @@ public:
   // If aGlobalObject represents a web-visible global, errors reported by this
   // AutoJSAPI as it comes off the stack will fire the relevant error events and
   // show up in the corresponding web console.
-  MOZ_MUST_USE bool Init(nsIGlobalObject* aGlobalObject, JSContext* aCx);
+  [[nodiscard]] bool Init(nsIGlobalObject* aGlobalObject, JSContext* aCx);
 
   // Convenience functions to take an nsPIDOMWindow* or nsGlobalWindow*,
   // when it is more easily available than an nsIGlobalObject.
-  MOZ_MUST_USE bool Init(nsPIDOMWindowInner* aWindow);
-  MOZ_MUST_USE bool Init(nsPIDOMWindowInner* aWindow, JSContext* aCx);
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow);
+  [[nodiscard]] bool Init(nsPIDOMWindowInner* aWindow, JSContext* aCx);
 
-  MOZ_MUST_USE bool Init(nsGlobalWindow* aWindow);
-  MOZ_MUST_USE bool Init(nsGlobalWindow* aWindow, JSContext* aCx);
+  [[nodiscard]] bool Init(nsGlobalWindow* aWindow);
+  [[nodiscard]] bool Init(nsGlobalWindow* aWindow, JSContext* aCx);
 
   JSContext* cx() const {
     MOZ_ASSERT(mCx, "Must call Init before using an AutoJSAPI");
@@ -272,12 +272,12 @@ public:
   //
   // Note that this fails if and only if we OOM while wrapping the exception
   // into the current compartment.
-  MOZ_MUST_USE bool StealException(JS::MutableHandle<JS::Value> aVal);
+  [[nodiscard]] bool StealException(JS::MutableHandle<JS::Value> aVal);
 
   // As for StealException(), but put the saved frames for any stack trace
   // associated with the point the exception was thrown into aStack.
   // aVal will be in the current compartment, but aStack might not be.
-  MOZ_MUST_USE bool StealExceptionAndStack(JS::MutableHandle<JS::Value> aVal,
+  [[nodiscard]] bool StealExceptionAndStack(JS::MutableHandle<JS::Value> aVal,
                                            JS::MutableHandle<JSObject*> aStack);
 
   // Peek the current exception from the JS engine, without stealing it.
@@ -286,7 +286,7 @@ public:
   //
   // Note that this fails if and only if we OOM while wrapping the exception
   // into the current compartment.
-  MOZ_MUST_USE bool PeekException(JS::MutableHandle<JS::Value> aVal);
+  [[nodiscard]] bool PeekException(JS::MutableHandle<JS::Value> aVal);
 
   void ClearException() {
     MOZ_ASSERT(IsStackTop());

dom/xhr/XMLHttpRequestString.h

@@ -44,7 +44,7 @@ public:
   // This method should be called only when the string is really needed because
   // it can cause the duplication of the strings in case the loading of the XHR
   // is not completed yet.
-  MOZ_MUST_USE bool GetAsString(nsAString& aString) const;
+  [[nodiscard]] bool GetAsString(nsAString& aString) const;
 
   size_t SizeOfThis(MallocSizeOf aMallocSizeOf) const;
 
@@ -121,7 +121,7 @@ public:
     return !mLength;
   }
 
-  MOZ_MUST_USE bool GetAsString(DOMString& aString) const;
+  [[nodiscard]] bool GetAsString(DOMString& aString) const;
 
 private:
   XMLHttpRequestStringSnapshot(const XMLHttpRequestStringSnapshot&) = delete;
@@ -193,7 +193,7 @@ public:
     mData.Append(aString);
   }
 
-  MOZ_MUST_USE bool
+  [[nodiscard]] bool
   GetAsString(nsAString& aString)
   {
     MutexAutoLock lock(mMutex);
@@ -206,7 +206,7 @@ public:
     return mData.SizeOfExcludingThisIfUnshared(aMallocSizeOf);
   }
 
-  MOZ_MUST_USE bool
+  [[nodiscard]] bool
   GetAsString(DOMString& aString, uint32_t aLength)
   {
     MutexAutoLock lock(mMutex);

dom/xslt/xslt/txExecutionState.h

@@ -60,7 +60,7 @@ public:
     {
     }
     ~txLoadedDocumentsHash();
-    MOZ_MUST_USE nsresult init(const txXPathNode& aSource);
+    [[nodiscard]] nsresult init(const txXPathNode& aSource);
 
 private:
     friend class txExecutionState;

gfx/2d/BaseRect.h

@@ -111,7 +111,7 @@ struct BaseRect {
   // Intersection with an empty Rect may not produce an empty Rect if overflow
   // occurs. e.g. {INT_MIN, 0, 0, 20} Intersect { 5000, 0, 500, 20 } gives:
   // the non-emtpy {5000, 0, 500, 20 } instead of {5000, 0, 0, 0}
-  MOZ_MUST_USE Sub Intersect(const Sub& aRect) const
+  [[nodiscard]] Sub Intersect(const Sub& aRect) const
   {
     Sub result;
     result.x = std::max<T>(x, aRect.x);
@@ -127,7 +127,7 @@ struct BaseRect {
   // better. This comes at a tiny cost in performance.
   // e.g. {INT_MIN, 0, 0, 20} Intersect { 5000, 0, 500, 20 } gives:
   // {5000, 0, 0, 0}
-  MOZ_MUST_USE Sub SafeIntersect(const Sub& aRect) const
+  [[nodiscard]] Sub SafeIntersect(const Sub& aRect) const
   {
     Sub result;
     result.x = std::max<T>(x, aRect.x);
@@ -161,7 +161,7 @@ struct BaseRect {
   // If both rectangles are empty, returns this.
   // WARNING! This is not safe against overflow, prefer using SafeUnion instead
   // when dealing with int-based rects.
-  MOZ_MUST_USE Sub Union(const Sub& aRect) const
+  [[nodiscard]] Sub Union(const Sub& aRect) const
   {
     if (IsEmpty()) {
       return aRect;
@@ -176,7 +176,7 @@ struct BaseRect {
   // Thus, empty input rectangles are allowed to affect the result.
   // WARNING! This is not safe against overflow, prefer using SafeUnionEdges
   // instead when dealing with int-based rects.
-  MOZ_MUST_USE Sub UnionEdges(const Sub& aRect) const
+  [[nodiscard]] Sub UnionEdges(const Sub& aRect) const
   {
     Sub result;
     result.x = std::min(x, aRect.x);
@@ -588,7 +588,7 @@ struct BaseRect {
    * Clamp aPoint to this rectangle. It is allowed to end up on any
    * edge of the rectangle.
    */
-  MOZ_MUST_USE Point ClampPoint(const Point& aPoint) const
+  [[nodiscard]] Point ClampPoint(const Point& aPoint) const
   {
     return Point(std::max(x, std::min(XMost(), aPoint.x)),
                  std::max(y, std::min(YMost(), aPoint.y)));
@@ -599,7 +599,7 @@ struct BaseRect {
    * aRect then the dimensions that don't fit will be shrunk so that they
    * do fit. The resulting rect is returned.
    */
-  MOZ_MUST_USE Sub MoveInsideAndClamp(const Sub& aRect) const
+  [[nodiscard]] Sub MoveInsideAndClamp(const Sub& aRect) const
   {
     Sub rect(std::max(aRect.x, x),
              std::max(aRect.y, y),

gfx/2d/Rect.h

@@ -166,7 +166,7 @@ struct IntRectTyped :
     // Same as Union(), but in the cases where aRect is non-empty, the union is
     // done while guarding against overflow. If an overflow is detected, Nothing
     // is returned.
-    MOZ_MUST_USE Maybe<Self> SafeUnion(const Self& aRect) const
+    [[nodiscard]] Maybe<Self> SafeUnion(const Self& aRect) const
     {
       if (this->IsEmpty()) {
         return aRect.Overflows() ? Nothing() : Some(aRect);
@@ -179,7 +179,7 @@ struct IntRectTyped :
 
     // Same as UnionEdges, but guards against overflow. If an overflow is detected,
     // Nothing is returned.
-    MOZ_MUST_USE Maybe<Self> SafeUnionEdges(const Self& aRect) const
+    [[nodiscard]] Maybe<Self> SafeUnionEdges(const Self& aRect) const
     {
       if (this->Overflows() || aRect.Overflows()) {
         return Nothing();

gfx/angle/src/compiler/preprocessor/Tokenizer.cpp

@@ -823,9 +823,9 @@ extern int pplex \
  */
 YY_DECL
 {
-	register yy_state_type yy_current_state;
-	register char *yy_cp, *yy_bp;
-	register int yy_act;
+	yy_state_type yy_current_state;
+	char *yy_cp, *yy_bp;
+	int yy_act;
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
 
     yylval = yylval_param;
@@ -878,7 +878,7 @@ YY_DECL
 yy_match:
 		do
 			{
-			register YY_CHAR yy_c = yy_ec[YY_SC_TO_UI(*yy_cp)] ;
+			YY_CHAR yy_c = yy_ec[YY_SC_TO_UI(*yy_cp)] ;
 			if ( yy_accept[yy_current_state] )
 				{
 				yyg->yy_last_accepting_state = yy_current_state;
@@ -1336,9 +1336,9 @@ ECHO;
 static int yy_get_next_buffer (yyscan_t yyscanner)
 {
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
-	register char *dest = YY_CURRENT_BUFFER_LVALUE->yy_ch_buf;
-	register char *source = yyg->yytext_ptr;
-	register int number_to_move, i;
+	char *dest = YY_CURRENT_BUFFER_LVALUE->yy_ch_buf;
+	char *source = yyg->yytext_ptr;
+	int number_to_move, i;
 	int ret_val;
 
 	if ( yyg->yy_c_buf_p > &YY_CURRENT_BUFFER_LVALUE->yy_ch_buf[yyg->yy_n_chars + 1] )
@@ -1470,15 +1470,15 @@ static int yy_get_next_buffer (yyscan_t yyscanner)
 
     static yy_state_type yy_get_previous_state (yyscan_t yyscanner)
 {
-	register yy_state_type yy_current_state;
-	register char *yy_cp;
+	yy_state_type yy_current_state;
+	char *yy_cp;
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
 
 	yy_current_state = yyg->yy_start;
 
 	for ( yy_cp = yyg->yytext_ptr + YY_MORE_ADJ; yy_cp < yyg->yy_c_buf_p; ++yy_cp )
 		{
-		register YY_CHAR yy_c = (*yy_cp ? yy_ec[YY_SC_TO_UI(*yy_cp)] : 1);
+		YY_CHAR yy_c = (*yy_cp ? yy_ec[YY_SC_TO_UI(*yy_cp)] : 1);
 		if ( yy_accept[yy_current_state] )
 			{
 			yyg->yy_last_accepting_state = yy_current_state;
@@ -1503,11 +1503,11 @@ static int yy_get_next_buffer (yyscan_t yyscanner)
  */
     static yy_state_type yy_try_NUL_trans  (yy_state_type yy_current_state , yyscan_t yyscanner)
 {
-	register int yy_is_jam;
+	int yy_is_jam;
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner; /* This var may be unused depending upon options. */
-	register char *yy_cp = yyg->yy_c_buf_p;
+	char *yy_cp = yyg->yy_c_buf_p;
 
-	register YY_CHAR yy_c = 1;
+	YY_CHAR yy_c = 1;
 	if ( yy_accept[yy_current_state] )
 		{
 		yyg->yy_last_accepting_state = yy_current_state;

gfx/angle/src/compiler/translator/glslang_lex.cpp

@@ -1286,9 +1286,9 @@ extern int yylex \
  */
 YY_DECL
 {
-	register yy_state_type yy_current_state;
-	register char *yy_cp, *yy_bp;
-	register int yy_act;
+	yy_state_type yy_current_state;
+	char *yy_cp, *yy_bp;
+	int yy_act;
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
 
     yylval = yylval_param;
@@ -1341,7 +1341,7 @@ YY_DECL
 yy_match:
 		do
 			{
-			register YY_CHAR yy_c = yy_ec[YY_SC_TO_UI(*yy_cp)] ;
+			YY_CHAR yy_c = yy_ec[YY_SC_TO_UI(*yy_cp)] ;
 			if ( yy_accept[yy_current_state] )
 				{
 				yyg->yy_last_accepting_state = yy_current_state;
@@ -2237,9 +2237,9 @@ ECHO;
 static int yy_get_next_buffer (yyscan_t yyscanner)
 {
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
-	register char *dest = YY_CURRENT_BUFFER_LVALUE->yy_ch_buf;
-	register char *source = yyg->yytext_ptr;
-	register int number_to_move, i;
+	char *dest = YY_CURRENT_BUFFER_LVALUE->yy_ch_buf;
+	char *source = yyg->yytext_ptr;
+	int number_to_move, i;
 	int ret_val;
 
 	if ( yyg->yy_c_buf_p > &YY_CURRENT_BUFFER_LVALUE->yy_ch_buf[yyg->yy_n_chars + 1] )
@@ -2371,15 +2371,15 @@ static int yy_get_next_buffer (yyscan_t yyscanner)
 
     static yy_state_type yy_get_previous_state (yyscan_t yyscanner)
 {
-	register yy_state_type yy_current_state;
-	register char *yy_cp;
+	yy_state_type yy_current_state;
+	char *yy_cp;
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner;
 
 	yy_current_state = yyg->yy_start;
 
 	for ( yy_cp = yyg->yytext_ptr + YY_MORE_ADJ; yy_cp < yyg->yy_c_buf_p; ++yy_cp )
 		{
-		register YY_CHAR yy_c = (*yy_cp ? yy_ec[YY_SC_TO_UI(*yy_cp)] : 1);
+		YY_CHAR yy_c = (*yy_cp ? yy_ec[YY_SC_TO_UI(*yy_cp)] : 1);
 		if ( yy_accept[yy_current_state] )
 			{
 			yyg->yy_last_accepting_state = yy_current_state;
@@ -2404,11 +2404,11 @@ static int yy_get_next_buffer (yyscan_t yyscanner)
  */
     static yy_state_type yy_try_NUL_trans  (yy_state_type yy_current_state , yyscan_t yyscanner)
 {
-	register int yy_is_jam;
+	int yy_is_jam;
     struct yyguts_t * yyg = (struct yyguts_t*)yyscanner; /* This var may be unused depending upon options. */
-	register char *yy_cp = yyg->yy_c_buf_p;
+	char *yy_cp = yyg->yy_c_buf_p;
 
-	register YY_CHAR yy_c = 1;
+	YY_CHAR yy_c = 1;
 	if ( yy_accept[yy_current_state] )
 		{
 		yyg->yy_last_accepting_state = yy_current_state;

gfx/src/nsPoint.h

@@ -35,12 +35,12 @@ struct nsPoint : public mozilla::gfx::BasePoint<nscoord, nsPoint> {
    * @param aFromAPP the APP to scale from
    * @param aToAPP the APP to scale to
    */
-  MOZ_MUST_USE inline nsPoint
+  [[nodiscard]] inline nsPoint
     ScaleToOtherAppUnits(int32_t aFromAPP, int32_t aToAPP) const;
 
-  MOZ_MUST_USE inline nsPoint
+  [[nodiscard]] inline nsPoint
     RemoveResolution(const float resolution) const;
-  MOZ_MUST_USE inline nsPoint
+  [[nodiscard]] inline nsPoint
     ApplyResolution(const float resolution) const;
 };
 

gfx/src/nsRect.h

@@ -56,7 +56,7 @@ struct nsRect :
   // overflowing nscoord values in the 'width' and 'height' fields by
   // clamping the width and height values to nscoord_MAX if necessary.
 
-  MOZ_MUST_USE nsRect SaturatingUnion(const nsRect& aRect) const
+  [[nodiscard]] nsRect SaturatingUnion(const nsRect& aRect) const
   {
     if (IsEmpty()) {
       return aRect;
@@ -67,7 +67,7 @@ struct nsRect :
     }
   }
 
-  MOZ_MUST_USE nsRect SaturatingUnionEdges(const nsRect& aRect) const
+  [[nodiscard]] nsRect SaturatingUnionEdges(const nsRect& aRect) const
   {
 #ifdef NS_COORD_IS_FLOAT
     return UnionEdges(aRect);
@@ -102,7 +102,7 @@ struct nsRect :
 
 #ifndef NS_COORD_IS_FLOAT
   // Make all nsRect Union methods be saturating.
-  MOZ_MUST_USE nsRect UnionEdges(const nsRect& aRect) const
+  [[nodiscard]] nsRect UnionEdges(const nsRect& aRect) const
   {
     return SaturatingUnionEdges(aRect);
   }
@@ -110,7 +110,7 @@ struct nsRect :
   {
     *this = aRect1.UnionEdges(aRect2);
   }
-  MOZ_MUST_USE nsRect Union(const nsRect& aRect) const
+  [[nodiscard]] nsRect Union(const nsRect& aRect) const
   {
     return SaturatingUnion(aRect);
   }
@@ -141,32 +141,32 @@ struct nsRect :
    * @param aToAPP the APP to scale to
    * @note this can turn an empty rectangle into a non-empty rectangle
    */
-  MOZ_MUST_USE inline nsRect
+  [[nodiscard]] inline nsRect
     ScaleToOtherAppUnitsRoundOut(int32_t aFromAPP, int32_t aToAPP) const;
-  MOZ_MUST_USE inline nsRect
+  [[nodiscard]] inline nsRect
     ScaleToOtherAppUnitsRoundIn(int32_t aFromAPP, int32_t aToAPP) const;
 
-  MOZ_MUST_USE inline mozilla::gfx::IntRect
+  [[nodiscard]] inline mozilla::gfx::IntRect
   ScaleToNearestPixels(float aXScale, float aYScale,
                        nscoord aAppUnitsPerPixel) const;
 
-  MOZ_MUST_USE inline mozilla::gfx::IntRect
+  [[nodiscard]] inline mozilla::gfx::IntRect
   ToNearestPixels(nscoord aAppUnitsPerPixel) const;
 
   // Note: this can turn an empty rectangle into a non-empty rectangle
-  MOZ_MUST_USE inline mozilla::gfx::IntRect
+  [[nodiscard]] inline mozilla::gfx::IntRect
   ScaleToOutsidePixels(float aXScale, float aYScale,
                        nscoord aAppUnitsPerPixel) const;
 
   // Note: this can turn an empty rectangle into a non-empty rectangle
-  MOZ_MUST_USE inline mozilla::gfx::IntRect
+  [[nodiscard]] inline mozilla::gfx::IntRect
   ToOutsidePixels(nscoord aAppUnitsPerPixel) const;
 
-  MOZ_MUST_USE inline mozilla::gfx::IntRect
+  [[nodiscard]] inline mozilla::gfx::IntRect
   ScaleToInsidePixels(float aXScale, float aYScale,
                       nscoord aAppUnitsPerPixel) const;
 
-  MOZ_MUST_USE inline mozilla::gfx::IntRect
+  [[nodiscard]] inline mozilla::gfx::IntRect
   ToInsidePixels(nscoord aAppUnitsPerPixel) const;
 
   // This is here only to keep IPDL-generated code happy. DO NOT USE.
@@ -175,7 +175,7 @@ struct nsRect :
     return IsEqualEdges(aRect);
   }
 
-  MOZ_MUST_USE inline nsRect RemoveResolution(const float aResolution) const;
+  [[nodiscard]] inline nsRect RemoveResolution(const float aResolution) const;
 };
 
 /*

gfx/src/nsRegion.h

@@ -296,9 +296,9 @@ public:
    * @param aToAPP the APP to scale to
    * @note this can turn an empty region into a non-empty region
    */
-  MOZ_MUST_USE nsRegion
+  [[nodiscard]] nsRegion
     ScaleToOtherAppUnitsRoundOut (int32_t aFromAPP, int32_t aToAPP) const;
-  MOZ_MUST_USE nsRegion
+  [[nodiscard]] nsRegion
     ScaleToOtherAppUnitsRoundIn (int32_t aFromAPP, int32_t aToAPP) const;
   nsRegion& ScaleRoundOut(float aXScale, float aYScale);
   nsRegion& ScaleInverseRoundOut(float aXScale, float aYScale);

gfx/src/nsSize.h

@@ -30,7 +30,7 @@ struct nsSize : public mozilla::gfx::BaseSize<nscoord, nsSize> {
    * @param aFromAPP the APP to scale from
    * @param aToAPP the APP to scale to
    */
-  MOZ_MUST_USE inline nsSize
+  [[nodiscard]] inline nsSize
     ScaleToOtherAppUnits(int32_t aFromAPP, int32_t aToAPP) const;
 };
 

hal/Hal.h

@@ -333,7 +333,7 @@ void NotifyScreenConfigurationChange(const hal::ScreenConfiguration& aScreenConf
  * Lock the screen orientation to the specific orientation.
  * @return Whether the lock has been accepted.
  */
-MOZ_MUST_USE bool LockScreenOrientation(const dom::ScreenOrientationInternal& aOrientation);
+[[nodiscard]] bool LockScreenOrientation(const dom::ScreenOrientationInternal& aOrientation);
 
 /**
  * Unlock the screen orientation.
@@ -346,7 +346,7 @@ void UnlockScreenOrientation();
  *
  * Currently, there can only be 0 or 1 alarm observers.
  */
-MOZ_MUST_USE bool RegisterTheOneAlarmObserver(hal::AlarmObserver* aObserver);
+[[nodiscard]] bool RegisterTheOneAlarmObserver(hal::AlarmObserver* aObserver);
 
 /**
  * Unregister the alarm observer.  Doing so will implicitly cancel any
@@ -373,7 +373,7 @@ void NotifyAlarmFired();
  * This API is currently only allowed to be used from non-sandboxed
  * contexts.
  */
-MOZ_MUST_USE bool SetAlarm(int32_t aSeconds, int32_t aNanoseconds);
+[[nodiscard]] bool SetAlarm(int32_t aSeconds, int32_t aNanoseconds);
 
 /**
  * Set the priority of the given process.

hal/HalInternal.h

@@ -56,7 +56,7 @@ void DisableScreenConfigurationNotifications();
 /**
  * Enable alarm notifications from the backend.
  */
-MOZ_MUST_USE bool EnableAlarm();
+[[nodiscard]] bool EnableAlarm();
 
 /**
  * Disable alarm notifications from the backend.

image/imgLoader.h

@@ -287,22 +287,22 @@ public:
   imgLoader();
   nsresult Init();
 
-  MOZ_MUST_USE nsresult LoadImage(nsIURI* aURI,
-                                  nsIURI* aInitialDocumentURI,
-                                  nsIURI* aReferrerURI,
-                                  ReferrerPolicy aReferrerPolicy,
-                                  nsIPrincipal* aLoadingPrincipal,
-                                  nsILoadGroup* aLoadGroup,
-                                  imgINotificationObserver* aObserver,
-                                  nsINode* aContext,
-                                  nsIDocument* aLoadingDocument,
-                                  nsLoadFlags aLoadFlags,
-                                  nsISupports* aCacheKey,
-                                  nsContentPolicyType aContentPolicyType,
-                                  const nsAString& initiatorType,
-                                  imgRequestProxy** _retval);
+  [[nodiscard]] nsresult LoadImage(nsIURI* aURI,
+                                   nsIURI* aInitialDocumentURI,
+                                   nsIURI* aReferrerURI,
+                                   ReferrerPolicy aReferrerPolicy,
+                                   nsIPrincipal* aLoadingPrincipal,
+                                   nsILoadGroup* aLoadGroup,
+                                   imgINotificationObserver* aObserver,
+                                   nsINode* aContext,
+                                   nsIDocument* aLoadingDocument,
+                                   nsLoadFlags aLoadFlags,
+                                   nsISupports* aCacheKey,
+                                   nsContentPolicyType aContentPolicyType,
+                                   const nsAString& initiatorType,
+                                   imgRequestProxy** _retval);
 
-  MOZ_MUST_USE nsresult
+  [[nodiscard]] nsresult
   LoadImageWithChannel(nsIChannel* channel,
                        imgINotificationObserver* aObserver,
                        nsISupports* aCX,

image/imgRequest.h

@@ -65,16 +65,16 @@ public:
   NS_DECL_NSIINTERFACEREQUESTOR
   NS_DECL_NSIASYNCVERIFYREDIRECTCALLBACK
 
-  MOZ_MUST_USE nsresult Init(nsIURI* aURI,
-                             nsIURI* aCurrentURI,
-                             bool aHadInsecureRedirect,
-                             nsIRequest* aRequest,
-                             nsIChannel* aChannel,
-                             imgCacheEntry* aCacheEntry,
-                             nsISupports* aCX,
-                             nsIPrincipal* aLoadingPrincipal,
-                             int32_t aCORSMode,
-                             ReferrerPolicy aReferrerPolicy);
+  [[nodiscard]] nsresult Init(nsIURI* aURI,
+                              nsIURI* aCurrentURI,
+                              bool aHadInsecureRedirect,
+                              nsIRequest* aRequest,
+                              nsIChannel* aChannel,
+                              imgCacheEntry* aCacheEntry,
+                              nsISupports* aCX,
+                              nsIPrincipal* aLoadingPrincipal,
+                              int32_t aCORSMode,
+                              ReferrerPolicy aReferrerPolicy);
 
   void ClearLoader();
 

intl/uconv/nsIUnicodeDecoder.h

@@ -110,9 +110,9 @@ public:
    *                    NS_ERROR_OUT_OF_MEMORY if OOM
    *                    NS_OK is all we have is an approximation
    */
-   MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char* aSrc,
-                                        int32_t aSrcLength,
-                                        int32_t* aDestLength) = 0;
+   [[nodiscard]] NS_IMETHOD GetMaxLength(const char* aSrc,
+                                         int32_t aSrcLength,
+                                         int32_t* aDestLength) = 0;
 
   /**
    * Resets the charset converter so it may be recycled for a completely 

intl/uconv/nsIUnicodeEncoder.h

@@ -135,9 +135,9 @@ public:
    *                    NS_ERROR_OUT_OF_MEMORY if OOM
    *                    NS_OK if all we have is an approximation
    */
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char16_t* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength) = 0;
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char16_t* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength) = 0;
 
   /**
    * Resets the charset converter so it may be recycled for a completely 

intl/uconv/nsReplacementToUnicode.h

@@ -24,9 +24,9 @@ public:
                      char16_t* aDest,
                      int32_t* aDestLength) override;
 
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength) override;
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength) override;
 
   NS_IMETHOD Reset() override;
 

intl/uconv/nsUTF8ToUnicode.h

@@ -49,9 +49,9 @@ protected:
   //--------------------------------------------------------------------
   // Subclassing of nsDecoderSupport class [declaration]
 
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength) override;
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength) override;
 
   //--------------------------------------------------------------------
   // Subclassing of nsBasicDecoderSupport class [declaration]

intl/uconv/nsUnicodeToUTF8.h

@@ -48,9 +48,9 @@ public:
 
   NS_IMETHOD Finish(char* aDest, int32_t* aDestLength) override;
 
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char16_t* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength) override;
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char16_t* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength) override;
 
   NS_IMETHOD Reset() override {mHighSurrogate = 0; return NS_OK;}
 

intl/uconv/ucvlatin/nsUTF16ToUnicode.h

@@ -24,9 +24,9 @@ public:
   //--------------------------------------------------------------------
   // Subclassing of nsDecoderSupport class [declaration]
 
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength) override;
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength) override;
   NS_IMETHOD Reset() override;
 
 protected:

intl/uconv/ucvlatin/nsUnicodeToUTF16.h

@@ -19,9 +19,9 @@ public:
 
   NS_IMETHOD Convert(const char16_t* aSrc, int32_t* aSrcLength, 
       char* aDest, int32_t* aDestLength);
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char16_t* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength);
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char16_t* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength);
   NS_IMETHOD Finish(char* aDest, int32_t* aDestLength);
   NS_IMETHOD Reset();
   NS_IMETHOD SetOutputErrorBehavior(int32_t aBehavior, 

intl/uconv/ucvtw/nsUnicodeToBIG5.h

@@ -29,9 +29,9 @@ public:
   NS_IMETHOD Finish(char* aDest,
                     int32_t* aDestLength) override;
 
-  MOZ_MUST_USE NS_IMETHOD GetMaxLength(const char16_t* aSrc,
-                                       int32_t aSrcLength,
-                                       int32_t* aDestLength) override;
+  [[nodiscard]] NS_IMETHOD GetMaxLength(const char16_t* aSrc,
+                                        int32_t aSrcLength,
+                                        int32_t* aDestLength) override;
 
   NS_IMETHOD Reset() override;
 

ipc/chromium/src/base/pickle.h

@@ -1,5 +1,6 @@
 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
+// Copyright (c) 2023, Moonchild Productions.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -92,32 +93,32 @@ class Pickle {
   // the Pickle, initialize *iter to NULL.  If successful, these methods return
   // true.  Otherwise, false is returned to indicate that the result could not
   // be extracted.
-  MOZ_MUST_USE bool ReadBool(PickleIterator* iter, bool* result) const;
-  MOZ_MUST_USE bool ReadInt16(PickleIterator* iter, int16_t* result) const;
-  MOZ_MUST_USE bool ReadUInt16(PickleIterator* iter, uint16_t* result) const;
-  MOZ_MUST_USE bool ReadShort(PickleIterator* iter, short* result) const;
-  MOZ_MUST_USE bool ReadInt(PickleIterator* iter, int* result) const;
-  MOZ_MUST_USE bool ReadLong(PickleIterator* iter, long* result) const;
-  MOZ_MUST_USE bool ReadULong(PickleIterator* iter, unsigned long* result) const;
-  MOZ_MUST_USE bool ReadSize(PickleIterator* iter, size_t* result) const;
-  MOZ_MUST_USE bool ReadInt32(PickleIterator* iter, int32_t* result) const;
-  MOZ_MUST_USE bool ReadUInt32(PickleIterator* iter, uint32_t* result) const;
-  MOZ_MUST_USE bool ReadInt64(PickleIterator* iter, int64_t* result) const;
-  MOZ_MUST_USE bool ReadUInt64(PickleIterator* iter, uint64_t* result) const;
-  MOZ_MUST_USE bool ReadDouble(PickleIterator* iter, double* result) const;
-  MOZ_MUST_USE bool ReadIntPtr(PickleIterator* iter, intptr_t* result) const;
-  MOZ_MUST_USE bool ReadUnsignedChar(PickleIterator* iter, unsigned char* result) const;
-  MOZ_MUST_USE bool ReadString(PickleIterator* iter, std::string* result) const;
-  MOZ_MUST_USE bool ReadWString(PickleIterator* iter, std::wstring* result) const;
-  MOZ_MUST_USE bool ReadBytesInto(PickleIterator* iter, void* data, uint32_t length) const;
-  MOZ_MUST_USE bool ExtractBuffers(PickleIterator* iter, size_t length, BufferList* buffers,
-                                   uint32_t alignment = sizeof(memberAlignmentType)) const;
+  [[nodiscard]] bool ReadBool(PickleIterator* iter, bool* result) const;
+  [[nodiscard]] bool ReadInt16(PickleIterator* iter, int16_t* result) const;
+  [[nodiscard]] bool ReadUInt16(PickleIterator* iter, uint16_t* result) const;
+  [[nodiscard]] bool ReadShort(PickleIterator* iter, short* result) const;
+  [[nodiscard]] bool ReadInt(PickleIterator* iter, int* result) const;
+  [[nodiscard]] bool ReadLong(PickleIterator* iter, long* result) const;
+  [[nodiscard]] bool ReadULong(PickleIterator* iter, unsigned long* result) const;
+  [[nodiscard]] bool ReadSize(PickleIterator* iter, size_t* result) const;
+  [[nodiscard]] bool ReadInt32(PickleIterator* iter, int32_t* result) const;
+  [[nodiscard]] bool ReadUInt32(PickleIterator* iter, uint32_t* result) const;
+  [[nodiscard]] bool ReadInt64(PickleIterator* iter, int64_t* result) const;
+  [[nodiscard]] bool ReadUInt64(PickleIterator* iter, uint64_t* result) const;
+  [[nodiscard]] bool ReadDouble(PickleIterator* iter, double* result) const;
+  [[nodiscard]] bool ReadIntPtr(PickleIterator* iter, intptr_t* result) const;
+  [[nodiscard]] bool ReadUnsignedChar(PickleIterator* iter, unsigned char* result) const;
+  [[nodiscard]] bool ReadString(PickleIterator* iter, std::string* result) const;
+  [[nodiscard]] bool ReadWString(PickleIterator* iter, std::wstring* result) const;
+  [[nodiscard]] bool ReadBytesInto(PickleIterator* iter, void* data, uint32_t length) const;
+  [[nodiscard]] bool ExtractBuffers(PickleIterator* iter, size_t length, BufferList* buffers,
+                                    uint32_t alignment = sizeof(memberAlignmentType)) const;
 
   // Safer version of ReadInt() checks for the result not being negative.
   // Use it for reading the object sizes.
-  MOZ_MUST_USE bool ReadLength(PickleIterator* iter, int* result) const;
+  [[nodiscard]] bool ReadLength(PickleIterator* iter, int* result) const;
 
-  MOZ_MUST_USE bool ReadSentinel(PickleIterator* iter, uint32_t sentinel) const
+  [[nodiscard]] bool ReadSentinel(PickleIterator* iter, uint32_t sentinel) const
 #ifdef MOZ_PICKLE_SENTINEL_CHECKING
     ;
 #else

ipc/ipdl/ipdl/cxx/cgen.py

@@ -185,7 +185,7 @@ class CxxCodeGen(CodePrinter, Visitor):
             self.printdent()
 
         if md.warn_unused:
-            self.write('MOZ_MUST_USE ')
+            self.write('[[nodiscard]] ')
         if md.inline:
             self.write('inline ')
         if md.never_inline:

ipc/mscom/COMPtrHolder.h

@@ -37,7 +37,7 @@ public:
     return mPtr.get();
   }
 
-  MOZ_MUST_USE Interface* Release()
+  [[nodiscard]] Interface* Release()
   {
     return mPtr.release();
   }

js/public/HashTable.h

@@ -88,7 +88,7 @@ class HashMap
     // HashMap construction is fallible (due to OOM); thus the user must call
     // init after constructing a HashMap and check the return value.
     explicit HashMap(AllocPolicy a = AllocPolicy()) : impl(a)  {}
-    MOZ_MUST_USE bool init(uint32_t len = 16) { return impl.init(len); }
+    [[nodiscard]] bool init(uint32_t len = 16) { return impl.init(len); }
     bool initialized() const                  { return impl.initialized(); }
 
     // Return whether the given lookup value is present in the map. E.g.:
@@ -151,19 +151,19 @@ class HashMap
     }
 
     template<typename KeyInput, typename ValueInput>
-    MOZ_MUST_USE bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    [[nodiscard]] bool add(AddPtr& p, KeyInput&& k, ValueInput&& v) {
         return impl.add(p,
                         mozilla::Forward<KeyInput>(k),
                         mozilla::Forward<ValueInput>(v));
     }
 
     template<typename KeyInput>
-    MOZ_MUST_USE bool add(AddPtr& p, KeyInput&& k) {
+    [[nodiscard]] bool add(AddPtr& p, KeyInput&& k) {
         return impl.add(p, mozilla::Forward<KeyInput>(k), Value());
     }
 
     template<typename KeyInput, typename ValueInput>
-    MOZ_MUST_USE bool relookupOrAdd(AddPtr& p, KeyInput&& k, ValueInput&& v) {
+    [[nodiscard]] bool relookupOrAdd(AddPtr& p, KeyInput&& k, ValueInput&& v) {
         return impl.relookupOrAdd(p, k,
                                   mozilla::Forward<KeyInput>(k),
                                   mozilla::Forward<ValueInput>(v));
@@ -232,7 +232,7 @@ class HashMap
 
     // Overwrite existing value with v. Return false on oom.
     template<typename KeyInput, typename ValueInput>
-    MOZ_MUST_USE bool put(KeyInput&& k, ValueInput&& v) {
+    [[nodiscard]] bool put(KeyInput&& k, ValueInput&& v) {
         AddPtr p = lookupForAdd(k);
         if (p) {
             p->value() = mozilla::Forward<ValueInput>(v);
@@ -243,7 +243,7 @@ class HashMap
 
     // Like put, but assert that the given key is not already present.
     template<typename KeyInput, typename ValueInput>
-    MOZ_MUST_USE bool putNew(KeyInput&& k, ValueInput&& v) {
+    [[nodiscard]] bool putNew(KeyInput&& k, ValueInput&& v) {
         return impl.putNew(k, mozilla::Forward<KeyInput>(k), mozilla::Forward<ValueInput>(v));
     }
 
@@ -341,7 +341,7 @@ class HashSet
     // HashSet construction is fallible (due to OOM); thus the user must call
     // init after constructing a HashSet and check the return value.
     explicit HashSet(AllocPolicy a = AllocPolicy()) : impl(a)  {}
-    MOZ_MUST_USE bool init(uint32_t len = 16) { return impl.init(len); }
+    [[nodiscard]] bool init(uint32_t len = 16) { return impl.init(len); }
     bool initialized() const                  { return impl.initialized(); }
 
     // Return whether the given lookup value is present in the map. E.g.:
@@ -399,12 +399,12 @@ class HashSet
     AddPtr lookupForAdd(const Lookup& l) const        { return impl.lookupForAdd(l); }
 
     template <typename U>
-    MOZ_MUST_USE bool add(AddPtr& p, U&& u) {
+    [[nodiscard]] bool add(AddPtr& p, U&& u) {
         return impl.add(p, mozilla::Forward<U>(u));
     }
 
     template <typename U>
-    MOZ_MUST_USE bool relookupOrAdd(AddPtr& p, const Lookup& l, U&& u) {
+    [[nodiscard]] bool relookupOrAdd(AddPtr& p, const Lookup& l, U&& u) {
         return impl.relookupOrAdd(p, l, mozilla::Forward<U>(u));
     }
 
@@ -471,19 +471,19 @@ class HashSet
 
     // Add |u| if it is not present already. Return false on oom.
     template <typename U>
-    MOZ_MUST_USE bool put(U&& u) {
+    [[nodiscard]] bool put(U&& u) {
         AddPtr p = lookupForAdd(u);
         return p ? true : add(p, mozilla::Forward<U>(u));
     }
 
     // Like put, but assert that the given key is not already present.
     template <typename U>
-    MOZ_MUST_USE bool putNew(U&& u) {
+    [[nodiscard]] bool putNew(U&& u) {
         return impl.putNew(u, mozilla::Forward<U>(u));
     }
 
     template <typename U>
-    MOZ_MUST_USE bool putNew(const Lookup& l, U&& u) {
+    [[nodiscard]] bool putNew(const Lookup& l, U&& u) {
         return impl.putNew(l, mozilla::Forward<U>(u));
     }
 
@@ -1277,7 +1277,7 @@ class HashTable : private AllocPolicy
 #endif
     {}
 
-    MOZ_MUST_USE bool init(uint32_t length)
+    [[nodiscard]] bool init(uint32_t length)
     {
         MOZ_ASSERT(!initialized());
 
@@ -1775,7 +1775,7 @@ class HashTable : private AllocPolicy
     }
 
     template <typename... Args>
-    MOZ_MUST_USE bool add(AddPtr& p, Args&&... args)
+    [[nodiscard]] bool add(AddPtr& p, Args&&... args)
     {
         mozilla::ReentrancyGuard g(*this);
         MOZ_ASSERT(table);
@@ -1828,7 +1828,7 @@ class HashTable : private AllocPolicy
     // Note: |l| may be alias arguments in |args|, so this function must take
     // care not to use |l| after moving |args|.
     template <typename... Args>
-    MOZ_MUST_USE bool putNew(const Lookup& l, Args&&... args)
+    [[nodiscard]] bool putNew(const Lookup& l, Args&&... args)
     {
         if (!this->checkSimulatedOOM())
             return false;
@@ -1846,7 +1846,7 @@ class HashTable : private AllocPolicy
     // Note: |l| may be a reference to a piece of |u|, so this function
     // must take care not to use |l| after moving |u|.
     template <typename... Args>
-    MOZ_MUST_USE bool relookupOrAdd(AddPtr& p, const Lookup& l, Args&&... args)
+    [[nodiscard]] bool relookupOrAdd(AddPtr& p, const Lookup& l, Args&&... args)
     {
         // Check for error from ensureHash() here.
         if (p.isValid())

js/public/RootingAPI.h

@@ -1274,7 +1274,7 @@ class MutableWrappedPtrOperations<UniquePtr<T, D>, Container>
     UniquePtr<T, D>& uniquePtr() { return static_cast<Container*>(this)->get(); }
 
   public:
-    MOZ_MUST_USE typename UniquePtr<T, D>::Pointer release() { return uniquePtr().release(); }
+    [[nodiscard]] typename UniquePtr<T, D>::Pointer release() { return uniquePtr().release(); }
     void reset(T* ptr = T()) { uniquePtr().reset(ptr); }
 };
 

js/public/UbiNode.h

@@ -285,7 +285,7 @@ class BaseStackFrame {
     // simplifies the principals check into the boolean isSystem() state. This
     // is fine because we only expose JS::ubi::Stack to devtools and chrome
     // code, and not to the web platform.
-    virtual MOZ_MUST_USE bool constructSavedFrameStack(JSContext* cx,
+    [[nodiscard]] virtual bool constructSavedFrameStack(JSContext* cx,
                                                        MutableHandleObject outSavedFrameStack)
         const = 0;
 
@@ -416,7 +416,7 @@ class StackFrame {
     StackFrame parent() const { return base()->parent(); }
     bool isSystem() const { return base()->isSystem(); }
     bool isSelfHosted(JSContext* cx) const { return base()->isSelfHosted(cx); }
-    MOZ_MUST_USE bool constructSavedFrameStack(JSContext* cx,
+    [[nodiscard]] bool constructSavedFrameStack(JSContext* cx,
                                                MutableHandleObject outSavedFrameStack) const {
         return base()->constructSavedFrameStack(cx, outSavedFrameStack);
     }
@@ -449,7 +449,7 @@ class ConcreteStackFrame<void> : public BaseStackFrame {
 
     uint64_t identifier() const override { return 0; }
     void trace(JSTracer* trc) override { }
-    MOZ_MUST_USE bool constructSavedFrameStack(JSContext* cx, MutableHandleObject out)
+    [[nodiscard]] bool constructSavedFrameStack(JSContext* cx, MutableHandleObject out)
         const override
     {
         out.set(nullptr);
@@ -465,7 +465,7 @@ class ConcreteStackFrame<void> : public BaseStackFrame {
     bool isSelfHosted(JSContext* cx) const override { MOZ_CRASH("null JS::ubi::StackFrame"); }
 };
 
-MOZ_MUST_USE JS_PUBLIC_API(bool)
+[[nodiscard]] JS_PUBLIC_API(bool)
 ConstructSavedFrameStackSlow(JSContext* cx,
                              JS::ubi::StackFrame& frame,
                              MutableHandleObject outSavedFrameStack);
@@ -628,7 +628,7 @@ class JS_PUBLIC_API(Base) {
     // Otherwise, place nullptr in the out parameter. Caller maintains ownership
     // of the out parameter. True is returned on success, false is returned on
     // OOM.
-    virtual MOZ_MUST_USE bool jsObjectConstructorName(JSContext* cx, UniqueTwoByteChars& outName)
+    [[nodiscard]] virtual bool jsObjectConstructorName(JSContext* cx, UniqueTwoByteChars& outName)
         const
     {
         outName.reset(nullptr);
@@ -778,7 +778,7 @@ class Node {
     JS::Zone* zone()                const { return base()->zone(); }
     JSCompartment* compartment()    const { return base()->compartment(); }
     const char* jsObjectClassName() const { return base()->jsObjectClassName(); }
-    MOZ_MUST_USE bool jsObjectConstructorName(JSContext* cx, UniqueTwoByteChars& outName) const {
+    [[nodiscard]] bool jsObjectConstructorName(JSContext* cx, UniqueTwoByteChars& outName) const {
         return base()->jsObjectConstructorName(cx, outName);
     }
 
@@ -981,12 +981,12 @@ class MOZ_STACK_CLASS JS_PUBLIC_API(RootList) {
     RootList(JSContext* cx, Maybe<AutoCheckCannotGC>& noGC, bool wantNames = false);
 
     // Find all GC roots.
-    MOZ_MUST_USE bool init();
+    [[nodiscard]] bool init();
     // Find only GC roots in the provided set of |JSCompartment|s.
-    MOZ_MUST_USE bool init(CompartmentSet& debuggees);
+    [[nodiscard]] bool init(CompartmentSet& debuggees);
     // Find only GC roots in the given Debugger object's set of debuggee
     // compartments.
-    MOZ_MUST_USE bool init(HandleObject debuggees);
+    [[nodiscard]] bool init(HandleObject debuggees);
 
     // Returns true if the RootList has been initialized successfully, false
     // otherwise.
@@ -995,7 +995,7 @@ class MOZ_STACK_CLASS JS_PUBLIC_API(RootList) {
     // Explicitly add the given Node as a root in this RootList. If wantNames is
     // true, you must pass an edgeName. The RootList does not take ownership of
     // edgeName.
-    MOZ_MUST_USE bool addRoot(Node node, const char16_t* edgeName = nullptr);
+    [[nodiscard]] bool addRoot(Node node, const char16_t* edgeName = nullptr);
 };
 
 
@@ -1100,7 +1100,7 @@ class JS_PUBLIC_API(Concrete<JSObject>) : public TracerConcreteWithCompartment<J
     }
 
     const char* jsObjectClassName() const override;
-    MOZ_MUST_USE bool jsObjectConstructorName(JSContext* cx, UniqueTwoByteChars& outName)
+    [[nodiscard]] bool jsObjectConstructorName(JSContext* cx, UniqueTwoByteChars& outName)
         const override;
     Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
 

js/public/UbiNodeCensus.h

@@ -104,14 +104,14 @@ struct CountType {
 
     // Implement the 'count' method for counts returned by this CountType
     // instance's 'newCount' method.
-    virtual MOZ_MUST_USE bool count(CountBase& count,
-                                    mozilla::MallocSizeOf mallocSizeOf,
-                                    const Node& node) = 0;
+    [[nodiscard]] virtual bool count(CountBase& count,
+                                     mozilla::MallocSizeOf mallocSizeOf,
+                                     const Node& node) = 0;
 
     // Implement the 'report' method for counts returned by this CountType
     // instance's 'newCount' method.
-    virtual MOZ_MUST_USE bool report(JSContext* cx, CountBase& count,
-                                     MutableHandleValue report) = 0;
+    [[nodiscard]] virtual bool report(JSContext* cx, CountBase& count,
+                                      MutableHandleValue report) = 0;
 };
 
 using CountTypePtr = js::UniquePtr<CountType>;
@@ -135,7 +135,7 @@ class CountBase {
     { }
 
     // Categorize and count |node| as appropriate for this count's type.
-    MOZ_MUST_USE bool count(mozilla::MallocSizeOf mallocSizeOf, const Node& node) {
+    [[nodiscard]] bool count(mozilla::MallocSizeOf mallocSizeOf, const Node& node) {
         total_++;
 
         auto id = node.identifier();
@@ -158,7 +158,7 @@ class CountBase {
     // Construct a JavaScript object reporting the counts recorded in this
     // count, and store it in |report|. Return true on success, or false on
     // failure.
-    MOZ_MUST_USE bool report(JSContext* cx, MutableHandleValue report) {
+    [[nodiscard]] bool report(JSContext* cx, MutableHandleValue report) {
         return type.report(cx, *this, report);
     }
 
@@ -202,7 +202,7 @@ struct Census {
 
     explicit Census(JSContext* cx) : cx(cx), atomsZone(nullptr) { }
 
-    MOZ_MUST_USE JS_PUBLIC_API(bool) init();
+    [[nodiscard]] JS_PUBLIC_API(bool) init();
 };
 
 // A BreadthFirst handler type that conducts a census, using a CountBase to
@@ -219,14 +219,14 @@ class CensusHandler {
         mallocSizeOf(mallocSizeOf)
     { }
 
-    MOZ_MUST_USE bool report(JSContext* cx, MutableHandleValue report) {
+    [[nodiscard]] bool report(JSContext* cx, MutableHandleValue report) {
         return rootCount->report(cx, report);
     }
 
     // This class needs to retain no per-node data.
     class NodeData { };
 
-    MOZ_MUST_USE JS_PUBLIC_API(bool) operator() (BreadthFirst<CensusHandler>& traversal,
+    [[nodiscard]] JS_PUBLIC_API(bool) operator() (BreadthFirst<CensusHandler>& traversal,
                                                  Node origin, const Edge& edge,
                                                  NodeData* referentData, bool first);
 };
@@ -235,7 +235,7 @@ using CensusTraversal = BreadthFirst<CensusHandler>;
 
 // Examine the census options supplied by the API consumer, and (among other
 // things) use that to build a CountType tree.
-MOZ_MUST_USE JS_PUBLIC_API(bool) ParseCensusOptions(JSContext* cx,
+[[nodiscard]] JS_PUBLIC_API(bool) ParseCensusOptions(JSContext* cx,
                                                     Census& census, HandleObject options,
                                                     CountTypePtr& outResult);
 

js/public/UbiNodeDominatorTree.h

@@ -322,9 +322,9 @@ class DominatorTree
 
     // Do the post order traversal of the heap graph and populate our
     // predecessor sets.
-    static MOZ_MUST_USE bool doTraversal(JSContext* cx, AutoCheckCannotGC& noGC, const Node& root,
-                                         JS::ubi::Vector<Node>& postOrder,
-                                         PredecessorSets& predecessorSets) {
+    [[nodiscard]] static bool doTraversal(JSContext* cx, AutoCheckCannotGC& noGC, const Node& root,
+                                          JS::ubi::Vector<Node>& postOrder,
+                                          PredecessorSets& predecessorSets) {
         uint32_t nodeCount = 0;
         auto onNode = [&](const Node& node) {
             nodeCount++;
@@ -356,8 +356,8 @@ class DominatorTree
 
     // Populates the given `map` with an entry for each node to its index in
     // `postOrder`.
-    static MOZ_MUST_USE bool mapNodesToTheirIndices(JS::ubi::Vector<Node>& postOrder,
-                                                    NodeToIndexMap& map) {
+    [[nodiscard]] static bool mapNodesToTheirIndices(JS::ubi::Vector<Node>& postOrder,
+                                                     NodeToIndexMap& map) {
         MOZ_ASSERT(!map.initialized());
         MOZ_ASSERT(postOrder.length() < UINT32_MAX);
         uint32_t length = postOrder.length();
@@ -370,7 +370,7 @@ class DominatorTree
 
     // Convert the Node -> NodeSet predecessorSets to a index -> Vector<index>
     // form.
-    static MOZ_MUST_USE bool convertPredecessorSetsToVectors(
+    [[nodiscard]] static bool convertPredecessorSetsToVectors(
         const Node& root,
         JS::ubi::Vector<Node>& postOrder,
         PredecessorSets& predecessorSets,
@@ -409,7 +409,7 @@ class DominatorTree
 
     // Initialize `doms` such that the immediate dominator of the `root` is the
     // `root` itself and all others are `UNDEFINED`.
-    static MOZ_MUST_USE bool initializeDominators(JS::ubi::Vector<uint32_t>& doms,
+    [[nodiscard]] static bool initializeDominators(JS::ubi::Vector<uint32_t>& doms,
                                                   uint32_t length) {
         MOZ_ASSERT(doms.length() == 0);
         if (!doms.growByUninitialized(length))
@@ -426,7 +426,7 @@ class DominatorTree
         MOZ_ASSERT_IF(retainedSizes.isSome(), postOrder.length() == retainedSizes->length());
     }
 
-    MOZ_MUST_USE bool computeRetainedSizes(mozilla::MallocSizeOf mallocSizeOf) {
+    [[nodiscard]] bool computeRetainedSizes(mozilla::MallocSizeOf mallocSizeOf) {
         MOZ_ASSERT(retainedSizes.isNothing());
         auto length = postOrder.length();
 
@@ -651,7 +651,7 @@ class DominatorTree
      * `outSize`, or 0 if `node` is not a member of the dominator tree. Returns
      * false on OOM failure, leaving `outSize` unchanged.
      */
-    MOZ_MUST_USE bool getRetainedSize(const Node& node, mozilla::MallocSizeOf mallocSizeOf,
+    [[nodiscard]] bool getRetainedSize(const Node& node, mozilla::MallocSizeOf mallocSizeOf,
                                       Node::Size& outSize) {
         assertSanity();
         auto ptr = nodeToPostOrderIndex.lookup(node);

js/public/UbiNodePostOrder.h

@@ -90,7 +90,7 @@ struct PostOrder {
 #endif
 
   private:
-    MOZ_MUST_USE bool fillEdgesFromRange(EdgeVector& edges, js::UniquePtr<EdgeRange>& range) {
+    [[nodiscard]] bool fillEdgesFromRange(EdgeVector& edges, js::UniquePtr<EdgeRange>& range) {
         MOZ_ASSERT(range);
         for ( ; !range->empty(); range->popFront()) {
             if (!edges.append(mozilla::Move(range->front())))
@@ -99,7 +99,7 @@ struct PostOrder {
         return true;
     }
 
-    MOZ_MUST_USE bool pushForTraversing(const Node& node) {
+    [[nodiscard]] bool pushForTraversing(const Node& node) {
         EdgeVector edges;
         auto range = node.edges(cx, /* wantNames */ false);
         return range &&
@@ -124,11 +124,11 @@ struct PostOrder {
     { }
 
     // Initialize this traversal object. Return false on OOM.
-    MOZ_MUST_USE bool init() { return seen.init(); }
+    [[nodiscard]] bool init() { return seen.init(); }
 
     // Add `node` as a starting point for the traversal. You may add
     // as many starting points as you like. Returns false on OOM.
-    MOZ_MUST_USE bool addStart(const Node& node) {
+    [[nodiscard]] bool addStart(const Node& node) {
         if (!seen.put(node))
             return false;
         return pushForTraversing(node);
@@ -144,7 +144,7 @@ struct PostOrder {
     // Return false on OOM or error return from `onNode::operator()` or
     // `onEdge::operator()`.
     template<typename NodeVisitor, typename EdgeVisitor>
-    MOZ_MUST_USE bool traverse(NodeVisitor onNode, EdgeVisitor onEdge) {
+    [[nodiscard]] bool traverse(NodeVisitor onNode, EdgeVisitor onEdge) {
 #ifdef DEBUG
         MOZ_ASSERT(!traversed, "Can only traverse() once!");
         traversed = true;

js/public/UbiNodeShortestPaths.h

@@ -32,7 +32,7 @@ struct JS_PUBLIC_API(BackEdge)
 
     BackEdge() : predecessor_(), name_(nullptr) { }
 
-    MOZ_MUST_USE bool init(const Node& predecessor, Edge& edge) {
+    [[nodiscard]] bool init(const Node& predecessor, Edge& edge) {
         MOZ_ASSERT(!predecessor_);
         MOZ_ASSERT(!name_);
 
@@ -289,7 +289,7 @@ struct JS_PUBLIC_API(ShortestPaths)
      * the given target, in which case `func` will not be invoked.
      */
     template <class Func>
-    MOZ_MUST_USE bool forEachPath(const Node& target, Func func) {
+    [[nodiscard]] bool forEachPath(const Node& target, Func func) {
         MOZ_ASSERT(initialized());
         MOZ_ASSERT(targets_.has(target));
 

js/public/Utility.h

@@ -352,7 +352,7 @@ namespace js {
  * instances of type |T|.  Return false if the calculation overflowed.
  */
 template <typename T>
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 CalculateAllocSize(size_t numElems, size_t* bytesOut)
 {
     *bytesOut = numElems * sizeof(T);
@@ -365,7 +365,7 @@ CalculateAllocSize(size_t numElems, size_t* bytesOut)
  * false if the calculation overflowed.
  */
 template <typename T, typename Extra>
-MOZ_MUST_USE inline bool
+[[nodiscard]] inline bool
 CalculateAllocSizeWithExtra(size_t numExtra, size_t* bytesOut)
 {
     *bytesOut = sizeof(T) + numExtra * sizeof(Extra);

js/src/builtin/AtomicsObject.h

@@ -21,21 +21,21 @@ class AtomicsObject : public JSObject
   public:
     static const Class class_;
     static JSObject* initClass(JSContext* cx, Handle<GlobalObject*> global);
-    static MOZ_MUST_USE bool toString(JSContext* cx, unsigned int argc, Value* vp);
+    [[nodiscard]] static bool toString(JSContext* cx, unsigned int argc, Value* vp);
 };
 
-MOZ_MUST_USE bool atomics_compareExchange(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_exchange(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_load(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_store(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_add(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_sub(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_and(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_or(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_xor(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_isLockFree(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_wait(JSContext* cx, unsigned argc, Value* vp);
-MOZ_MUST_USE bool atomics_notify(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_compareExchange(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_exchange(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_load(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_store(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_add(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_sub(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_and(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_or(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_xor(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_isLockFree(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_wait(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool atomics_notify(JSContext* cx, unsigned argc, Value* vp);
 
 /* asm.js callouts */
 namespace wasm { class Instance; }
@@ -52,14 +52,14 @@ class FutexRuntime
     friend class AutoLockFutexAPI;
 
 public:
-    static MOZ_MUST_USE bool initialize();
+    [[nodiscard]] static bool initialize();
     static void destroy();
 
     static void lock();
     static void unlock();
 
     FutexRuntime();
-    MOZ_MUST_USE bool initInstance();
+    [[nodiscard]] bool initInstance();
     void destroyInstance();
 
     // Parameters to notify().
@@ -83,8 +83,8 @@ public:
     // wait.
     //
     // wait() will not wake up spuriously.
-    MOZ_MUST_USE bool wait(JSContext* cx, js::UniqueLock<js::Mutex>& locked,
-                           mozilla::Maybe<mozilla::TimeDuration>& timeout, WaitResult* result);
+    [[nodiscard]] bool wait(JSContext* cx, js::UniqueLock<js::Mutex>& locked,
+                            mozilla::Maybe<mozilla::TimeDuration>& timeout, WaitResult* result);
 
     // Notify the thread represented by this Runtime.
     //

js/src/builtin/Eval.h

@@ -16,17 +16,17 @@ namespace js {
 // JSOP_EVAL which in turn calls DirectEval. Thus, even though IndirectEval is
 // the callee function object for *all* calls to eval, it is by construction
 // only ever called in the case indirect eval.
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 IndirectEval(JSContext* cx, unsigned argc, Value* vp);
 
 // Performs a direct eval of |v| (a string containing code, or another value
 // that will be vacuously returned), which must correspond to the currently-
 // executing stack frame, which must be a script frame.
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 DirectEval(JSContext* cx, HandleValue v, MutableHandleValue vp);
 
 // Performs a direct eval called from Ion code.
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 DirectEvalStringFromIon(JSContext* cx,
                         HandleObject scopeObj, HandleScript callerScript,
                         HandleValue newTargetValue, HandleString str,

js/src/builtin/MapObject.cpp

@@ -476,7 +476,7 @@ class js::OrderedHashTableRef : public gc::BufferableRef
 };
 
 template <typename ObjectT>
-inline static MOZ_MUST_USE bool
+[[nodiscard]] inline static bool
 WriteBarrierPostImpl(JSRuntime* rt, ObjectT* obj, const Value& keyValue)
 {
     if (MOZ_LIKELY(!keyValue.isObject()))
@@ -501,13 +501,13 @@ WriteBarrierPostImpl(JSRuntime* rt, ObjectT* obj, const Value& keyValue)
     return true;
 }
 
-inline static MOZ_MUST_USE bool
+[[nodiscard]] inline static bool
 WriteBarrierPost(JSRuntime* rt, MapObject* map, const Value& key)
 {
     return WriteBarrierPostImpl(rt, map, key);
 }
 
-inline static MOZ_MUST_USE bool
+[[nodiscard]] inline static bool
 WriteBarrierPost(JSRuntime* rt, SetObject* set, const Value& key)
 {
     return WriteBarrierPostImpl(rt, set, key);

js/src/builtin/MapObject.h

@@ -41,7 +41,7 @@ class HashableValue
 
     HashableValue() : value(UndefinedValue()) {}
 
-    MOZ_MUST_USE bool setValue(JSContext* cx, HandleValue v);
+    [[nodiscard]] bool setValue(JSContext* cx, HandleValue v);
     HashNumber hash(const mozilla::HashCodeScrambler& hcs) const;
     bool operator==(const HashableValue& other) const;
     HashableValue mark(JSTracer* trc) const;
@@ -66,7 +66,7 @@ class MutableWrappedPtrOperations<HashableValue, Wrapper>
   : public WrappedPtrOperations<HashableValue, Wrapper>
 {
   public:
-    MOZ_MUST_USE bool setValue(JSContext* cx, HandleValue v) {
+    [[nodiscard]] bool setValue(JSContext* cx, HandleValue v) {
         return static_cast<Wrapper*>(this)->get().setValue(cx, v);
     }
 };
@@ -107,26 +107,26 @@ class MapObject : public NativeObject {
 
     enum { NurseryKeysSlot, SlotCount };
 
-    static MOZ_MUST_USE bool getKeysAndValuesInterleaved(JSContext* cx, HandleObject obj,
+    [[nodiscard]] static bool getKeysAndValuesInterleaved(JSContext* cx, HandleObject obj,
                                             JS::MutableHandle<GCVector<JS::Value>> entries);
-    static MOZ_MUST_USE bool entries(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool has(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool entries(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool has(JSContext* cx, unsigned argc, Value* vp);
     static MapObject* create(JSContext* cx, HandleObject proto = nullptr);
 
     // Publicly exposed Map calls for JSAPI access (webidl maplike/setlike
     // interfaces, etc.)
     static uint32_t size(JSContext *cx, HandleObject obj);
-    static MOZ_MUST_USE bool get(JSContext *cx, HandleObject obj, HandleValue key,
+    [[nodiscard]] static bool get(JSContext *cx, HandleObject obj, HandleValue key,
                                  MutableHandleValue rval);
-    static MOZ_MUST_USE bool has(JSContext *cx, HandleObject obj, HandleValue key, bool* rval);
-    static MOZ_MUST_USE bool delete_(JSContext *cx, HandleObject obj, HandleValue key, bool* rval);
+    [[nodiscard]] static bool has(JSContext *cx, HandleObject obj, HandleValue key, bool* rval);
+    [[nodiscard]] static bool delete_(JSContext *cx, HandleObject obj, HandleValue key, bool* rval);
 
     // Set call for public JSAPI exposure. Does not actually return map object
     // as stated in spec, expects caller to return a value. for instance, with
     // webidl maplike/setlike, should return interface object.
-    static MOZ_MUST_USE bool set(JSContext *cx, HandleObject obj, HandleValue key, HandleValue val);
-    static MOZ_MUST_USE bool clear(JSContext *cx, HandleObject obj);
-    static MOZ_MUST_USE bool iterator(JSContext *cx, IteratorKind kind, HandleObject obj,
+    [[nodiscard]] static bool set(JSContext *cx, HandleObject obj, HandleValue key, HandleValue val);
+    [[nodiscard]] static bool clear(JSContext *cx, HandleObject obj);
+    [[nodiscard]] static bool iterator(JSContext *cx, IteratorKind kind, HandleObject obj,
                                       MutableHandleValue iter);
 
     using UnbarrieredTable = OrderedHashMap<Value, Value, UnbarrieredHashPolicy, RuntimeAllocPolicy>;
@@ -143,29 +143,29 @@ class MapObject : public NativeObject {
     static ValueMap& extract(const CallArgs& args);
     static void mark(JSTracer* trc, JSObject* obj);
     static void finalize(FreeOp* fop, JSObject* obj);
-    static MOZ_MUST_USE bool construct(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool construct(JSContext* cx, unsigned argc, Value* vp);
 
     static bool is(HandleValue v);
     static bool is(HandleObject o);
 
-    static MOZ_MUST_USE bool iterator_impl(JSContext* cx, const CallArgs& args, IteratorKind kind);
+    [[nodiscard]] static bool iterator_impl(JSContext* cx, const CallArgs& args, IteratorKind kind);
 
-    static MOZ_MUST_USE bool size_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool size(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool get_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool get(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool has_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool set_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool set(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool delete_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool delete_(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool keys_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool keys(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool values_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool values(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool entries_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool clear_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool clear(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool size_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool size(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool get_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool get(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool has_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool set_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool set(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool delete_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool delete_(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool keys_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool keys(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool values_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool values(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool entries_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool clear_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool clear(JSContext* cx, unsigned argc, Value* vp);
 };
 
 class MapIteratorObject : public NativeObject
@@ -187,7 +187,7 @@ class MapIteratorObject : public NativeObject
                                      MapObject::IteratorKind kind);
     static void finalize(FreeOp* fop, JSObject* obj);
 
-    static MOZ_MUST_USE bool next(Handle<MapIteratorObject*> mapIterator,
+    [[nodiscard]] static bool next(Handle<MapIteratorObject*> mapIterator,
                                   HandleArrayObject resultPairObj, JSContext* cx);
 
     static JSObject* createResultPair(JSContext* cx);
@@ -213,21 +213,21 @@ class SetObject : public NativeObject {
 
     enum { NurseryKeysSlot, SlotCount };
 
-    static MOZ_MUST_USE bool keys(JSContext *cx, HandleObject obj,
+    [[nodiscard]] static bool keys(JSContext *cx, HandleObject obj,
                                   JS::MutableHandle<GCVector<JS::Value>> keys);
-    static MOZ_MUST_USE bool values(JSContext *cx, unsigned argc, Value *vp);
-    static MOZ_MUST_USE bool add(JSContext *cx, HandleObject obj, HandleValue key);
-    static MOZ_MUST_USE bool has(JSContext *cx, unsigned argc, Value *vp);
+    [[nodiscard]] static bool values(JSContext *cx, unsigned argc, Value *vp);
+    [[nodiscard]] static bool add(JSContext *cx, HandleObject obj, HandleValue key);
+    [[nodiscard]] static bool has(JSContext *cx, unsigned argc, Value *vp);
 
     // Publicly exposed Set calls for JSAPI access (webidl maplike/setlike
     // interfaces, etc.)
     static SetObject* create(JSContext *cx, HandleObject proto = nullptr);
     static uint32_t size(JSContext *cx, HandleObject obj);
-    static MOZ_MUST_USE bool has(JSContext *cx, HandleObject obj, HandleValue key, bool* rval);
-    static MOZ_MUST_USE bool clear(JSContext *cx, HandleObject obj);
-    static MOZ_MUST_USE bool iterator(JSContext *cx, IteratorKind kind, HandleObject obj,
+    [[nodiscard]] static bool has(JSContext *cx, HandleObject obj, HandleValue key, bool* rval);
+    [[nodiscard]] static bool clear(JSContext *cx, HandleObject obj);
+    [[nodiscard]] static bool iterator(JSContext *cx, IteratorKind kind, HandleObject obj,
                                       MutableHandleValue iter);
-    static MOZ_MUST_USE bool delete_(JSContext *cx, HandleObject obj, HandleValue key, bool *rval);
+    [[nodiscard]] static bool delete_(JSContext *cx, HandleObject obj, HandleValue key, bool *rval);
 
     using UnbarrieredTable = OrderedHashSet<Value, UnbarrieredHashPolicy, RuntimeAllocPolicy>;
     friend class OrderedHashTableRef<SetObject>;
@@ -251,20 +251,20 @@ class SetObject : public NativeObject {
 
     static bool isBuiltinAdd(HandleValue add, JSContext* cx);
 
-    static MOZ_MUST_USE bool iterator_impl(JSContext* cx, const CallArgs& args, IteratorKind kind);
+    [[nodiscard]] static bool iterator_impl(JSContext* cx, const CallArgs& args, IteratorKind kind);
 
-    static MOZ_MUST_USE bool size_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool size(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool has_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool add_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool add(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool delete_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool delete_(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool values_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool entries_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool entries(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool clear_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool clear(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool size_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool size(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool has_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool add_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool add(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool delete_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool delete_(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool values_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool entries_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool entries(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool clear_impl(JSContext* cx, const CallArgs& args);
+    [[nodiscard]] static bool clear(JSContext* cx, unsigned argc, Value* vp);
 };
 
 class SetIteratorObject : public NativeObject
@@ -286,7 +286,7 @@ class SetIteratorObject : public NativeObject
                                      SetObject::IteratorKind kind);
     static void finalize(FreeOp* fop, JSObject* obj);
 
-    static MOZ_MUST_USE bool next(Handle<SetIteratorObject*> setIterator,
+    [[nodiscard]] static bool next(Handle<SetIteratorObject*> setIterator,
                                   HandleArrayObject resultObj, JSContext* cx);
 
     static JSObject* createResult(JSContext* cx);
@@ -299,7 +299,7 @@ using SetInitGetPrototypeOp = NativeObject* (*)(JSContext*, Handle<GlobalObject*
 using SetInitIsBuiltinOp = bool (*)(HandleValue, JSContext*);
 
 template <SetInitGetPrototypeOp getPrototypeOp, SetInitIsBuiltinOp isBuiltinOp>
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 IsOptimizableInitForSet(JSContext* cx, HandleObject setObject, HandleValue iterable, bool* optimized)
 {
     MOZ_ASSERT(!*optimized);

js/src/builtin/Object.h

@@ -18,10 +18,10 @@ class Value;
 namespace js {
 
 // Object constructor native. Exposed only so the JIT can know its address.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_construct(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_propertyIsEnumerable(JSContext* cx, unsigned argc, Value* vp);
 
 PlainObject*
@@ -32,32 +32,32 @@ PlainObject*
 ObjectCreateWithTemplate(JSContext* cx, HandlePlainObject templateObj);
 
 // Object methods exposed so they can be installed in the self-hosting global.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_create(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_defineProperty(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_getOwnPropertyNames(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_getOwnPropertyDescriptor(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_getPrototypeOf(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_hasOwnProperty(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_isExtensible(JSContext* cx, unsigned argc, JS::Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 obj_toString(JSContext* cx, unsigned argc, JS::Value* vp);
 
 // Exposed so SelfHosting.cpp can use it in the OwnPropertyKeys intrinsic
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 GetOwnPropertyKeys(JSContext* cx, const JS::CallArgs& args, unsigned flags);
 
 /*
@@ -65,7 +65,7 @@ GetOwnPropertyKeys(JSContext* cx, const JS::CallArgs& args, unsigned flags);
  * exposing a jsid to script for Object.getOwnProperty{Names,Symbols}
  * or scriptable proxy traps.
  */
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 IdToStringOrSymbol(JSContext* cx, JS::HandleId id, JS::MutableHandleValue result);
 
 #if JS_HAS_TOSOURCE
@@ -74,7 +74,7 @@ JSString*
 ObjectToSource(JSContext* cx, JS::HandleObject obj);
 #endif // JS_HAS_TOSOURCE
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 WatchHandler(JSContext* cx, JSObject* obj, jsid id, const JS::Value& old,
              JS::Value* nvp, void* closure);
 

js/src/builtin/Profilers.h

@@ -29,21 +29,21 @@ typedef int pid_t;
  *
  * Returns true if no profilers fail to start.
  */
-extern MOZ_MUST_USE JS_PUBLIC_API(bool)
+[[nodiscard]] extern JS_PUBLIC_API(bool)
 JS_StartProfiling(const char* profileName, pid_t pid);
 
 /**
  * Stop any profilers that were previously started with JS_StartProfiling.
  * Returns true if no profilers fail to stop.
  */
-extern MOZ_MUST_USE JS_PUBLIC_API(bool)
+[[nodiscard]] extern JS_PUBLIC_API(bool)
 JS_StopProfiling(const char* profileName);
 
 /**
  * Write the current profile data to the given file, if applicable to whatever
  * profiler is being used.
  */
-extern MOZ_MUST_USE JS_PUBLIC_API(bool)
+[[nodiscard]] extern JS_PUBLIC_API(bool)
 JS_DumpProfile(const char* outfile, const char* profileName);
 
 /**
@@ -51,13 +51,13 @@ JS_DumpProfile(const char* outfile, const char* profileName);
  * whether any profilers failed to pause. (Profilers that do not support
  * pause/resume do not count.)
  */
-extern MOZ_MUST_USE JS_PUBLIC_API(bool)
+[[nodiscard]] extern JS_PUBLIC_API(bool)
 JS_PauseProfilers(const char* profileName);
 
 /**
  * Resume suspended profilers
  */
-extern MOZ_MUST_USE JS_PUBLIC_API(bool)
+[[nodiscard]] extern JS_PUBLIC_API(bool)
 JS_ResumeProfilers(const char* profileName);
 
 /**
@@ -70,23 +70,23 @@ JS_UnsafeGetLastProfilingError();
 
 #ifdef MOZ_CALLGRIND
 
-extern MOZ_MUST_USE JS_FRIEND_API(bool)
+[[nodiscard]] extern JS_FRIEND_API(bool)
 js_StopCallgrind();
 
-extern MOZ_MUST_USE JS_FRIEND_API(bool)
+[[nodiscard]] extern JS_FRIEND_API(bool)
 js_StartCallgrind();
 
-extern MOZ_MUST_USE JS_FRIEND_API(bool)
+[[nodiscard]] extern JS_FRIEND_API(bool)
 js_DumpCallgrind(const char* outfile);
 
 #endif /* MOZ_CALLGRIND */
 
 #ifdef __linux__
 
-extern MOZ_MUST_USE JS_FRIEND_API(bool)
+[[nodiscard]] extern JS_FRIEND_API(bool)
 js_StartPerf();
 
-extern MOZ_MUST_USE JS_FRIEND_API(bool)
+[[nodiscard]] extern JS_FRIEND_API(bool)
 js_StopPerf();
 
 #endif /* __linux__ */

js/src/builtin/Promise.cpp

@@ -311,7 +311,7 @@ class MutableWrappedPtrOperations<PromiseCombinatorElements, Wrapper>
     elements().setElementNeedsWrapping = needsWrapping;
   }
 
-  MOZ_MUST_USE bool pushUndefined(JSContext* cx) {
+  [[nodiscard]] bool pushUndefined(JSContext* cx) {
     // Helper for the AutoCompartment we need to work with |array|. We mostly do this
     // for performance; we could go ahead and do the define via a cross-
     // compartment proxy instead...
@@ -335,7 +335,7 @@ class MutableWrappedPtrOperations<PromiseCombinatorElements, Wrapper>
   // Promise.all/allSettled/any function, which isn't necessarily the same
   // compartment as unwrappedArray as explained in NewPromiseCombinatorElements.
   // So before storing |val| we may need to enter unwrappedArray's compartment.
-  MOZ_MUST_USE bool setElement(JSContext* cx, uint32_t index, HandleValue val) {
+  [[nodiscard]] bool setElement(JSContext* cx, uint32_t index, HandleValue val) {
     // The index is guaranteed to be initialized to `undefined`.
     MOZ_ASSERT(unwrappedArray()->getDenseElement(index).isUndefined());
 
@@ -560,7 +560,7 @@ MaybeGetAndClearException(JSContext* cx, MutableHandleValue rval)
     return GetAndClearException(cx, rval);
 }
 
-static MOZ_MUST_USE bool RunResolutionFunction(JSContext *cx, HandleObject resolutionFun,
+[[nodiscard]] static bool RunResolutionFunction(JSContext *cx, HandleObject resolutionFun,
                                                HandleValue result, ResolutionMode mode,
                                                HandleObject promiseObj);
 
@@ -778,7 +778,7 @@ static bool ResolvePromiseFunction(JSContext* cx, unsigned argc, Value* vp);
 static bool RejectPromiseFunction(JSContext* cx, unsigned argc, Value* vp);
 
 // ES2016, 25.4.1.3.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 CreateResolvingFunctions(JSContext* cx, HandleObject promise,
                          MutableHandleObject resolveFn,
                          MutableHandleObject rejectFn)
@@ -823,11 +823,11 @@ IsSettledMaybeWrappedPromise(JSObject* promise)
 }
 
 // ES2016, 25.4.1.7.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 RejectMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue reason);
 
 // ES2016, 25.4.1.7.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 RejectPromiseInternal(JSContext* cx, Handle<PromiseObject*> promise, HandleValue reason);
 
 // ES2016, 25.4.1.3.1.
@@ -874,15 +874,15 @@ RejectPromiseFunction(JSContext* cx, unsigned argc, Value* vp)
     return true;
 }
 
-static MOZ_MUST_USE bool FulfillMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj,
+[[nodiscard]] static bool FulfillMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj,
                                                     HandleValue value_);
 
-static MOZ_MUST_USE bool EnqueuePromiseResolveThenableJob(JSContext* cx,
+[[nodiscard]] static bool EnqueuePromiseResolveThenableJob(JSContext* cx,
                                                           HandleValue promiseToResolve,
                                                           HandleValue thenable,
                                                           HandleValue thenVal);
 
-static MOZ_MUST_USE bool EnqueuePromiseResolveThenableBuiltinJob(JSContext* cx,
+[[nodiscard]] static bool EnqueuePromiseResolveThenableBuiltinJob(JSContext* cx,
                                                                  HandleObject promiseToResolve,
                                                                  HandleObject thenable);
 
@@ -891,7 +891,7 @@ static bool Promise_then_impl(JSContext* cx, HandleValue promiseVal, HandleValue
                               HandleValue onRejected, MutableHandleValue rval, bool rvalUsed);
 
 // ES2016, 25.4.1.3.2, steps 6-13.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 ResolvePromiseInternal(JSContext* cx, HandleObject promise, HandleValue resolutionVal)
 {
     assertSameCompartment(cx, promise, resolutionVal);
@@ -1024,7 +1024,7 @@ static bool PromiseReactionJob(JSContext* cx, unsigned argc, Value* vp);
  * targetState - The PromiseState this reaction job targets. This decides
  *               whether the onFulfilled or onRejected handler is called.
  */
-MOZ_MUST_USE static bool
+[[nodiscard]] static bool
 EnqueuePromiseReactionJob(JSContext* cx, HandleObject reactionObj,
                           HandleValue handlerArg_, JS::PromiseState targetState)
 {
@@ -1130,11 +1130,11 @@ EnqueuePromiseReactionJob(JSContext* cx, HandleObject reactionObj,
     return cx->runtime()->enqueuePromiseJob(cx, job, promise, global);
 }
 
-static MOZ_MUST_USE bool TriggerPromiseReactions(JSContext* cx, HandleValue reactionsVal,
+[[nodiscard]] static bool TriggerPromiseReactions(JSContext* cx, HandleValue reactionsVal,
                                                  JS::PromiseState state, HandleValue valueOrReason);
 
 // ES2016, Commoned-out implementation of 25.4.1.4. and 25.4.1.7.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 ResolvePromise(JSContext* cx, Handle<PromiseObject*> promise, HandleValue valueOrReason,
                JS::PromiseState state)
 {
@@ -1177,14 +1177,14 @@ ResolvePromise(JSContext* cx, Handle<PromiseObject*> promise, HandleValue valueO
 }
 
 // ES2016, 25.4.1.7.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 RejectPromiseInternal(JSContext* cx, Handle<PromiseObject*> promise, HandleValue reason)
 {
     return ResolvePromise(cx, promise, reason, JS::PromiseState::Rejected);
 }
 
 // ES2016, 25.4.1.4.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 FulfillMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue value_)
 {
     Rooted<PromiseObject*> promise(cx);
@@ -1210,7 +1210,7 @@ FulfillMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue v
 
 static bool GetCapabilitiesExecutor(JSContext* cx, unsigned argc, Value* vp);
 static bool PromiseConstructor(JSContext* cx, unsigned argc, Value* vp);
-static MOZ_MUST_USE PromiseObject* CreatePromiseObjectInternal(JSContext* cx,
+[[nodiscard]] static PromiseObject* CreatePromiseObjectInternal(JSContext* cx,
                                                                HandleObject proto = nullptr,
                                                                bool protoIsWrapped = false,
                                                                bool informDebugger = true);
@@ -1220,7 +1220,7 @@ enum GetCapabilitiesExecutorSlots {
     GetCapabilitiesExecutorSlots_Reject
 };
 
-static MOZ_MUST_USE PromiseObject*
+[[nodiscard]] static PromiseObject*
 CreatePromiseObjectWithoutResolutionFunctions(JSContext* cx)
 {
     Rooted<PromiseObject*> promise(cx, CreatePromiseObjectInternal(cx));
@@ -1231,7 +1231,7 @@ CreatePromiseObjectWithoutResolutionFunctions(JSContext* cx)
     return promise;
 }
 
-static MOZ_MUST_USE PromiseObject*
+[[nodiscard]] static PromiseObject*
 CreatePromiseWithDefaultResolutionFunctions(JSContext* cx, MutableHandleObject resolve,
                                             MutableHandleObject reject)
 {
@@ -1258,7 +1258,7 @@ CreatePromiseWithDefaultResolutionFunctions(JSContext* cx, MutableHandleObject r
 }
 
 // ES2016, 25.4.1.5.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 NewPromiseCapability(JSContext* cx, HandleObject C, MutableHandle<PromiseCapability> capability,
                      bool canOmitResolutionFunctions)
 {
@@ -1369,7 +1369,7 @@ GetCapabilitiesExecutor(JSContext* cx, unsigned argc, Value* vp)
 }
 
 // ES2016, 25.4.1.7.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 RejectMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue reason_)
 {
     Rooted<PromiseObject*> promise(cx);
@@ -1410,7 +1410,7 @@ RejectMaybeWrappedPromise(JSContext *cx, HandleObject promiseObj, HandleValue re
 }
 
 // ES2016, 25.4.1.8.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 TriggerPromiseReactions(JSContext* cx, HandleValue reactionsVal, JS::PromiseState state,
                         HandleValue valueOrReason)
 {
@@ -1440,7 +1440,7 @@ TriggerPromiseReactions(JSContext* cx, HandleValue reactionsVal, JS::PromiseStat
 // Implements PromiseReactionJob optimized for the case when the reaction
 // handler is one of the default resolving functions as created by the
 // CreateResolvingFunctions abstract operation.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 DefaultResolvingPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> reaction,
                                    MutableHandleValue rval)
 {
@@ -1485,7 +1485,7 @@ DefaultResolvingPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*>
     return true;
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AsyncFunctionPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> reaction,
                                 MutableHandleValue rval)
 {
@@ -1513,7 +1513,7 @@ AsyncFunctionPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> re
     return true;
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AsyncGeneratorPromiseReactionJob(JSContext* cx, Handle<PromiseReactionRecord*> reaction,
                                  MutableHandleValue rval)
 {
@@ -1723,7 +1723,7 @@ PromiseResolveThenableJob(JSContext* cx, unsigned argc, Value* vp)
     return Call(cx, rejectVal, UndefinedHandleValue, rval, &rval);
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 OriginalPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                          Handle<PromiseObject*> promiseToResolve);
 
@@ -1785,7 +1785,7 @@ PromiseResolveBuiltinThenableJob(JSContext* cx, unsigned argc, Value* vp)
  * thenable_ - The thenable to resolve the Promise with.
  * thenVal - The `then` function to invoke with the `thenable` as the receiver.
  */
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 EnqueuePromiseResolveThenableJob(JSContext* cx, HandleValue promiseToResolve_,
                                  HandleValue thenable_, HandleValue thenVal)
 {
@@ -1847,7 +1847,7 @@ EnqueuePromiseResolveThenableJob(JSContext* cx, HandleValue promiseToResolve_,
  * promiseToResolve - The promise to resolve, obviously.
  * thenable - The thenable to resolve the Promise with.
  */
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 EnqueuePromiseResolveThenableBuiltinJob(JSContext* cx, HandleObject promiseToResolve,
                                         HandleObject thenable)
 {
@@ -1869,11 +1869,11 @@ EnqueuePromiseResolveThenableBuiltinJob(JSContext* cx, HandleObject promiseToRes
     return cx->runtime()->enqueuePromiseJob(cx, job, promiseToResolve, incumbentGlobal);
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AddDummyPromiseReactionForDebugger(JSContext* cx, Handle<PromiseObject*> promise,
                                    HandleObject dependentPromise);
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AddPromiseReaction(JSContext* cx, Handle<PromiseObject*> promise,
                    Handle<PromiseReactionRecord*> reaction);
 
@@ -1947,7 +1947,7 @@ ClearResolutionFunctionSlots(JSFunction* resolutionFun)
 }
 
 // ES2016, 25.4.3.1. steps 3-7.
-static MOZ_MUST_USE PromiseObject*
+[[nodiscard]] static PromiseObject*
 CreatePromiseObjectInternal(JSContext* cx, HandleObject proto /* = nullptr */,
                             bool protoIsWrapped /* = false */, bool informDebugger /* = true */)
 {
@@ -2175,19 +2175,19 @@ class MOZ_STACK_CLASS PromiseForOfIterator : public JS::ForOfIterator {
     }
 };
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseAll(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                   Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseAllSettled(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                          Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseAny(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                   Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseRace(JSContext* cx, PromiseForOfIterator& iterator, HandleObject C,
                    Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done);
 
@@ -2204,7 +2204,7 @@ enum class CombinatorKind { All, AllSettled, Any, Race };
 // https://tc39.es/proposal-promise-any/
 //
 // Promise.any ( iterable )
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 CommonPromiseCombinator(JSContext* cx, CallArgs& args, CombinatorKind mode)
 {
     HandleValue iterable = args.get(0);
@@ -2340,11 +2340,11 @@ static bool Promise_static_all(JSContext* cx, unsigned argc, Value* vp) {
   return CommonPromiseCombinator(cx, args, CombinatorKind::All);
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseThen(JSContext* cx, Handle<PromiseObject*> promise, HandleValue onFulfilled_,
                    HandleValue onRejected_, Handle<PromiseCapability> resultCapability);
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                         Handle<PromiseObject*> promiseToResolve,
                                         Handle<PromiseCapability> resultCapability);
@@ -2356,7 +2356,7 @@ static JSFunction* NewPromiseCombinatorElementFunction(
 static bool PromiseAllResolveElementFunction(JSContext* cx, unsigned argc, Value* vp);
 
 // Unforgeable version of ES2016, 25.4.4.1.
-MOZ_MUST_USE JSObject*
+[[nodiscard]] JSObject*
 js::GetWaitForAllPromise(JSContext* cx, const JS::AutoObjectVector& promises)
 {
 #ifdef DEBUG
@@ -2476,7 +2476,7 @@ js::GetWaitForAllPromise(JSContext* cx, const JS::AutoObjectVector& promises)
     return resultCapability.promise();
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 RunResolutionFunction(JSContext *cx, HandleObject resolutionFun, HandleValue result,
                       ResolutionMode mode, HandleObject promiseObj)
 {
@@ -2528,7 +2528,7 @@ RunResolutionFunction(JSContext *cx, HandleObject resolutionFun, HandleValue res
     return RejectPromiseInternal(cx, promise, result);
 }
 
-static MOZ_MUST_USE JSObject*
+[[nodiscard]] static JSObject*
 CommonStaticResolveRejectImpl(JSContext* cx, HandleValue thisVal, HandleValue argVal,
                               ResolutionMode mode);
 
@@ -2544,7 +2544,7 @@ IsPromiseSpecies(JSContext* cx, JSFunction* species);
 // https://tc39.es/proposal-promise-any/
 // Runtime Semantics: PerformPromiseAny, steps 6-8.
 template <typename T>
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 CommonPerformPromiseCombinator(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                                HandleObject resultPromise, HandleValue promiseResolve, bool* done,
                                bool resolveReturnsUndefined, T getResolveAndReject)
@@ -2810,7 +2810,7 @@ CommonPerformPromiseCombinator(JSContext *cx, PromiseForOfIterator& iterator, Ha
 
 // Create the elements for the Promise combinators Promise.all and
 // Promise.allSettled.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 NewPromiseCombinatorElements(JSContext* cx, Handle<PromiseCapability> resultCapability,
                              MutableHandle<PromiseCombinatorElements> elements)
 {
@@ -2862,7 +2862,7 @@ NewPromiseCombinatorElements(JSContext* cx, Handle<PromiseCapability> resultCapa
 }
 
 // Retrieve the combinator elements from the data holder.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 GetPromiseCombinatorElements(JSContext* cx, Handle<PromiseCombinatorDataHolder*> data,
                              MutableHandle<PromiseCombinatorElements> elements)
 {
@@ -2947,7 +2947,7 @@ PromiseCombinatorElementFunctionAlreadyCalled(const CallArgs& args,
 
 // ES2020 draft rev a09fc232c137800dbf51b6204f37fdede4ba1646
 // 25.6.4.1.1 PerformPromiseAll (iteratorRecord, constructor, resultCapability)
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseAll(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                   Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done)
 {
@@ -3083,7 +3083,7 @@ Promise_static_race(JSContext* cx, unsigned argc, Value* vp)
 
 // ES2020 draft rev a09fc232c137800dbf51b6204f37fdede4ba1646
 // 25.6.4.3.1 PerformPromiseRace (iteratorRecord, constructor, resultCapability)
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseRace(JSContext *cx, PromiseForOfIterator& iterator, HandleObject C,
                    Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done)
 {
@@ -3138,7 +3138,7 @@ static bool Promise_static_allSettled(JSContext* cx, unsigned argc, Value* vp) {
 // 25.6.4.2 Promise.allSettled ( iterable )
 //
 // PerformPromiseAllSettled ( iteratorRecord, constructor, resultCapability )
-static MOZ_MUST_USE bool PerformPromiseAllSettled(
+[[nodiscard]] static bool PerformPromiseAllSettled(
     JSContext* cx, PromiseForOfIterator& iterator, HandleObject C,
     Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done) {
   *done = false;
@@ -3349,7 +3349,7 @@ static void ThrowAggregateError(JSContext* cx,
 // https://tc39.es/proposal-promise-any/
 //
 // PerformPromiseAny ( iteratorRecord, constructor, resultCapability )
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseAny(JSContext* cx, PromiseForOfIterator& iterator, HandleObject C,
                   Handle<PromiseCapability> resultCapability, HandleValue promiseResolve, bool* done)
 {
@@ -3553,7 +3553,7 @@ ThrowAggregateError(JSContext* cx, Handle<PromiseCombinatorElements> errors, Han
 // 25.6.4.4 Promise.reject ( r )
 // 25.6.4.5 Promise.resolve ( x )
 // 25.6.4.5.1 PromiseResolve ( C, x )
-static MOZ_MUST_USE JSObject*
+[[nodiscard]] static JSObject*
 CommonStaticResolveRejectImpl(JSContext* cx, HandleValue thisVal, HandleValue argVal,
                               ResolutionMode mode)
 {
@@ -3610,7 +3610,7 @@ CommonStaticResolveRejectImpl(JSContext* cx, HandleValue thisVal, HandleValue ar
     return capability.promise();
 }
 
-MOZ_MUST_USE JSObject*
+[[nodiscard]] JSObject*
 js::PromiseResolve(JSContext* cx, HandleObject constructor, HandleValue value)
 {
     RootedValue C(cx, ObjectValue(*constructor));
@@ -3789,7 +3789,7 @@ PromiseThenNewPromiseCapability(JSContext* cx, HandleObject promiseObj,
 }
 
 // ES2016, 25.4.5.3., steps 3-5.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::OriginalPromiseThen(JSContext* cx, Handle<PromiseObject*> promise,
                         HandleValue onFulfilled, HandleValue onRejected,
                         MutableHandleObject dependent, CreateDependentPromise createDependent)
@@ -3813,7 +3813,7 @@ js::OriginalPromiseThen(JSContext* cx, Handle<PromiseObject*> promise,
     return true;
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 OriginalPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                          Handle<PromiseObject*> promiseToResolve)
 {
@@ -3870,7 +3870,7 @@ OriginalPromiseThenBuiltin(JSContext* cx, HandleValue promiseVal, HandleValue on
     return true;
 }
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::RejectPromiseWithPendingError(JSContext* cx, Handle<PromiseObject*> promise)
 {
     // Not much we can do about uncatchable exceptions, just bail.
@@ -3880,7 +3880,7 @@ js::RejectPromiseWithPendingError(JSContext* cx, Handle<PromiseObject*> promise)
     return PromiseObject::reject(cx, promise, exn);
 }
 
-static MOZ_MUST_USE bool PerformPromiseThenWithReaction(JSContext* cx,
+[[nodiscard]] static bool PerformPromiseThenWithReaction(JSContext* cx,
                                                         Handle<PromiseObject*> promise,
                                                         Handle<PromiseReactionRecord*> reaction);
 
@@ -3888,7 +3888,7 @@ static MOZ_MUST_USE bool PerformPromiseThenWithReaction(JSContext* cx,
 // js/src/builtin/AsyncFunction.cpp, to call Promise internal functions.
 
 // ES 2018 draft 14.6.11 and 14.7.14 step 1.
-MOZ_MUST_USE PromiseObject*
+[[nodiscard]] PromiseObject*
 js::CreatePromiseObjectForAsync(JSContext* cx, HandleValue generatorVal)
 {
     // Step 1.
@@ -3909,7 +3909,7 @@ js::IsPromiseForAsync(JSObject* promise)
 }
 
 // ES 2018 draft 25.5.5.2 steps 3.f, 3.g.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncFunctionThrown(JSContext* cx, Handle<PromiseObject*> resultPromise)
 {
     // Step 3.f.
@@ -3925,7 +3925,7 @@ js::AsyncFunctionThrown(JSContext* cx, Handle<PromiseObject*> resultPromise)
 }
 
 // ES 2018 draft 25.5.5.2 steps 3.d-e, 3.g.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncFunctionReturned(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value)
 {
     // Steps 3.d-e.
@@ -3939,7 +3939,7 @@ js::AsyncFunctionReturned(JSContext* cx, Handle<PromiseObject*> resultPromise, H
 // Helper function that performs the equivalent steps as
 // Async Iteration proposal 4.1 Await steps 2-3, 6-9 or similar.
 template <typename T>
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 InternalAwait(JSContext* cx, HandleValue value, HandleObject resultPromise,
               HandleValue onFulfilled, HandleValue onRejected, T extraStep)
 {
@@ -3972,7 +3972,7 @@ InternalAwait(JSContext* cx, HandleValue value, HandleObject resultPromise,
 }
 
 // ES 2018 draft 25.5.5.3 steps 2-10.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncFunctionAwait(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value)
 {
     // Steps 4-5.
@@ -3987,7 +3987,7 @@ js::AsyncFunctionAwait(JSContext* cx, Handle<PromiseObject*> resultPromise, Hand
 }
 
 // Async Iteration proposal 4.1 Await steps 2-9.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncGeneratorAwait(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                         HandleValue value)
 {
@@ -4146,13 +4146,13 @@ enum class ResumeNextKind {
     Enqueue, Reject, Resolve
 };
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                          ResumeNextKind kind, HandleValue valueOrException = UndefinedHandleValue,
                          bool done = false);
 
 // Async Iteration proposal 11.4.3.3.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncGeneratorResolve(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                           HandleValue value, bool done)
 {
@@ -4160,7 +4160,7 @@ js::AsyncGeneratorResolve(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenO
 }
 
 // Async Iteration proposal 11.4.3.4.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncGeneratorReject(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                          HandleValue exception)
 {
@@ -4168,7 +4168,7 @@ js::AsyncGeneratorReject(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenOb
 }
 
 // Async Iteration proposal 11.4.3.5.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                          ResumeNextKind kind,
                          HandleValue valueOrException_ /* = UndefinedHandleValue */,
@@ -4349,7 +4349,7 @@ AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenOb
 }
 
 // Async Iteration proposal 11.4.3.6.
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 js::AsyncGeneratorEnqueue(JSContext* cx, HandleValue asyncGenVal,
                           CompletionKind completionKind, HandleValue completionValue,
                           MutableHandleValue result)
@@ -4540,7 +4540,7 @@ Promise_then(JSContext* cx, unsigned argc, Value* vp)
 }
 
 // ES2016, 25.4.5.3.1.
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseThen(JSContext* cx, Handle<PromiseObject*> promise, HandleValue onFulfilled_,
                    HandleValue onRejected_, Handle<PromiseCapability> resultCapability)
 {
@@ -4567,7 +4567,7 @@ PerformPromiseThen(JSContext* cx, Handle<PromiseObject*> promise, HandleValue on
     return PerformPromiseThenWithReaction(cx, promise, reaction);
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> promise,
                                         Handle<PromiseObject*> promiseToResolve,
                                         Handle<PromiseCapability> resultCapability)
@@ -4593,7 +4593,7 @@ PerformPromiseThenWithoutSettleHandlers(JSContext* cx, Handle<PromiseObject*> pr
     return PerformPromiseThenWithReaction(cx, promise, reaction);
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 PerformPromiseThenWithReaction(JSContext* cx, Handle<PromiseObject*> promise,
                                Handle<PromiseReactionRecord*> reaction)
 {
@@ -4637,7 +4637,7 @@ PerformPromiseThenWithReaction(JSContext* cx, Handle<PromiseObject*> promise,
     return true;
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AddPromiseReaction(JSContext* cx, Handle<PromiseObject*> promise,
                    Handle<PromiseReactionRecord*> reaction)
 {
@@ -4705,7 +4705,7 @@ AddPromiseReaction(JSContext* cx, Handle<PromiseObject*> promise,
     return true;
 }
 
-static MOZ_MUST_USE bool
+[[nodiscard]] static bool
 AddDummyPromiseReactionForDebugger(JSContext* cx, Handle<PromiseObject*> promise,
                                    HandleObject dependentPromise)
 {

js/src/builtin/Promise.h

@@ -73,10 +73,10 @@ class PromiseObject : public NativeObject
         return getFixedSlot(PromiseSlot_ReactionsOrResult);
     }
 
-    static MOZ_MUST_USE bool resolve(JSContext* cx, Handle<PromiseObject*> promise,
-                                     HandleValue resolutionValue);
-    static MOZ_MUST_USE bool reject(JSContext* cx, Handle<PromiseObject*> promise,
-                                    HandleValue rejectionValue);
+    [[nodiscard]] static bool resolve(JSContext* cx, Handle<PromiseObject*> promise,
+                                      HandleValue resolutionValue);
+    [[nodiscard]] static bool reject(JSContext* cx, Handle<PromiseObject*> promise,
+                                     HandleValue rejectionValue);
 
     static void onSettled(JSContext* cx, Handle<PromiseObject*> promise);
 
@@ -89,7 +89,7 @@ class PromiseObject : public NativeObject
         MOZ_ASSERT(state() != JS::PromiseState::Pending);
         return resolutionTime() - allocationTime();
     }
-    MOZ_MUST_USE bool dependentPromises(JSContext* cx, MutableHandle<GCVector<Value>> values);
+    [[nodiscard]] bool dependentPromises(JSContext* cx, MutableHandle<GCVector<Value>> values);
     uint64_t getID();
     bool isUnhandled() {
         MOZ_ASSERT(state() == JS::PromiseState::Rejected);
@@ -113,7 +113,7 @@ class PromiseObject : public NativeObject
  * Asserts that all objects in the `promises` vector are, maybe wrapped,
  * instances of `Promise` or a subclass of `Promise`.
  */
-MOZ_MUST_USE JSObject*
+[[nodiscard]] JSObject*
 GetWaitForAllPromise(JSContext* cx, const JS::AutoObjectVector& promises);
 
 enum class CreateDependentPromise {
@@ -132,7 +132,7 @@ enum class CreateDependentPromise {
  * `promise` field that can contain null. That field is only ever used by
  * devtools, which have to treat these reactions specially.
  */
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 OriginalPromiseThen(JSContext* cx, Handle<PromiseObject*> promise,
                     HandleValue onFulfilled, HandleValue onRejected,
                     MutableHandleObject dependent, CreateDependentPromise createDependent);
@@ -143,41 +143,41 @@ OriginalPromiseThen(JSContext* cx, Handle<PromiseObject*> promise,
  * The abstract operation PromiseResolve, given a constructor and a value,
  * returns a new promise resolved with that value.
  */
-MOZ_MUST_USE JSObject*
+[[nodiscard]] JSObject*
 PromiseResolve(JSContext* cx, HandleObject constructor, HandleValue value);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 RejectPromiseWithPendingError(JSContext* cx, Handle<PromiseObject*> promise);
 
-MOZ_MUST_USE PromiseObject*
+[[nodiscard]] PromiseObject*
 CreatePromiseObjectForAsync(JSContext* cx, HandleValue generatorVal);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 IsPromiseForAsync(JSObject* promise);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncFunctionReturned(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncFunctionThrown(JSContext* cx, Handle<PromiseObject*> resultPromise);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncFunctionAwait(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value);
 
 class AsyncGeneratorObject;
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncGeneratorAwait(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj, HandleValue value);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncGeneratorResolve(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                       HandleValue value, bool done);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncGeneratorReject(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
                      HandleValue exception);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 AsyncGeneratorEnqueue(JSContext* cx, HandleValue asyncGenVal, CompletionKind completionKind,
                       HandleValue completionValue, MutableHandleValue result);
 

js/src/builtin/Reflect.h

@@ -17,10 +17,10 @@ InitReflect(JSContext* cx, js::HandleObject obj);
 
 namespace js {
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 Reflect_getPrototypeOf(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 Reflect_isExtensible(JSContext* cx, unsigned argc, Value* vp);
 
 }

js/src/builtin/ReflectParse.cpp

@@ -256,7 +256,7 @@ class NodeBuilder
           userv(c)
     {}
 
-    MOZ_MUST_USE bool init(HandleObject userobj = nullptr) {
+    [[nodiscard]] bool init(HandleObject userobj = nullptr) {
         if (src) {
             if (!atomValue(src, &srcval))
                 return false;
@@ -307,7 +307,7 @@ class NodeBuilder
     }
 
   private:
-    MOZ_MUST_USE bool callbackHelper(HandleValue fun, const InvokeArgs& args, size_t i,
+    [[nodiscard]] bool callbackHelper(HandleValue fun, const InvokeArgs& args, size_t i,
                                      TokenPos* pos, MutableHandleValue dst)
     {
         // The end of the implementation of callback(). All arguments except
@@ -324,7 +324,7 @@ class NodeBuilder
     // that convert to HandleValue, so this isn't as template-y as it seems,
     // just variadic.
     template <typename... Arguments>
-    MOZ_MUST_USE bool callbackHelper(HandleValue fun, const InvokeArgs& args, size_t i,
+    [[nodiscard]] bool callbackHelper(HandleValue fun, const InvokeArgs& args, size_t i,
                                      HandleValue head, Arguments&&... tail)
     {
         // Recursive loop to store the arguments into args. This eventually
@@ -338,7 +338,7 @@ class NodeBuilder
     //     bool callback(HandleValue fun, HandleValue... args, TokenPos* pos,
     //                   MutableHandleValue dst);
     template <typename... Arguments>
-    MOZ_MUST_USE bool callback(HandleValue fun, Arguments&&... args) {
+    [[nodiscard]] bool callback(HandleValue fun, Arguments&&... args) {
         InvokeArgs iargs(cx);
         if (!iargs.init(cx, sizeof...(args) - 2 + size_t(saveLoc)))
             return false;
@@ -355,7 +355,7 @@ class NodeBuilder
         return v.isMagic(JS_SERIALIZE_NO_NODE) ? JS::UndefinedHandleValue : v;
     }
 
-    MOZ_MUST_USE bool atomValue(const char* s, MutableHandleValue dst) {
+    [[nodiscard]] bool atomValue(const char* s, MutableHandleValue dst) {
         /*
          * Bug 575416: instead of Atomize, lookup constant atoms in tbl file
          */
@@ -367,7 +367,7 @@ class NodeBuilder
         return true;
     }
 
-    MOZ_MUST_USE bool newObject(MutableHandleObject dst) {
+    [[nodiscard]] bool newObject(MutableHandleObject dst) {
         RootedPlainObject nobj(cx, NewBuiltinClassInstance<PlainObject>(cx));
         if (!nobj)
             return false;
@@ -376,11 +376,11 @@ class NodeBuilder
         return true;
     }
 
-    MOZ_MUST_USE bool newArray(NodeVector& elts, MutableHandleValue dst);
+    [[nodiscard]] bool newArray(NodeVector& elts, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool createNode(ASTType type, TokenPos* pos, MutableHandleObject dst);
+    [[nodiscard]] bool createNode(ASTType type, TokenPos* pos, MutableHandleObject dst);
 
-    MOZ_MUST_USE bool newNodeHelper(HandleObject obj, MutableHandleValue dst) {
+    [[nodiscard]] bool newNodeHelper(HandleObject obj, MutableHandleValue dst) {
         // The end of the implementation of newNode().
         MOZ_ASSERT(obj);
         dst.setObject(*obj);
@@ -388,7 +388,7 @@ class NodeBuilder
     }
 
     template <typename... Arguments>
-    MOZ_MUST_USE bool newNodeHelper(HandleObject obj, const char *name, HandleValue value,
+    [[nodiscard]] bool newNodeHelper(HandleObject obj, const char *name, HandleValue value,
                                     Arguments&&... rest)
     {
         // Recursive loop to define properties. Note that the newNodeHelper()
@@ -407,13 +407,13 @@ class NodeBuilder
     //                  {const char *name0, HandleValue value0,}...
     //                  MutableHandleValue dst);
     template <typename... Arguments>
-    MOZ_MUST_USE bool newNode(ASTType type, TokenPos* pos, Arguments&&... args) {
+    [[nodiscard]] bool newNode(ASTType type, TokenPos* pos, Arguments&&... args) {
         RootedObject node(cx);
         return createNode(type, pos, &node) &&
                newNodeHelper(node, Forward<Arguments>(args)...);
     }
 
-    MOZ_MUST_USE bool listNode(ASTType type, const char* propName, NodeVector& elts, TokenPos* pos,
+    [[nodiscard]] bool listNode(ASTType type, const char* propName, NodeVector& elts, TokenPos* pos,
                                MutableHandleValue dst) {
         RootedValue array(cx);
         if (!newArray(elts, &array))
@@ -426,7 +426,7 @@ class NodeBuilder
         return newNode(type, pos, propName, array, dst);
     }
 
-    MOZ_MUST_USE bool defineProperty(HandleObject obj, const char* name, HandleValue val) {
+    [[nodiscard]] bool defineProperty(HandleObject obj, const char* name, HandleValue val) {
         MOZ_ASSERT_IF(val.isMagic(), val.whyMagic() == JS_SERIALIZE_NO_NODE);
 
         /*
@@ -441,9 +441,9 @@ class NodeBuilder
         return DefineProperty(cx, obj, atom->asPropertyName(), optVal);
     }
 
-    MOZ_MUST_USE bool newNodeLoc(TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool newNodeLoc(TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool setNodeLoc(HandleObject node, TokenPos* pos);
+    [[nodiscard]] bool setNodeLoc(HandleObject node, TokenPos* pos);
 
   public:
     /*
@@ -459,27 +459,27 @@ class NodeBuilder
      * misc nodes
      */
 
-    MOZ_MUST_USE bool program(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool program(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool literal(HandleValue val, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool literal(HandleValue val, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool identifier(HandleValue name, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool identifier(HandleValue name, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool function(ASTType type, TokenPos* pos,
+    [[nodiscard]] bool function(ASTType type, TokenPos* pos,
                                HandleValue id, NodeVector& args, NodeVector& defaults,
                                HandleValue body, HandleValue rest, GeneratorStyle generatorStyle,
                                bool isAsync, bool isExpression, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool variableDeclarator(HandleValue id, HandleValue init, TokenPos* pos,
+    [[nodiscard]] bool variableDeclarator(HandleValue id, HandleValue init, TokenPos* pos,
                                          MutableHandleValue dst);
 
-    MOZ_MUST_USE bool switchCase(HandleValue expr, NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool switchCase(HandleValue expr, NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool catchClause(HandleValue var, HandleValue guard, HandleValue body, TokenPos* pos,
+    [[nodiscard]] bool catchClause(HandleValue var, HandleValue guard, HandleValue body, TokenPos* pos,
                                   MutableHandleValue dst);
 
-    MOZ_MUST_USE bool prototypeMutation(HandleValue val, TokenPos* pos, MutableHandleValue dst);
-    MOZ_MUST_USE bool propertyInitializer(HandleValue key, HandleValue val, PropKind kind,
+    [[nodiscard]] bool prototypeMutation(HandleValue val, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool propertyInitializer(HandleValue key, HandleValue val, PropKind kind,
                                           bool isShorthand, bool isMethod, TokenPos* pos,
                                           MutableHandleValue dst);
 
@@ -488,165 +488,165 @@ class NodeBuilder
      * statements
      */
 
-    MOZ_MUST_USE bool blockStatement(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool blockStatement(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool expressionStatement(HandleValue expr, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool expressionStatement(HandleValue expr, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool emptyStatement(TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool emptyStatement(TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool ifStatement(HandleValue test, HandleValue cons, HandleValue alt, TokenPos* pos,
+    [[nodiscard]] bool ifStatement(HandleValue test, HandleValue cons, HandleValue alt, TokenPos* pos,
                      MutableHandleValue dst);
 
-    MOZ_MUST_USE bool breakStatement(HandleValue label, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool breakStatement(HandleValue label, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool continueStatement(HandleValue label, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool continueStatement(HandleValue label, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool labeledStatement(HandleValue label, HandleValue stmt, TokenPos* pos,
+    [[nodiscard]] bool labeledStatement(HandleValue label, HandleValue stmt, TokenPos* pos,
                           MutableHandleValue dst);
 
-    MOZ_MUST_USE bool throwStatement(HandleValue arg, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool throwStatement(HandleValue arg, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool returnStatement(HandleValue arg, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool returnStatement(HandleValue arg, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool forStatement(HandleValue init, HandleValue test, HandleValue update, HandleValue stmt,
+    [[nodiscard]] bool forStatement(HandleValue init, HandleValue test, HandleValue update, HandleValue stmt,
                       TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool forInStatement(HandleValue var, HandleValue expr, HandleValue stmt,
+    [[nodiscard]] bool forInStatement(HandleValue var, HandleValue expr, HandleValue stmt,
                                      bool isForEach, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool forOfStatement(HandleValue var, HandleValue expr, HandleValue stmt, TokenPos* pos,
+    [[nodiscard]] bool forOfStatement(HandleValue var, HandleValue expr, HandleValue stmt, TokenPos* pos,
                                      MutableHandleValue dst);
 
-    MOZ_MUST_USE bool withStatement(HandleValue expr, HandleValue stmt, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool withStatement(HandleValue expr, HandleValue stmt, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool whileStatement(HandleValue test, HandleValue stmt, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool whileStatement(HandleValue test, HandleValue stmt, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool doWhileStatement(HandleValue stmt, HandleValue test, TokenPos* pos,
+    [[nodiscard]] bool doWhileStatement(HandleValue stmt, HandleValue test, TokenPos* pos,
                                        MutableHandleValue dst);
 
-    MOZ_MUST_USE bool switchStatement(HandleValue disc, NodeVector& elts, bool lexical, TokenPos* pos,
+    [[nodiscard]] bool switchStatement(HandleValue disc, NodeVector& elts, bool lexical, TokenPos* pos,
                                       MutableHandleValue dst);
 
-    MOZ_MUST_USE bool tryStatement(HandleValue body, NodeVector& guarded, HandleValue unguarded,
+    [[nodiscard]] bool tryStatement(HandleValue body, NodeVector& guarded, HandleValue unguarded,
                                    HandleValue finally, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool debuggerStatement(TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool debuggerStatement(TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool importDeclaration(NodeVector& elts, HandleValue moduleSpec, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool importDeclaration(NodeVector& elts, HandleValue moduleSpec, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool importSpecifier(HandleValue importName, HandleValue bindingName, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool importSpecifier(HandleValue importName, HandleValue bindingName, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool exportDeclaration(HandleValue decl, NodeVector& elts, HandleValue moduleSpec,
+    [[nodiscard]] bool exportDeclaration(HandleValue decl, NodeVector& elts, HandleValue moduleSpec,
                                         HandleValue isDefault, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool exportSpecifier(HandleValue bindingName, HandleValue exportName, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool exportSpecifier(HandleValue bindingName, HandleValue exportName, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool exportBatchSpecifier(TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool exportBatchSpecifier(TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool classDefinition(bool expr, HandleValue name, HandleValue heritage,
+    [[nodiscard]] bool classDefinition(bool expr, HandleValue name, HandleValue heritage,
                                       HandleValue block, TokenPos* pos, MutableHandleValue dst);
-    MOZ_MUST_USE bool classMembers(NodeVector& members, MutableHandleValue dst);
-    MOZ_MUST_USE bool classMethod(HandleValue name, HandleValue body, PropKind kind, bool isStatic,
+    [[nodiscard]] bool classMembers(NodeVector& members, MutableHandleValue dst);
+    [[nodiscard]] bool classMethod(HandleValue name, HandleValue body, PropKind kind, bool isStatic,
                                   TokenPos* pos, MutableHandleValue dst);
-    MOZ_MUST_USE bool classField(HandleValue name, HandleValue initializer,
+    [[nodiscard]] bool classField(HandleValue name, HandleValue initializer,
                                  TokenPos* pos, MutableHandleValue dst);
-    MOZ_MUST_USE bool staticClassBlock(HandleValue body, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool staticClassBlock(HandleValue body, TokenPos* pos, MutableHandleValue dst);
 
     /*
      * expressions
      */
 
-    MOZ_MUST_USE bool binaryExpression(BinaryOperator op, HandleValue left, HandleValue right,
+    [[nodiscard]] bool binaryExpression(BinaryOperator op, HandleValue left, HandleValue right,
                                        TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool unaryExpression(UnaryOperator op, HandleValue expr, TokenPos* pos,
+    [[nodiscard]] bool unaryExpression(UnaryOperator op, HandleValue expr, TokenPos* pos,
                                       MutableHandleValue dst);
 
-    MOZ_MUST_USE bool assignmentExpression(AssignmentOperator op, HandleValue lhs, HandleValue rhs,
+    [[nodiscard]] bool assignmentExpression(AssignmentOperator op, HandleValue lhs, HandleValue rhs,
                                            TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool updateExpression(HandleValue expr, bool incr, bool prefix, TokenPos* pos,
+    [[nodiscard]] bool updateExpression(HandleValue expr, bool incr, bool prefix, TokenPos* pos,
                                        MutableHandleValue dst);
 
-    MOZ_MUST_USE bool logicalExpression(ParseNodeKind kind, HandleValue left, HandleValue right, TokenPos* pos,
+    [[nodiscard]] bool logicalExpression(ParseNodeKind kind, HandleValue left, HandleValue right, TokenPos* pos,
                                         MutableHandleValue dst);
 
-    MOZ_MUST_USE bool conditionalExpression(HandleValue test, HandleValue cons, HandleValue alt,
+    [[nodiscard]] bool conditionalExpression(HandleValue test, HandleValue cons, HandleValue alt,
                                             TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool sequenceExpression(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool sequenceExpression(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool newExpression(HandleValue callee, NodeVector& args, TokenPos* pos,
+    [[nodiscard]] bool newExpression(HandleValue callee, NodeVector& args, TokenPos* pos,
                                     MutableHandleValue dst);
 
-    MOZ_MUST_USE bool callExpression(HandleValue callee, NodeVector& args, TokenPos* pos,
+    [[nodiscard]] bool callExpression(HandleValue callee, NodeVector& args, TokenPos* pos,
                                      MutableHandleValue dst, bool isOptional = false);
 
-    MOZ_MUST_USE bool memberExpression(bool computed, HandleValue expr, HandleValue member,
+    [[nodiscard]] bool memberExpression(bool computed, HandleValue expr, HandleValue member,
                                        TokenPos* pos, MutableHandleValue dst,
                                        bool isOptional = false);
 
-    MOZ_MUST_USE bool arrayExpression(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool arrayExpression(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool templateLiteral(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool templateLiteral(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool taggedTemplate(HandleValue callee, NodeVector& args, TokenPos* pos,
+    [[nodiscard]] bool taggedTemplate(HandleValue callee, NodeVector& args, TokenPos* pos,
                                      MutableHandleValue dst);
 
-    MOZ_MUST_USE bool callSiteObj(NodeVector& raw, NodeVector& cooked, TokenPos* pos,
+    [[nodiscard]] bool callSiteObj(NodeVector& raw, NodeVector& cooked, TokenPos* pos,
                                   MutableHandleValue dst);
 
-    MOZ_MUST_USE bool spreadExpression(HandleValue expr, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool spreadExpression(HandleValue expr, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool optionalExpression(HandleValue expr, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool optionalExpression(HandleValue expr, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool deleteOptionalExpression(HandleValue expr, TokenPos* pos,
+    [[nodiscard]] bool deleteOptionalExpression(HandleValue expr, TokenPos* pos,
                                                MutableHandleValue dst);
 
-    MOZ_MUST_USE bool computedName(HandleValue name, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool computedName(HandleValue name, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool objectExpression(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool objectExpression(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool thisExpression(TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool thisExpression(TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool yieldExpression(HandleValue arg, YieldKind kind, TokenPos* pos,
+    [[nodiscard]] bool yieldExpression(HandleValue arg, YieldKind kind, TokenPos* pos,
                                       MutableHandleValue dst);
 
-    MOZ_MUST_USE bool comprehensionBlock(HandleValue patt, HandleValue src, bool isForEach,
+    [[nodiscard]] bool comprehensionBlock(HandleValue patt, HandleValue src, bool isForEach,
                                          bool isForOf, TokenPos* pos, MutableHandleValue dst);
-    MOZ_MUST_USE bool comprehensionIf(HandleValue test, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool comprehensionIf(HandleValue test, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool comprehensionExpression(HandleValue body, NodeVector& blocks,
+    [[nodiscard]] bool comprehensionExpression(HandleValue body, NodeVector& blocks,
                                               HandleValue filter, bool isLegacy, TokenPos* pos,
                                               MutableHandleValue dst);
 
-    MOZ_MUST_USE bool generatorExpression(HandleValue body, NodeVector& blocks, HandleValue filter,
+    [[nodiscard]] bool generatorExpression(HandleValue body, NodeVector& blocks, HandleValue filter,
                                           bool isLegacy, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool metaProperty(HandleValue meta, HandleValue property, TokenPos* pos,
+    [[nodiscard]] bool metaProperty(HandleValue meta, HandleValue property, TokenPos* pos,
                                    MutableHandleValue dst);
 
-    MOZ_MUST_USE bool callImportExpression(HandleValue ident, HandleValue arg, TokenPos* pos,
+    [[nodiscard]] bool callImportExpression(HandleValue ident, HandleValue arg, TokenPos* pos,
                                            MutableHandleValue dst);
 
-    MOZ_MUST_USE bool super(TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool super(TokenPos* pos, MutableHandleValue dst);
 
     /*
      * declarations
      */
 
-    MOZ_MUST_USE bool variableDeclaration(NodeVector& elts, VarDeclKind kind, TokenPos* pos,
+    [[nodiscard]] bool variableDeclaration(NodeVector& elts, VarDeclKind kind, TokenPos* pos,
                                           MutableHandleValue dst);
 
     /*
      * patterns
      */
 
-    MOZ_MUST_USE bool arrayPattern(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool arrayPattern(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool objectPattern(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
+    [[nodiscard]] bool objectPattern(NodeVector& elts, TokenPos* pos, MutableHandleValue dst);
 
-    MOZ_MUST_USE bool propertyPattern(HandleValue key, HandleValue patt, bool isShorthand,
+    [[nodiscard]] bool propertyPattern(HandleValue key, HandleValue patt, bool isShorthand,
                                       TokenPos* pos, MutableHandleValue dst);
 };
 

js/src/builtin/RegExp.h

@@ -25,42 +25,42 @@ InitRegExpClass(JSContext* cx, HandleObject obj);
  * |input| may be nullptr if there is no JSString corresponding to
  * |chars| and |length|.
  */
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 ExecuteRegExpLegacy(JSContext* cx, RegExpStatics* res, Handle<RegExpObject*> reobj,
                     HandleLinearString input, size_t* lastIndex, bool test,
                     MutableHandleValue rval);
 
 /* Translation from MatchPairs to a JS array in regexp_exec()'s output format. */
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CreateRegExpMatchResult(JSContext* cx, RegExpShared& re,
                         HandleString input, const MatchPairs& matches,
                         MutableHandleValue rval);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpMatcher(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpMatcherRaw(JSContext* cx, HandleObject regexp, HandleString input,
                  int32_t lastIndex, MatchPairs* maybeMatches, MutableHandleValue output);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpSearcher(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpSearcherRaw(JSContext* cx, HandleObject regexp, HandleString input,
                   int32_t lastIndex, MatchPairs* maybeMatches, int32_t* result);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpTester(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpTesterRaw(JSContext* cx, HandleObject regexp, HandleString input,
                 int32_t lastIndex, int32_t* endIndex);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intrinsic_GetElemBaseForLambda(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intrinsic_GetStringDataProperty(JSContext* cx, unsigned argc, Value* vp);
 
 /*
@@ -73,67 +73,67 @@ intrinsic_GetStringDataProperty(JSContext* cx, unsigned argc, Value* vp);
  * Must be called without |new|.
  * Dedicated function for RegExp.prototype[@@split] optimized path.
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_construct_raw_flags(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Clone given RegExp object, inheriting pattern and flags, ignoring other
  * properties.
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_clone(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 IsRegExp(JSContext* cx, HandleValue value, bool* result);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpCreate(JSContext* cx, HandleValue pattern, HandleValue flags, MutableHandleValue rval);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpPrototypeOptimizable(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpPrototypeOptimizableRaw(JSContext* cx, JSObject* proto);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpInstanceOptimizable(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpInstanceOptimizableRaw(JSContext* cx, JSObject* obj, JSObject* proto);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 RegExpGetSubstitution(JSContext* cx, HandleArrayObject matchResult, HandleLinearString string,
                       size_t position, HandleLinearString replacement, size_t firstDollarIndex,
                       HandleValue namedCaptures, MutableHandleValue rval);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 GetFirstDollarIndex(JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 GetFirstDollarIndexRaw(JSContext* cx, HandleString str, int32_t* index);
 
 extern int32_t
 GetFirstDollarIndexRawFlat(JSLinearString* text);
 
 // RegExp ClassSpec members used in RegExpObject.cpp.
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_construct(JSContext* cx, unsigned argc, Value* vp);
 extern const JSPropertySpec regexp_static_props[];
 extern const JSPropertySpec regexp_properties[];
 extern const JSFunctionSpec regexp_methods[];
 
 // Used in RegExpObject::isOriginalFlagGetter.
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_global(JSContext* cx, unsigned argc, JS::Value* vp);
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_ignoreCase(JSContext* cx, unsigned argc, JS::Value* vp);
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_multiline(JSContext* cx, unsigned argc, JS::Value* vp);
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_sticky(JSContext* cx, unsigned argc, JS::Value* vp);
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_unicode(JSContext* cx, unsigned argc, JS::Value* vp);
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 regexp_dotAll(JSContext* cx, unsigned argc, JS::Value* vp);
 
 } /* namespace js */

js/src/builtin/Stream.h

@@ -37,31 +37,31 @@ class ReadableStream : public NativeObject
 
     JS::ReadableStreamMode mode() const;
 
-    static MOZ_MUST_USE bool close(JSContext* cx, Handle<ReadableStream*> stream);
-    static MOZ_MUST_USE JSObject* cancel(JSContext* cx, Handle<ReadableStream*> stream,
-                                         HandleValue reason);
-    static MOZ_MUST_USE bool error(JSContext* cx, Handle<ReadableStream*> stream,
-                                   HandleValue error);
+    [[nodiscard]] static bool close(JSContext* cx, Handle<ReadableStream*> stream);
+    [[nodiscard]] static JSObject* cancel(JSContext* cx, Handle<ReadableStream*> stream,
+                                          HandleValue reason);
+    [[nodiscard]] static bool error(JSContext* cx, Handle<ReadableStream*> stream,
+                                    HandleValue error);
 
-    static MOZ_MUST_USE NativeObject* getReader(JSContext* cx, Handle<ReadableStream*> stream,
-                                                JS::ReadableStreamReaderMode mode);
+    [[nodiscard]] static NativeObject* getReader(JSContext* cx, Handle<ReadableStream*> stream,
+                                                 JS::ReadableStreamReaderMode mode);
 
-    static MOZ_MUST_USE bool tee(JSContext* cx,
-                                 Handle<ReadableStream*> stream, bool cloneForBranch2,
-                                 MutableHandle<ReadableStream*> branch1Stream,
-                                 MutableHandle<ReadableStream*> branch2Stream);
+    [[nodiscard]] static bool tee(JSContext* cx,
+                                  Handle<ReadableStream*> stream, bool cloneForBranch2,
+                                  MutableHandle<ReadableStream*> branch1Stream,
+                                  MutableHandle<ReadableStream*> branch2Stream);
 
-    static MOZ_MUST_USE bool enqueue(JSContext* cx, Handle<ReadableStream*> stream,
-                                     HandleValue chunk);
-    static MOZ_MUST_USE bool enqueueBuffer(JSContext* cx, Handle<ReadableStream*> stream,
-                                           Handle<ArrayBufferObject*> chunk);
-    static MOZ_MUST_USE bool getExternalSource(JSContext* cx, Handle<ReadableStream*> stream,
-                                               void** source);
+    [[nodiscard]] static bool enqueue(JSContext* cx, Handle<ReadableStream*> stream,
+                                      HandleValue chunk);
+    [[nodiscard]] static bool enqueueBuffer(JSContext* cx, Handle<ReadableStream*> stream,
+                                            Handle<ArrayBufferObject*> chunk);
+    [[nodiscard]] static bool getExternalSource(JSContext* cx, Handle<ReadableStream*> stream,
+                                                void** source);
     void releaseExternalSource();
     uint8_t embeddingFlags() const;
-    static MOZ_MUST_USE bool updateDataAvailableFromSource(JSContext* cx,
-                                                           Handle<ReadableStream*> stream,
-                                                           uint32_t availableData);
+    [[nodiscard]] static bool updateDataAvailableFromSource(JSContext* cx,
+                                                            Handle<ReadableStream*> stream,
+                                                            uint32_t availableData);
 
     enum State {
          Readable  = 1 << 0,
@@ -71,7 +71,7 @@ class ReadableStream : public NativeObject
     };
 
   private:
-    static MOZ_MUST_USE ReadableStream* createStream(JSContext* cx, HandleObject proto = nullptr);
+    [[nodiscard]] static ReadableStream* createStream(JSContext* cx, HandleObject proto = nullptr);
 
   public:
     static bool constructor(JSContext* cx, unsigned argc, Value* vp);
@@ -84,7 +84,7 @@ class ReadableStream : public NativeObject
 class ReadableStreamDefaultReader : public NativeObject
 {
   public:
-    static MOZ_MUST_USE JSObject* read(JSContext* cx, Handle<ReadableStreamDefaultReader*> reader);
+    [[nodiscard]] static JSObject* read(JSContext* cx, Handle<ReadableStreamDefaultReader*> reader);
 
     static bool constructor(JSContext* cx, unsigned argc, Value* vp);
     static const ClassSpec classSpec_;
@@ -96,8 +96,8 @@ class ReadableStreamDefaultReader : public NativeObject
 class ReadableStreamBYOBReader : public NativeObject
 {
   public:
-    static MOZ_MUST_USE JSObject* read(JSContext* cx, Handle<ReadableStreamBYOBReader*> reader,
-                                       Handle<ArrayBufferViewObject*> view);
+    [[nodiscard]] static JSObject* read(JSContext* cx, Handle<ReadableStreamBYOBReader*> reader,
+                                        Handle<ArrayBufferViewObject*> view);
 
     static bool constructor(JSContext* cx, unsigned argc, Value* vp);
     static const ClassSpec classSpec_;
@@ -108,10 +108,10 @@ class ReadableStreamBYOBReader : public NativeObject
 
 bool ReadableStreamReaderIsClosed(const JSObject* reader);
 
-MOZ_MUST_USE bool ReadableStreamReaderCancel(JSContext* cx, HandleObject reader,
+[[nodiscard]] bool ReadableStreamReaderCancel(JSContext* cx, HandleObject reader,
                                              HandleValue reason);
 
-MOZ_MUST_USE bool ReadableStreamReaderReleaseLock(JSContext* cx, HandleObject reader);
+[[nodiscard]] bool ReadableStreamReaderReleaseLock(JSContext* cx, HandleObject reader);
 
 class ReadableStreamDefaultController : public NativeObject
 {

js/src/builtin/SymbolObject.h

@@ -38,22 +38,22 @@ class SymbolObject : public NativeObject
         setFixedSlot(PRIMITIVE_VALUE_SLOT, SymbolValue(symbol));
     }
 
-    static MOZ_MUST_USE bool construct(JSContext* cx, unsigned argc, Value* vp);
+    static [[nodiscard]] bool construct(JSContext* cx, unsigned argc, Value* vp);
 
     // Static methods.
-    static MOZ_MUST_USE bool for_(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool keyFor(JSContext* cx, unsigned argc, Value* vp);
+    static [[nodiscard]] bool for_(JSContext* cx, unsigned argc, Value* vp);
+    static [[nodiscard]] bool keyFor(JSContext* cx, unsigned argc, Value* vp);
 
     // Methods defined on Symbol.prototype.
-    static MOZ_MUST_USE bool toString_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool toString(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool valueOf_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool valueOf(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool toPrimitive(JSContext* cx, unsigned argc, Value* vp);
+    static [[nodiscard]] bool toString_impl(JSContext* cx, const CallArgs& args);
+    static [[nodiscard]] bool toString(JSContext* cx, unsigned argc, Value* vp);
+    static [[nodiscard]] bool valueOf_impl(JSContext* cx, const CallArgs& args);
+    static [[nodiscard]] bool valueOf(JSContext* cx, unsigned argc, Value* vp);
+    static [[nodiscard]] bool toPrimitive(JSContext* cx, unsigned argc, Value* vp);
 
     // Properties defined on Symbol.prototype.
-    static MOZ_MUST_USE bool descriptionGetter_impl(JSContext* cx, const CallArgs& args);
-    static MOZ_MUST_USE bool descriptionGetter(JSContext* cx, unsigned argc, Value *vp);
+    static [[nodiscard]] bool descriptionGetter_impl(JSContext* cx, const CallArgs& args);
+    static [[nodiscard]] bool descriptionGetter(JSContext* cx, unsigned argc, Value *vp);
 
     static const JSPropertySpec properties[];
     static const JSFunctionSpec methods[];

js/src/builtin/TestingFunctions.h

@@ -10,13 +10,13 @@
 
 namespace js {
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 DefineTestingFunctions(JSContext* cx, HandleObject obj, bool fuzzingSafe, bool disableOOMFunctions);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 testingFunc_assertFloat32(JSContext* cx, unsigned argc, Value* vp);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 testingFunc_assertRecoveredOnBailout(JSContext* cx, unsigned argc, Value* vp);
 
 } /* namespace js */

js/src/builtin/TypedObject.h

@@ -176,7 +176,7 @@ class TypeDescr : public NativeObject
     }
 
     // Whether id is an 'own' property of objects with this descriptor.
-    MOZ_MUST_USE bool hasProperty(const JSAtomState& names, jsid id);
+    [[nodiscard]] bool hasProperty(const JSAtomState& names, jsid id);
 
     // Type descriptors may contain a list of their references for use during
     // scanning. Marking code is optimized to use this list to mark inline
@@ -188,7 +188,7 @@ class TypeDescr : public NativeObject
     // The list is three consecutive arrays of int32_t offsets, with each array
     // terminated by -1. The arrays store offsets of string, object, and value
     // references in the descriptor, in that order.
-    MOZ_MUST_USE bool hasTraceList() const {
+    [[nodiscard]] bool hasTraceList() const {
         return !getFixedSlot(JS_DESCR_SLOT_TRACE_LIST).isUndefined();
     }
     const int32_t* traceList() const {
@@ -257,7 +257,7 @@ class ScalarTypeDescr : public SimpleTypeDescr
         return Type(getReservedSlot(JS_DESCR_SLOT_TYPE).toInt32());
     }
 
-    static MOZ_MUST_USE bool call(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool call(JSContext* cx, unsigned argc, Value* vp);
 };
 
 // Enumerates the cases of ScalarTypeDescr::Type which have
@@ -310,7 +310,7 @@ class ReferenceTypeDescr : public SimpleTypeDescr
         return typeName(type());
     }
 
-    static MOZ_MUST_USE bool call(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool call(JSContext* cx, unsigned argc, Value* vp);
 };
 
 #define JS_FOR_EACH_REFERENCE_TYPE_REPR(macro_) \
@@ -333,7 +333,7 @@ class ComplexTypeDescr : public TypeDescr
 bool IsTypedObjectClass(const Class* clasp); // Defined below
 bool IsTypedObjectArray(JSObject& obj);
 
-MOZ_MUST_USE bool CreateUserSizeAndAlignmentProperties(JSContext* cx, HandleTypeDescr obj);
+[[nodiscard]] bool CreateUserSizeAndAlignmentProperties(JSContext* cx, HandleTypeDescr obj);
 
 class ArrayTypeDescr;
 
@@ -371,7 +371,7 @@ class ArrayMetaTypeDescr : public NativeObject
 
     // This is the function that gets called when the user
     // does `new ArrayType(elem)`. It produces an array type object.
-    static MOZ_MUST_USE bool construct(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool construct(JSContext* cx, unsigned argc, Value* vp);
 };
 
 /*
@@ -422,7 +422,7 @@ class StructMetaTypeDescr : public NativeObject
 
     // This is the function that gets called when the user
     // does `new StructType(...)`. It produces a struct type object.
-    static MOZ_MUST_USE bool construct(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool construct(JSContext* cx, unsigned argc, Value* vp);
 };
 
 class StructTypeDescr : public ComplexTypeDescr
@@ -435,7 +435,7 @@ class StructTypeDescr : public ComplexTypeDescr
 
     // Set `*out` to the index of the field named `id` and returns true,
     // or return false if no such field exists.
-    MOZ_MUST_USE bool fieldIndex(jsid id, size_t* out) const;
+    [[nodiscard]] bool fieldIndex(jsid id, size_t* out) const;
 
     // Return the name of the field at index `index`.
     JSAtom& fieldName(size_t index) const;
@@ -475,45 +475,45 @@ class TypedObject : public ShapedObject
 {
     static const bool IsTypedObjectClass = true;
 
-    static MOZ_MUST_USE bool obj_getArrayElement(JSContext* cx,
-                                                 Handle<TypedObject*> typedObj,
-                                                 Handle<TypeDescr*> typeDescr,
-                                                 uint32_t index,
-                                                 MutableHandleValue vp);
+    [[nodiscard]] static bool obj_getArrayElement(JSContext* cx,
+                                                  Handle<TypedObject*> typedObj,
+                                                  Handle<TypeDescr*> typeDescr,
+                                                  uint32_t index,
+                                                  MutableHandleValue vp);
 
   protected:
     static const ObjectOps objectOps_;
 
-    static MOZ_MUST_USE bool obj_lookupProperty(JSContext* cx, HandleObject obj,
-                                                HandleId id, MutableHandleObject objp,
-                                                MutableHandle<PropertyResult> propp);
+    [[nodiscard]] static bool obj_lookupProperty(JSContext* cx, HandleObject obj,
+                                                 HandleId id, MutableHandleObject objp,
+                                                 MutableHandle<PropertyResult> propp);
 
-    static MOZ_MUST_USE bool obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
-                                                Handle<PropertyDescriptor> desc,
-                                                ObjectOpResult& result);
+    [[nodiscard]] static bool obj_defineProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                                 Handle<PropertyDescriptor> desc,
+                                                 ObjectOpResult& result);
 
-    static MOZ_MUST_USE bool obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id,
-                                             bool* foundp);
+    [[nodiscard]] static bool obj_hasProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                              bool* foundp);
 
-    static MOZ_MUST_USE bool obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
-                                             HandleId id, MutableHandleValue vp);
+    [[nodiscard]] static bool obj_getProperty(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                              HandleId id, MutableHandleValue vp);
 
-    static MOZ_MUST_USE bool obj_getElement(JSContext* cx, HandleObject obj, HandleValue receiver,
-                                            uint32_t index, MutableHandleValue vp);
+    [[nodiscard]] static bool obj_getElement(JSContext* cx, HandleObject obj, HandleValue receiver,
+                                             uint32_t index, MutableHandleValue vp);
 
-    static MOZ_MUST_USE bool obj_setProperty(JSContext* cx, HandleObject obj, HandleId id,
-                                             HandleValue v, HandleValue receiver,
-                                             ObjectOpResult& result);
+    [[nodiscard]] static bool obj_setProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                              HandleValue v, HandleValue receiver,
+                                              ObjectOpResult& result);
 
-    static MOZ_MUST_USE bool obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj,
-                                                          HandleId id,
-                                                          MutableHandle<PropertyDescriptor> desc);
+    [[nodiscard]] static bool obj_getOwnPropertyDescriptor(JSContext* cx, HandleObject obj,
+                                                           HandleId id,
+                                                           MutableHandle<PropertyDescriptor> desc);
 
-    static MOZ_MUST_USE bool obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
-                                                ObjectOpResult& result);
+    [[nodiscard]] static bool obj_deleteProperty(JSContext* cx, HandleObject obj, HandleId id,
+                                                 ObjectOpResult& result);
 
-    static MOZ_MUST_USE bool obj_enumerate(JSContext* cx, HandleObject obj,
-                                           AutoIdVector& properties, bool enumerableOnly);
+    [[nodiscard]] static bool obj_enumerate(JSContext* cx, HandleObject obj,
+                                            AutoIdVector& properties, bool enumerableOnly);
 
 
     uint8_t* typedMem() const;
@@ -548,7 +548,7 @@ class TypedObject : public ShapedObject
         return typedMem(nogc) + offset;
     }
 
-    inline MOZ_MUST_USE bool opaque() const;
+    [[nodiscard]] inline bool opaque() const;
 
     // Creates a new typed object whose memory is freshly allocated and
     // initialized with zeroes (or, in the case of references, an appropriate
@@ -558,11 +558,11 @@ class TypedObject : public ShapedObject
 
     // User-accessible constructor (`new TypeDescriptor(...)`). Note that the
     // callee here is the type descriptor.
-    static MOZ_MUST_USE bool construct(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool construct(JSContext* cx, unsigned argc, Value* vp);
 
     /* Accessors for self hosted code. */
-    static MOZ_MUST_USE bool GetBuffer(JSContext* cx, unsigned argc, Value* vp);
-    static MOZ_MUST_USE bool GetByteOffset(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool GetBuffer(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool GetByteOffset(JSContext* cx, unsigned argc, Value* vp);
 
     Shape** addressOfShapeFromGC() { return shape_.unsafeUnbarrieredForTracing(); }
 };
@@ -733,14 +733,14 @@ class InlineOpaqueTypedObject : public InlineTypedObject
  *
  * Constructs a new, unattached instance of `Handle`.
  */
-MOZ_MUST_USE bool NewOpaqueTypedObject(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool NewOpaqueTypedObject(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: NewDerivedTypedObject(typeObj, owner, offset)
  *
  * Constructs a new, unattached instance of `Handle`.
  */
-MOZ_MUST_USE bool NewDerivedTypedObject(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool NewDerivedTypedObject(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: AttachTypedObject(typedObj, newDatum, newOffset)
@@ -748,7 +748,7 @@ MOZ_MUST_USE bool NewDerivedTypedObject(JSContext* cx, unsigned argc, Value* vp)
  * Moves `typedObj` to point at the memory referenced by `newDatum` with
  * the offset `newOffset`.
  */
-MOZ_MUST_USE bool AttachTypedObject(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool AttachTypedObject(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: SetTypedObjectOffset(typedObj, offset)
@@ -756,41 +756,41 @@ MOZ_MUST_USE bool AttachTypedObject(JSContext* cx, unsigned argc, Value* vp);
  * Changes the offset for `typedObj` within its buffer to `offset`.
  * `typedObj` must already be attached.
  */
-MOZ_MUST_USE bool SetTypedObjectOffset(JSContext*, unsigned argc, Value* vp);
+[[nodiscard]] bool SetTypedObjectOffset(JSContext*, unsigned argc, Value* vp);
 
 /*
  * Usage: ObjectIsTypeDescr(obj)
  *
  * True if `obj` is a type object.
  */
-MOZ_MUST_USE bool ObjectIsTypeDescr(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool ObjectIsTypeDescr(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: ObjectIsTypedObject(obj)
  *
  * True if `obj` is a transparent or opaque typed object.
  */
-MOZ_MUST_USE bool ObjectIsTypedObject(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool ObjectIsTypedObject(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: ObjectIsOpaqueTypedObject(obj)
  *
  * True if `obj` is an opaque typed object.
  */
-MOZ_MUST_USE bool ObjectIsOpaqueTypedObject(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool ObjectIsOpaqueTypedObject(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: ObjectIsTransparentTypedObject(obj)
  *
  * True if `obj` is a transparent typed object.
  */
-MOZ_MUST_USE bool ObjectIsTransparentTypedObject(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool ObjectIsTransparentTypedObject(JSContext* cx, unsigned argc, Value* vp);
 
 /* Predicates on type descriptor objects.  In all cases, 'obj' must be a type descriptor. */
 
-MOZ_MUST_USE bool TypeDescrIsSimpleType(JSContext*, unsigned argc, Value* vp);
+[[nodiscard]] bool TypeDescrIsSimpleType(JSContext*, unsigned argc, Value* vp);
 
-MOZ_MUST_USE bool TypeDescrIsArrayType(JSContext*, unsigned argc, Value* vp);
+[[nodiscard]] bool TypeDescrIsArrayType(JSContext*, unsigned argc, Value* vp);
 
 /*
  * Usage: TypedObjectIsAttached(obj)
@@ -798,21 +798,21 @@ MOZ_MUST_USE bool TypeDescrIsArrayType(JSContext*, unsigned argc, Value* vp);
  * Given a TypedObject `obj`, returns true if `obj` is
  * "attached" (i.e., its data pointer is nullptr).
  */
-MOZ_MUST_USE bool TypedObjectIsAttached(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool TypedObjectIsAttached(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: TypedObjectTypeDescr(obj)
  *
  * Given a TypedObject `obj`, returns the object's type descriptor.
  */
-MOZ_MUST_USE bool TypedObjectTypeDescr(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool TypedObjectTypeDescr(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: ClampToUint8(v)
  *
  * Same as the C function ClampDoubleToUint8. `v` must be a number.
  */
-MOZ_MUST_USE bool ClampToUint8(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool ClampToUint8(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: GetTypedObjectModule()
@@ -823,7 +823,7 @@ MOZ_MUST_USE bool ClampToUint8(JSContext* cx, unsigned argc, Value* vp);
  * to access them; eventually this should be linked into the module
  * system.
  */
-MOZ_MUST_USE bool GetTypedObjectModule(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] bool GetTypedObjectModule(JSContext* cx, unsigned argc, Value* vp);
 
 /*
  * Usage: Store_int8(targetDatum, targetOffset, value)
@@ -844,7 +844,7 @@ MOZ_MUST_USE bool GetTypedObjectModule(JSContext* cx, unsigned argc, Value* vp);
 #define JS_STORE_SCALAR_CLASS_DEFN(_constant, T, _name)                       \
 class StoreScalar##T {                                                        \
   public:                                                                     \
-    static MOZ_MUST_USE bool Func(JSContext* cx, unsigned argc, Value* vp);   \
+    [[nodiscard]] static bool Func(JSContext* cx, unsigned argc, Value* vp);   \
     static const JSJitInfo JitInfo;                                           \
 };
 
@@ -864,11 +864,11 @@ class StoreScalar##T {                                                        \
 #define JS_STORE_REFERENCE_CLASS_DEFN(_constant, T, _name)                    \
 class StoreReference##_name {                                                 \
   private:                                                                    \
-    static MOZ_MUST_USE bool store(JSContext* cx, T* heap, const Value& v,    \
-                                   TypedObject* obj, jsid id);                \
+    [[nodiscard]] static bool store(JSContext* cx, T* heap, const Value& v,   \
+                                    TypedObject* obj, jsid id);               \
                                                                               \
   public:                                                                     \
-    static MOZ_MUST_USE bool Func(JSContext* cx, unsigned argc, Value* vp);   \
+    [[nodiscard]] static bool Func(JSContext* cx, unsigned argc, Value* vp);  \
     static const JSJitInfo JitInfo;                                           \
 };
 
@@ -883,7 +883,7 @@ class StoreReference##_name {                                                 \
 #define JS_LOAD_SCALAR_CLASS_DEFN(_constant, T, _name)                        \
 class LoadScalar##T {                                                         \
   public:                                                                     \
-    static MOZ_MUST_USE bool Func(JSContext* cx, unsigned argc, Value* vp);   \
+    [[nodiscard]] static bool Func(JSContext* cx, unsigned argc, Value* vp);  \
     static const JSJitInfo JitInfo;                                           \
 };
 
@@ -901,7 +901,7 @@ class LoadReference##_name {                                                  \
     static void load(T* heap, MutableHandleValue v);                          \
                                                                               \
   public:                                                                     \
-    static MOZ_MUST_USE bool Func(JSContext* cx, unsigned argc, Value* vp);   \
+    [[nodiscard]] static bool Func(JSContext* cx, unsigned argc, Value* vp);  \
     static const JSJitInfo JitInfo;                                           \
 };
 

js/src/builtin/WeakSetObject.h

@@ -23,7 +23,7 @@ class WeakSetObject : public NativeObject
     static const JSFunctionSpec methods[];
 
     static WeakSetObject* create(JSContext* cx, HandleObject proto = nullptr);
-    static MOZ_MUST_USE bool construct(JSContext* cx, unsigned argc, Value* vp);
+    [[nodiscard]] static bool construct(JSContext* cx, unsigned argc, Value* vp);
 
     static bool isBuiltinAdd(HandleValue add, JSContext* cx);
 };

js/src/builtin/intl/Collator.h

@@ -49,7 +49,7 @@ CreateCollatorPrototype(JSContext* cx, JS::Handle<JSObject*> Intl,
  *
  * Usage: collator = intl_Collator(locales, options)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_Collator(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -60,7 +60,7 @@ intl_Collator(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: collations = intl_availableCollations(locale)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_availableCollations(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -73,7 +73,7 @@ intl_availableCollations(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: result = intl_CompareStrings(collator, x, y)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_CompareStrings(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -82,7 +82,7 @@ intl_CompareStrings(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: result = intl_isUpperCaseFirst(locale)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_isUpperCaseFirst(JSContext* cx, unsigned argc, Value* vp);
 
 

js/src/builtin/intl/DateTimeFormat.h

@@ -51,7 +51,7 @@ CreateDateTimeFormatPrototype(JSContext* cx, JS::Handle<JSObject*> Intl,
  *
  * Usage: dateTimeFormat = intl_DateTimeFormat(locales, options)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_DateTimeFormat(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -62,7 +62,7 @@ intl_DateTimeFormat(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: calendars = intl_availableCalendars(locale)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_availableCalendars(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -72,7 +72,7 @@ intl_availableCalendars(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: calendar = intl_defaultCalendar(locale)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_defaultCalendar(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -85,7 +85,7 @@ intl_defaultCalendar(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: ianaTimeZone = intl_IsValidTimeZoneName(timeZone)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_IsValidTimeZoneName(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -94,7 +94,7 @@ intl_IsValidTimeZoneName(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: ianaTimeZone = intl_canonicalizeTimeZone(timeZone)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_canonicalizeTimeZone(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -102,7 +102,7 @@ intl_canonicalizeTimeZone(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: icuDefaultTimeZone = intl_defaultTimeZone()
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_defaultTimeZone(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -110,7 +110,7 @@ intl_defaultTimeZone(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: defaultTimeZoneOffset = intl_defaultTimeZoneOffset()
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_defaultTimeZoneOffset(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -121,7 +121,7 @@ intl_defaultTimeZoneOffset(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: pattern = intl_patternForSkeleton(locale, skeleton, hourCycle)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_patternForSkeleton(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -155,7 +155,7 @@ intl_patternForSkeleton(JSContext* cx, unsigned argc, Value* vp);
  * Usage: pattern = intl_patternForStyle(locale, dateStyle, timeStyle, timeZone,
  *                                       hour12, hourCycle)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_patternForStyle(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -167,7 +167,7 @@ intl_patternForStyle(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: formatted = intl_FormatDateTime(dateTimeFormat, x, formatToParts)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_FormatDateTime(JSContext* cx, unsigned argc, Value* vp);
 
 

js/src/builtin/intl/IntlObject.h

@@ -50,7 +50,7 @@ InitIntlClass(JSContext* cx, JS::Handle<JSObject*> obj);
  *
  * NOTE: "calendar" and "locale" properties are *not* added to the object.
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_GetCalendarInfo(JSContext* cx, unsigned argc, JS::Value* vp);
 
 /**
@@ -92,7 +92,7 @@ intl_GetCalendarInfo(JSContext* cx, unsigned argc, JS::Value* vp);
  *   'AM'
  * ]
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_ComputeDisplayNames(JSContext* cx, unsigned argc, JS::Value* vp);
 
 /**
@@ -111,7 +111,7 @@ intl_ComputeDisplayNames(JSContext* cx, unsigned argc, JS::Value* vp);
  *
  * Usage: result = intl_BestAvailableLocale("Collator", locale, defaultOrNull)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_BestAvailableLocale(JSContext* cx, unsigned argc, JS::Value* vp);
 
 /**
@@ -121,7 +121,7 @@ intl_BestAvailableLocale(JSContext* cx, unsigned argc, JS::Value* vp);
  *
  * Usage: result = intl_supportedLocaleOrFallback(locale)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_supportedLocaleOrFallback(JSContext* cx, unsigned argc, JS::Value* vp);
 
 } // namespace js

js/src/builtin/intl/LanguageTag.h

@@ -220,9 +220,9 @@ class MOZ_STACK_CLASS LanguageTag final {
 
   void performComplexLanguageMappings();
   void performComplexRegionMappings();
-  MOZ_MUST_USE bool performVariantMappings(JSContext* cx);
+  [[nodiscard]] bool performVariantMappings(JSContext* cx);
 
-  MOZ_MUST_USE bool updateGrandfatheredMappings(JSContext* cx);
+  [[nodiscard]] bool updateGrandfatheredMappings(JSContext* cx);
 
   static const char* replaceTransformExtensionType(
       mozilla::Span<const char> key, mozilla::Span<const char> type);
@@ -716,21 +716,21 @@ MOZ_MAKE_ENUM_CLASS_BITWISE_OPERATORS(LanguageTagParser::TokenKind)
  * Parse a string as a standalone |language| tag. If |str| is a standalone
  * language tag, store it in |result| and return true. Otherwise return false.
  */
-MOZ_MUST_USE bool ParseStandaloneLanguageTag(JS::Handle<JSLinearString*> str,
+[[nodiscard]] bool ParseStandaloneLanguageTag(JS::Handle<JSLinearString*> str,
                                              LanguageSubtag& result);
 
 /**
  * Parse a string as a standalone |script| tag. If |str| is a standalone script
  * tag, store it in |result| and return true. Otherwise return false.
  */
-MOZ_MUST_USE bool ParseStandaloneScriptTag(JS::Handle<JSLinearString*> str,
+[[nodiscard]] bool ParseStandaloneScriptTag(JS::Handle<JSLinearString*> str,
                                            ScriptSubtag& result);
 
 /**
  * Parse a string as a standalone |region| tag. If |str| is a standalone region
  * tag, store it in |result| and return true. Otherwise return false.
  */
-MOZ_MUST_USE bool ParseStandaloneRegionTag(JS::Handle<JSLinearString*> str,
+[[nodiscard]] bool ParseStandaloneRegionTag(JS::Handle<JSLinearString*> str,
                                            RegionSubtag& result);
 
 /**
@@ -759,7 +759,7 @@ class UnicodeExtensionKeyword final {
   void trace(JSTracer* trc);
 };
 
-extern MOZ_MUST_USE bool ApplyUnicodeExtensionToTag(
+[[nodiscard]] extern bool ApplyUnicodeExtensionToTag(
     JSContext* cx, LanguageTag& tag,
     JS::HandleVector<UnicodeExtensionKeyword> keywords);
 

js/src/builtin/intl/Locale.h

@@ -48,14 +48,14 @@ extern JSObject* CreateLocalePrototype(JSContext* cx,
                                        JS::Handle<JSObject*> Intl,
                                        JS::Handle<GlobalObject*> global);
 
-extern MOZ_MUST_USE bool intl_ValidateAndCanonicalizeLanguageTag(JSContext* cx,
-                                                                 unsigned argc,
-                                                                 Value* vp);
+[[nodiscard]] extern bool intl_ValidateAndCanonicalizeLanguageTag(JSContext* cx,
+                                                                  unsigned argc,
+                                                                  Value* vp);
 
-extern MOZ_MUST_USE bool intl_TryValidateAndCanonicalizeLanguageTag(
+[[nodiscard]] extern bool intl_TryValidateAndCanonicalizeLanguageTag(
     JSContext* cx, unsigned argc, Value* vp);
 
-extern MOZ_MUST_USE bool intl_ValidateAndCanonicalizeUnicodeExtensionType(
+[[nodiscard]] extern bool intl_ValidateAndCanonicalizeUnicodeExtensionType(
     JSContext* cx, unsigned argc, Value* vp);
 
 }  // namespace js

js/src/builtin/intl/NumberFormat.h

@@ -47,7 +47,7 @@ CreateNumberFormatPrototype(JSContext* cx, HandleObject Intl, Handle<GlobalObjec
  *
  * Usage: numberFormat = intl_NumberFormat(locales, options)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_NumberFormat(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -57,7 +57,7 @@ intl_NumberFormat(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: defaultNumberingSystem = intl_numberingSystem(locale)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_numberingSystem(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -68,9 +68,9 @@ intl_numberingSystem(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: formatted = intl_FormatNumber(numberFormat, x, formatToParts)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_FormatNumber(JSContext* cx, unsigned argc, Value* vp);
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 FormatNumeric(JSContext* cx, UNumberFormat* nf, HandleValue x, MutableHandleValue result);
 
 } // namespace js

js/src/builtin/intl/PluralRules.h

@@ -46,7 +46,7 @@ CreatePluralRulesPrototype(JSContext* cx, JS::Handle<JSObject*> Intl,
  *
  * Usage: pluralRules = intl_PluralRules(locales, options)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_PluralRules(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -58,7 +58,7 @@ intl_PluralRules(JSContext* cx, unsigned argc, Value* vp);
  *
  * Usage: rule = intl_SelectPluralRule(pluralRules, x)
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_SelectPluralRule(JSContext* cx, unsigned argc, Value* vp);
 
 /**
@@ -71,7 +71,7 @@ intl_SelectPluralRule(JSContext* cx, unsigned argc, Value* vp);
  *
  * intl_getPluralCategories('pl', 'cardinal'); // ['one', 'few', 'many', 'other']
  */
-extern MOZ_MUST_USE bool
+[[nodiscard]] extern bool
 intl_GetPluralCategories(JSContext* cx, unsigned argc, Value* vp);
 
 } // namespace js

js/src/builtin/intl/RelativeTimeFormat.h

@@ -48,8 +48,7 @@ CreateRelativeTimeFormatPrototype(JSContext* cx, JS::Handle<JSObject*> Intl,
  *
  * Usage: formatted = intl_FormatRelativeTime(relativeTimeFormat, t, unit)
  */
-extern MOZ_MUST_USE bool
-intl_FormatRelativeTime(JSContext* cx, unsigned argc, Value* vp);
+[[nodiscard]] extern bool intl_FormatRelativeTime(JSContext* cx, unsigned argc, Value* vp);
 
 } // namespace js
 

js/src/builtin/intl/SharedIntlData.h

@@ -233,9 +233,9 @@ class SharedIntlData
     * Sets |supported| to true if |locale| is supported by the requested Intl
     * service constructor. Otherwise sets |supported| to false.
     */
-    MOZ_MUST_USE bool isSupportedLocale(JSContext* cx, SupportedLocaleKind kind,
-                                        JS::Handle<JSString*> locale,
-                                        bool* supported);
+    [[nodiscard]] bool isSupportedLocale(JSContext* cx, SupportedLocaleKind kind,
+                                         JS::Handle<JSString*> locale,
+                                         bool* supported);
 
   private:
     /**

js/src/ctypes/CTypes.h

@@ -178,7 +178,7 @@ GetDeflatedUTF8StringLength(JSContext* maybecx, const CharT* chars,
                             size_t charsLength);
 
 template <typename CharT>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 DeflateStringToUTF8Buffer(JSContext* maybecx, const CharT* src, size_t srclen,
                           char* dst, size_t* dstlenp);
 
@@ -456,7 +456,7 @@ namespace CType {
   TypeCode GetTypeCode(JSObject* typeObj);
   bool TypesEqual(JSObject* t1, JSObject* t2);
   size_t GetSize(JSObject* obj);
-  MOZ_MUST_USE bool GetSafeSize(JSObject* obj, size_t* result);
+  [[nodiscard]] bool GetSafeSize(JSObject* obj, size_t* result);
   bool IsSizeDefined(JSObject* obj);
   size_t GetAlignment(JSObject* obj);
   ffi_type* GetFFIType(JSContext* cx, JSObject* obj);
@@ -480,12 +480,12 @@ namespace ArrayType {
 
   JSObject* GetBaseType(JSObject* obj);
   size_t GetLength(JSObject* obj);
-  MOZ_MUST_USE bool GetSafeLength(JSObject* obj, size_t* result);
+  [[nodiscard]] bool GetSafeLength(JSObject* obj, size_t* result);
   UniquePtrFFIType BuildFFIType(JSContext* cx, JSObject* obj);
 } // namespace ArrayType
 
 namespace StructType {
-  MOZ_MUST_USE bool DefineInternal(JSContext* cx, JSObject* typeObj, JSObject* fieldsObj);
+  [[nodiscard]] bool DefineInternal(JSContext* cx, JSObject* typeObj, JSObject* fieldsObj);
 
   const FieldInfoHash* GetFieldInfo(JSObject* obj);
   const FieldInfo* LookupField(JSContext* cx, JSObject* obj, JSFlatString* name);
@@ -521,9 +521,9 @@ namespace CData {
   bool IsCDataProto(JSObject* obj);
 
   // Attached by JSAPI as the function 'ctypes.cast'
-  MOZ_MUST_USE bool Cast(JSContext* cx, unsigned argc, Value* vp);
+  [[nodiscard]] bool Cast(JSContext* cx, unsigned argc, Value* vp);
   // Attached by JSAPI as the function 'ctypes.getRuntime'
-  MOZ_MUST_USE bool GetRuntime(JSContext* cx, unsigned argc, Value* vp);
+  [[nodiscard]] bool GetRuntime(JSContext* cx, unsigned argc, Value* vp);
 } // namespace CData
 
 namespace Int64 {

js/src/ctypes/Library.h

@@ -23,14 +23,14 @@ enum LibrarySlot {
 
 namespace Library
 {
-  MOZ_MUST_USE bool Name(JSContext* cx, unsigned argc, JS::Value* vp);
+  [[nodiscard]] bool Name(JSContext* cx, unsigned argc, JS::Value* vp);
 
   JSObject* Create(JSContext* cx, JS::HandleValue path, const JSCTypesCallbacks* callbacks);
 
   bool IsLibrary(JSObject* obj);
   PRLibrary* GetLibrary(JSObject* obj);
 
-  MOZ_MUST_USE bool Open(JSContext* cx, unsigned argc, JS::Value* vp);
+  [[nodiscard]] bool Open(JSContext* cx, unsigned argc, JS::Value* vp);
 } // namespace Library
 
 } // namespace ctypes

js/src/ds/Fifo.h

@@ -94,7 +94,7 @@ class Fifo
     // Push an element to the back of the queue. This method can take either a
     // |const T&| or a |T&&|.
     template <typename U>
-    MOZ_MUST_USE bool pushBack(U&& u) {
+    [[nodiscard]] bool pushBack(U&& u) {
         if (!rear_.append(mozilla::Forward<U>(u)))
             return false;
         if (!fixup()) {
@@ -106,7 +106,7 @@ class Fifo
 
     // Construct a T in-place at the back of the queue.
     template <typename... Args>
-    MOZ_MUST_USE bool emplaceBack(Args&&... args) {
+    [[nodiscard]] bool emplaceBack(Args&&... args) {
         if (!rear_.emplaceBack(mozilla::Forward<Args>(args)...))
             return false;
         if (!fixup()) {
@@ -127,7 +127,7 @@ class Fifo
     }
 
     // Remove the front element from the queue.
-    MOZ_MUST_USE bool popFront() {
+    [[nodiscard]] bool popFront() {
         MOZ_ASSERT(!empty());
         T t(mozilla::Move(front()));
         front_.popBack();

js/src/ds/InlineTable.h

@@ -67,7 +67,7 @@ class InlineTable
         return inlNext_ > InlineEntries;
     }
 
-    MOZ_MUST_USE bool switchToTable() {
+    [[nodiscard]] bool switchToTable() {
         MOZ_ASSERT(inlNext_ == InlineEntries);
 
         if (table_.initialized()) {
@@ -91,7 +91,7 @@ class InlineTable
     }
 
     MOZ_NEVER_INLINE
-    MOZ_MUST_USE bool switchAndAdd(const InlineEntry& entry) {
+    [[nodiscard]] bool switchAndAdd(const InlineEntry& entry) {
         if (!switchToTable())
             return false;
 
@@ -285,7 +285,7 @@ class InlineTable
     template <typename KeyInput,
               typename... Args>
     MOZ_ALWAYS_INLINE
-    MOZ_MUST_USE bool add(AddPtr& p, KeyInput&& key, Args&&... args) {
+    [[nodiscard]] bool add(AddPtr& p, KeyInput&& key, Args&&... args) {
         MOZ_ASSERT(!p);
         MOZ_ASSERT(keyNonZero(key));
 
@@ -439,7 +439,7 @@ class InlineMap
             this->value = mozilla::Forward<ValueInput>(value);
         }
 
-        MOZ_MUST_USE bool moveTo(Map& map) {
+        [[nodiscard]] bool moveTo(Map& map) {
             return map.putNew(mozilla::Move(key), mozilla::Move(value));
         }
     };
@@ -531,12 +531,12 @@ class InlineMap
 
     template <typename KeyInput, typename ValueInput>
     MOZ_ALWAYS_INLINE
-    MOZ_MUST_USE bool add(AddPtr& p, KeyInput&& key, ValueInput&& value) {
+    [[nodiscard]] bool add(AddPtr& p, KeyInput&& key, ValueInput&& value) {
         return impl_.add(p, mozilla::Forward<KeyInput>(key), mozilla::Forward<ValueInput>(value));
     }
 
     template <typename KeyInput, typename ValueInput>
-    MOZ_MUST_USE bool put(KeyInput&& key, ValueInput&& value) {
+    [[nodiscard]] bool put(KeyInput&& key, ValueInput&& value) {
         AddPtr p = lookupForAdd(key);
         if (p) {
             p->value() = mozilla::Forward<ValueInput>(value);
@@ -577,7 +577,7 @@ class InlineSet
             this->key = mozilla::Forward<TInput>(key);
         }
 
-        MOZ_MUST_USE bool moveTo(Set& set) {
+        [[nodiscard]] bool moveTo(Set& set) {
             return set.putNew(mozilla::Move(key));
         }
     };
@@ -662,12 +662,12 @@ class InlineSet
 
     template <typename TInput>
     MOZ_ALWAYS_INLINE
-    MOZ_MUST_USE bool add(AddPtr& p, TInput&& key) {
+    [[nodiscard]] bool add(AddPtr& p, TInput&& key) {
         return impl_.add(p, mozilla::Forward<TInput>(key));
     }
 
     template <typename TInput>
-    MOZ_MUST_USE bool put(TInput&& key) {
+    [[nodiscard]] bool put(TInput&& key) {
         AddPtr p = lookupForAdd(key);
         return p ? true : add(p, mozilla::Forward<TInput>(key));
     }

js/src/ds/LifoAlloc.h

@@ -313,7 +313,7 @@ class LifoAlloc
     // allocation requests, not necessarily contiguous. Note that this does
     // not guarantee a successful single allocation of N bytes.
     MOZ_ALWAYS_INLINE
-    MOZ_MUST_USE bool ensureUnusedApproximate(size_t n) {
+    [[nodiscard]] bool ensureUnusedApproximate(size_t n) {
         AutoFallibleScope fallibleAllocator(this);
         size_t total = 0;
         for (BumpChunk* chunk = latest; chunk; chunk = chunk->next()) {
@@ -640,7 +640,7 @@ class LifoAllocPolicy
     }
     void reportAllocOverflow() const {
     }
-    MOZ_MUST_USE bool checkSimulatedOOM() const {
+    [[nodiscard]] bool checkSimulatedOOM() const {
         return fb == Infallible || !js::oom::ShouldFailWithOOM();
     }
 };

js/src/ds/OrderedHashTable.h

@@ -86,7 +86,7 @@ class OrderedHashTable
     OrderedHashTable(AllocPolicy& ap, mozilla::HashCodeScrambler hcs)
         : hashTable(nullptr), data(nullptr), dataLength(0), ranges(nullptr), alloc(ap), hcs(hcs) {}
 
-    MOZ_MUST_USE bool init() {
+    [[nodiscard]] bool init() {
         MOZ_ASSERT(!hashTable, "init must be called at most once");
 
         uint32_t buckets = initialBuckets();
@@ -153,7 +153,7 @@ class OrderedHashTable
      * means the element was not added to the table.
      */
     template <typename ElementInput>
-    MOZ_MUST_USE bool put(ElementInput&& element) {
+    [[nodiscard]] bool put(ElementInput&& element) {
         HashNumber h = prepareHash(Ops::getKey(element));
         if (Data* e = lookup(Ops::getKey(element), h)) {
             e->element = Forward<ElementInput>(element);
@@ -224,7 +224,7 @@ class OrderedHashTable
      * particular, those Ranges are still live and will see any entries added
      * after a successful clear().
      */
-    MOZ_MUST_USE bool clear() {
+    [[nodiscard]] bool clear() {
         if (dataLength != 0) {
             Data** oldHashTable = hashTable;
             Data* oldData = data;
@@ -633,7 +633,7 @@ class OrderedHashTable
      * empty elements in data[0:dataLength]. On allocation failure, this
      * leaves everything as it was and returns false.
      */
-    MOZ_MUST_USE bool rehash(uint32_t newHashShift) {
+    [[nodiscard]] bool rehash(uint32_t newHashShift) {
         // If the size of the table is not changing, rehash in place to avoid
         // allocating memory.
         if (newHashShift == hashShift) {
@@ -746,17 +746,17 @@ class OrderedHashMap
     typedef typename Impl::Range Range;
 
     OrderedHashMap(AllocPolicy ap, mozilla::HashCodeScrambler hcs) : impl(ap, hcs) {}
-    MOZ_MUST_USE bool init()                        { return impl.init(); }
+    [[nodiscard]] bool init()                        { return impl.init(); }
     uint32_t count() const                          { return impl.count(); }
     bool has(const Key& key) const                  { return impl.has(key); }
     Range all()                                     { return impl.all(); }
     const Entry* get(const Key& key) const          { return impl.get(key); }
     Entry* get(const Key& key)                      { return impl.get(key); }
     bool remove(const Key& key, bool* foundp)       { return impl.remove(key, foundp); }
-    MOZ_MUST_USE bool clear()                       { return impl.clear(); }
+    [[nodiscard]] bool clear()                       { return impl.clear(); }
 
     template <typename V>
-    MOZ_MUST_USE bool put(const Key& key, V&& value) {
+    [[nodiscard]] bool put(const Key& key, V&& value) {
         return impl.put(Entry(key, Forward<V>(value)));
     }
 
@@ -804,13 +804,13 @@ class OrderedHashSet
     typedef typename Impl::Range Range;
 
     explicit OrderedHashSet(AllocPolicy ap, mozilla::HashCodeScrambler hcs) : impl(ap, hcs) {}
-    MOZ_MUST_USE bool init()                        { return impl.init(); }
+    [[nodiscard]] bool init()                        { return impl.init(); }
     uint32_t count() const                          { return impl.count(); }
     bool has(const T& value) const                  { return impl.has(value); }
     Range all()                                     { return impl.all(); }
-    MOZ_MUST_USE bool put(const T& value)           { return impl.put(value); }
+    [[nodiscard]] bool put(const T& value)           { return impl.put(value); }
     bool remove(const T& value, bool* foundp)       { return impl.remove(value, foundp); }
-    MOZ_MUST_USE bool clear()                       { return impl.clear(); }
+    [[nodiscard]] bool clear()                       { return impl.clear(); }
 
     HashNumber hash(const T& value) const { return impl.prepareHash(value); }
 

js/src/ds/PageProtectingVector.h

@@ -223,7 +223,7 @@ class PageProtectingVector final
         unprotectedBytes = 0;
     }
 
-    MOZ_MUST_USE bool reserve(size_t size) {
+    [[nodiscard]] bool reserve(size_t size) {
         AutoUnprotect guard;
         if (size > vector.capacity())
             guard.emplace(this);
@@ -237,7 +237,7 @@ class PageProtectingVector final
     }
 
     template<typename U>
-    MOZ_MUST_USE bool append(const U* values, size_t size) {
+    [[nodiscard]] bool append(const U* values, size_t size) {
         bool ret;
         {
             AutoUnprotect guard;

js/src/ds/PriorityQueue.h

@@ -34,7 +34,7 @@ class PriorityQueue
       : heap(ap)
     {}
 
-    MOZ_MUST_USE bool reserve(size_t capacity) {
+    [[nodiscard]] bool reserve(size_t capacity) {
         return heap.reserve(capacity);
     }
 
@@ -56,7 +56,7 @@ class PriorityQueue
         return highest;
     }
 
-    MOZ_MUST_USE bool insert(const T& v) {
+    [[nodiscard]] bool insert(const T& v) {
         if (!heap.append(v))
             return false;
         siftUp(heap.length() - 1);

js/src/ds/Sort.h

@@ -77,7 +77,7 @@ MergeArrayRuns(T* dst, const T* src, size_t run1, size_t run2, Comparator c)
  * arbitrary.
  */
 template<typename T, typename Comparator>
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 MergeSort(T* array, size_t nelems, T* scratch, Comparator c)
 {
     const size_t INS_SORT_LIMIT = 3;

js/src/ds/SplayTree.h

@@ -89,7 +89,7 @@ class SplayTree
         return false;
     }
 
-    MOZ_MUST_USE bool insert(const T& v)
+    [[nodiscard]] bool insert(const T& v)
     {
         Node* element = allocateNode(v);
         if (!element)

js/src/frontend/BytecodeCompiler.h

@@ -49,7 +49,7 @@ CompileModule(ExclusiveContext* cx, const ReadOnlyCompileOptions& options,
               SourceBufferHolder& srcBuf, LifoAlloc& alloc,
               ScriptSourceObject** sourceObjectOut = nullptr);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CompileLazyFunction(JSContext* cx, Handle<LazyScript*> lazy, const char16_t* chars, size_t length);
 
 //
@@ -64,32 +64,32 @@ CompileLazyFunction(JSContext* cx, Handle<LazyScript*> lazy, const char16_t* cha
 //     Function("/*", "*/x) {")
 //     Function("x){ if (3", "return x;}")
 //
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CompileStandaloneFunction(JSContext* cx, MutableHandleFunction fun,
                           const ReadOnlyCompileOptions& options,
                           JS::SourceBufferHolder& srcBuf,
                           mozilla::Maybe<uint32_t> parameterListEnd,
                           HandleScope enclosingScope = nullptr);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CompileStandaloneGenerator(JSContext* cx, MutableHandleFunction fun,
                            const ReadOnlyCompileOptions& options,
                            JS::SourceBufferHolder& srcBuf,
                            mozilla::Maybe<uint32_t> parameterListEnd);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CompileStandaloneAsyncFunction(JSContext* cx, MutableHandleFunction fun,
                                const ReadOnlyCompileOptions& options,
                                JS::SourceBufferHolder& srcBuf,
                                mozilla::Maybe<uint32_t> parameterListEnd);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CompileStandaloneAsyncGenerator(JSContext* cx, MutableHandleFunction fun,
                                 const ReadOnlyCompileOptions& options,
                                 JS::SourceBufferHolder& srcBuf,
                                 mozilla::Maybe<uint32_t> parameterListEnd);
 
-MOZ_MUST_USE bool
+[[nodiscard]] bool
 CompileAsyncFunctionBody(JSContext* cx, MutableHandleFunction fun,
                          const ReadOnlyCompileOptions& options,
                          Handle<PropertyNameVector> formals, JS::SourceBufferHolder& srcBuf);

js/src/frontend/BytecodeControlStructures.h

@@ -64,7 +64,7 @@ class BreakableControl : public NestableControl
 
     BreakableControl(BytecodeEmitter* bce, StatementKind kind);
 
-    MOZ_MUST_USE bool patchBreaks(BytecodeEmitter* bce);
+    [[nodiscard]] bool patchBreaks(BytecodeEmitter* bce);
 };
 template <>
 inline bool
@@ -131,7 +131,7 @@ class LoopControl : public BreakableControl
         return canIonOsr_;
     }
 
-    MOZ_MUST_USE bool patchBreaksAndContinues(BytecodeEmitter* bce);
+    [[nodiscard]] bool patchBreaksAndContinues(BytecodeEmitter* bce);
 };
 template <>
 inline bool

js/src/frontend/BytecodeEmitter.cpp

@@ -144,13 +144,13 @@ class MOZ_RAII OptionalEmitter
     Other
   };
 
-  MOZ_MUST_USE bool emitJumpShortCircuit();
-  MOZ_MUST_USE bool emitJumpShortCircuitForCall();
+  [[nodiscard]] bool emitJumpShortCircuit();
+  [[nodiscard]] bool emitJumpShortCircuitForCall();
 
   // JSOp is the op code to be emitted, Kind is if we are dealing with a
   // reference (in which case we need two elements on the stack) or other value
   // (which needs one element on the stack)
-  MOZ_MUST_USE bool emitOptionalJumpTarget(JSOp op, Kind kind = Kind::Other);
+  [[nodiscard]] bool emitOptionalJumpTarget(JSOp op, Kind kind = Kind::Other);
 };
 
 BytecodeEmitter::BytecodeEmitter(BytecodeEmitter* parent,
@@ -708,7 +708,7 @@ class NonLocalExitControl
 
     NonLocalExitControl(const NonLocalExitControl&) = delete;
 
-    MOZ_MUST_USE bool leaveScope(EmitterScope* scope);
+    [[nodiscard]] bool leaveScope(EmitterScope* scope);
 
   public:
     NonLocalExitControl(BytecodeEmitter* bce, Kind kind)
@@ -725,9 +725,9 @@ class NonLocalExitControl
         bce_->stackDepth = savedDepth_;
     }
 
-    MOZ_MUST_USE bool prepareForNonLocalJump(NestableControl* target);
+    [[nodiscard]] bool prepareForNonLocalJump(NestableControl* target);
 
-    MOZ_MUST_USE bool prepareForNonLocalJumpToOutermost() {
+    [[nodiscard]] bool prepareForNonLocalJumpToOutermost() {
         return prepareForNonLocalJump(nullptr);
     }
 };

js/src/frontend/BytecodeEmitter.h

@@ -40,7 +40,7 @@ class CGConstList {
     explicit CGConstList(ExclusiveContext* cx)
       : vector(cx, ValueVector(cx))
     { }
-    MOZ_MUST_USE bool append(const Value& v) {
+    [[nodiscard]] bool append(const Value& v) {
         return vector.append(v);
     }
     size_t length() const { return vector.length(); }
@@ -75,7 +75,7 @@ struct CGTryNoteList {
     Vector<JSTryNote> list;
     explicit CGTryNoteList(ExclusiveContext* cx) : list(cx) {}
 
-    MOZ_MUST_USE bool append(JSTryNoteKind kind, uint32_t stackDepth, size_t start, size_t end);
+    [[nodiscard]] bool append(JSTryNoteKind kind, uint32_t stackDepth, size_t start, size_t end);
     size_t length() const { return list.length(); }
     void finish(TryNoteArray* array);
 };
@@ -97,7 +97,7 @@ struct CGScopeNoteList {
     Vector<CGScopeNote> list;
     explicit CGScopeNoteList(ExclusiveContext* cx) : list(cx) {}
 
-    MOZ_MUST_USE bool append(uint32_t scopeIndex, uint32_t offset, bool inPrologue,
+    [[nodiscard]] bool append(uint32_t scopeIndex, uint32_t offset, bool inPrologue,
                              uint32_t parent);
     void recordEnd(uint32_t index, uint32_t offset, bool inPrologue);
     size_t length() const { return list.length(); }
@@ -110,7 +110,7 @@ struct CGYieldAndAwaitOffsetList {
     uint32_t numAwaits;
     explicit CGYieldAndAwaitOffsetList(ExclusiveContext* cx) : list(cx), numYields(0), numAwaits(0) {}
 
-    MOZ_MUST_USE bool append(uint32_t offset) { return list.append(offset); }
+    [[nodiscard]] bool append(uint32_t offset) { return list.append(offset); }
     size_t length() const { return list.length(); }
     void finish(YieldAndAwaitOffsetArray& array, uint32_t prologueLength);
 };
@@ -264,7 +264,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
                     TokenPos bodyPosition, EmitterMode emitterMode = Normal,
                     FieldInitializers fieldInitializers = FieldInitializers::Invalid());
 
-    MOZ_MUST_USE bool init();
+    [[nodiscard]] bool init();
 
     template <typename T>
     T* findInnermostNestableControl() const;
@@ -300,7 +300,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     Scope* innermostScope() const;
 
     MOZ_ALWAYS_INLINE
-    MOZ_MUST_USE bool makeAtomIndex(JSAtom* atom, uint32_t* indexp) {
+    [[nodiscard]] bool makeAtomIndex(JSAtom* atom, uint32_t* indexp) {
         MOZ_ASSERT(atomIndices);
         AtomIndexMap::AddPtr p = atomIndices->lookupForAdd(atom);
         if (p) {
@@ -317,16 +317,16 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     }
 
     bool isInLoop();
-    MOZ_MUST_USE bool checkSingletonContext();
+    [[nodiscard]] bool checkSingletonContext();
 
     // Check whether our function is in a run-once context (a toplevel
     // run-one script or a run-once lambda).
-    MOZ_MUST_USE bool checkRunOnceContext();
+    [[nodiscard]] bool checkRunOnceContext();
 
     bool needsImplicitThis();
 
-    MOZ_MUST_USE bool maybeSetDisplayURL();
-    MOZ_MUST_USE bool maybeSetSourceMap();
+    [[nodiscard]] bool maybeSetDisplayURL();
+    [[nodiscard]] bool maybeSetSourceMap();
     void tellDebuggerAboutCompiledScript(ExclusiveContext* cx);
 
     inline TokenStream& tokenStream();
@@ -383,29 +383,29 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     // statement, we define useless code as code with no side effects, because
     // the main effect, the value left on the stack after the code executes,
     // will be discarded by a pop bytecode.
-    MOZ_MUST_USE bool checkSideEffects(ParseNode* pn, bool* answer);
+    [[nodiscard]] bool checkSideEffects(ParseNode* pn, bool* answer);
 
 #ifdef DEBUG
-    MOZ_MUST_USE bool checkStrictOrSloppy(JSOp op);
+    [[nodiscard]] bool checkStrictOrSloppy(JSOp op);
 #endif
 
     // Append a new source note of the given type (and therefore size) to the
     // notes dynamic array, updating noteCount. Return the new note's index
     // within the array pointed at by current->notes as outparam.
-    MOZ_MUST_USE bool newSrcNote(SrcNoteType type, unsigned* indexp = nullptr);
-    MOZ_MUST_USE bool newSrcNote2(SrcNoteType type, ptrdiff_t offset, unsigned* indexp = nullptr);
-    MOZ_MUST_USE bool newSrcNote3(SrcNoteType type, ptrdiff_t offset1, ptrdiff_t offset2,
+    [[nodiscard]] bool newSrcNote(SrcNoteType type, unsigned* indexp = nullptr);
+    [[nodiscard]] bool newSrcNote2(SrcNoteType type, ptrdiff_t offset, unsigned* indexp = nullptr);
+    [[nodiscard]] bool newSrcNote3(SrcNoteType type, ptrdiff_t offset1, ptrdiff_t offset2,
                                   unsigned* indexp = nullptr);
 
     void copySrcNotes(jssrcnote* destination, uint32_t nsrcnotes);
-    MOZ_MUST_USE bool setSrcNoteOffset(unsigned index, unsigned which, ptrdiff_t offset);
+    [[nodiscard]] bool setSrcNoteOffset(unsigned index, unsigned which, ptrdiff_t offset);
 
     // NB: this function can add at most one extra extended delta note.
-    MOZ_MUST_USE bool addToSrcNoteDelta(jssrcnote* sn, ptrdiff_t delta);
+    [[nodiscard]] bool addToSrcNoteDelta(jssrcnote* sn, ptrdiff_t delta);
 
     // Finish taking source notes in cx's notePool. If successful, the final
     // source note count is stored in the out outparam.
-    MOZ_MUST_USE bool finishTakingSrcNotes(uint32_t* out);
+    [[nodiscard]] bool finishTakingSrcNotes(uint32_t* out);
 
     // Control whether emitTree emits a line number note.
     enum EmitLineNumberNote {
@@ -414,242 +414,242 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     };
 
     // Emit code for the tree rooted at pn.
-    MOZ_MUST_USE bool emitTree(ParseNode* pn, ValueUsage valueUsage = ValueUsage::WantValue,
+    [[nodiscard]] bool emitTree(ParseNode* pn, ValueUsage valueUsage = ValueUsage::WantValue,
                                EmitLineNumberNote emitLineNote = EMIT_LINENOTE);
 
     // Emit code for the optional tree rooted at pn.
-    MOZ_MUST_USE bool emitOptionalTree(ParseNode* pn,
+    [[nodiscard]] bool emitOptionalTree(ParseNode* pn,
                                        OptionalEmitter& oe,
                                        ValueUsage valueUsage = ValueUsage::WantValue);
 
     // Emit code for the tree rooted at pn with its own TDZ cache.
-    MOZ_MUST_USE bool emitTreeInBranch(ParseNode* pn,
+    [[nodiscard]] bool emitTreeInBranch(ParseNode* pn,
                                        ValueUsage valueUsage = ValueUsage::WantValue);
 
     // Emit global, eval, or module code for tree rooted at body. Always
     // encompasses the entire source.
-    MOZ_MUST_USE bool emitScript(ParseNode* body);
+    [[nodiscard]] bool emitScript(ParseNode* body);
 
     // Emit function code for the tree rooted at body.
-    MOZ_MUST_USE bool emitFunctionScript(FunctionNode* funNode);
+    [[nodiscard]] bool emitFunctionScript(FunctionNode* funNode);
 
     void updateDepth(ptrdiff_t target);
-    MOZ_MUST_USE bool updateLineNumberNotes(uint32_t offset);
-    MOZ_MUST_USE bool updateSourceCoordNotes(uint32_t offset);
+    [[nodiscard]] bool updateLineNumberNotes(uint32_t offset);
+    [[nodiscard]] bool updateSourceCoordNotes(uint32_t offset);
 
     JSOp strictifySetNameOp(JSOp op);
 
-    MOZ_MUST_USE bool emitCheck(JSOp op, ptrdiff_t delta, ptrdiff_t* offset);
+    [[nodiscard]] bool emitCheck(JSOp op, ptrdiff_t delta, ptrdiff_t* offset);
 
     // Emit one bytecode.
-    MOZ_MUST_USE bool emit1(JSOp op);
+    [[nodiscard]] bool emit1(JSOp op);
 
     // Emit two bytecodes, an opcode (op) with a byte of immediate operand
     // (op1).
-    MOZ_MUST_USE bool emit2(JSOp op, uint8_t op1);
+    [[nodiscard]] bool emit2(JSOp op, uint8_t op1);
 
     // Emit three bytecodes, an opcode with two bytes of immediate operands.
-    MOZ_MUST_USE bool emit3(JSOp op, jsbytecode op1, jsbytecode op2);
+    [[nodiscard]] bool emit3(JSOp op, jsbytecode op1, jsbytecode op2);
 
     // Helper to emit JSOP_DUPAT. The argument is the value's depth on the
     // JS stack, as measured from the top.
-    MOZ_MUST_USE bool emitDupAt(unsigned slotFromTop);
+    [[nodiscard]] bool emitDupAt(unsigned slotFromTop);
 
     // Helper to emit JSOP_POP or JSOP_POPN.
-    MOZ_MUST_USE bool emitPopN(unsigned n);
+    [[nodiscard]] bool emitPopN(unsigned n);
 
     // Helper to emit JSOP_SWAP or JSOP_UNPICK.
-    MOZ_MUST_USE bool emitUnpickN(unsigned n);
+    [[nodiscard]] bool emitUnpickN(unsigned n);
 
     // Helper to emit JSOP_CHECKISOBJ.
-    MOZ_MUST_USE bool emitCheckIsObj(CheckIsObjectKind kind);
+    [[nodiscard]] bool emitCheckIsObj(CheckIsObjectKind kind);
 
     // Helper to emit JSOP_CHECKISCALLABLE.
-    MOZ_MUST_USE bool emitCheckIsCallable(CheckIsCallableKind kind);
+    [[nodiscard]] bool emitCheckIsCallable(CheckIsCallableKind kind);
 
     // Emit a bytecode followed by an uint16 immediate operand stored in
     // big-endian order.
-    MOZ_MUST_USE bool emitUint16Operand(JSOp op, uint32_t operand);
+    [[nodiscard]] bool emitUint16Operand(JSOp op, uint32_t operand);
 
     // Emit a bytecode followed by an uint32 immediate operand.
-    MOZ_MUST_USE bool emitUint32Operand(JSOp op, uint32_t operand);
+    [[nodiscard]] bool emitUint32Operand(JSOp op, uint32_t operand);
 
     // Emit (1 + extra) bytecodes, for N bytes of op and its immediate operand.
-    MOZ_MUST_USE bool emitN(JSOp op, size_t extra, ptrdiff_t* offset = nullptr);
+    [[nodiscard]] bool emitN(JSOp op, size_t extra, ptrdiff_t* offset = nullptr);
 
-    MOZ_MUST_USE bool emitNumberOp(double dval);
+    [[nodiscard]] bool emitNumberOp(double dval);
 
-    MOZ_MUST_USE bool emitBigIntOp(BigInt* bigint);
+    [[nodiscard]] bool emitBigIntOp(BigInt* bigint);
 
-    MOZ_MUST_USE bool emitThisLiteral(ThisLiteral* pn);
-    MOZ_MUST_USE bool emitGetFunctionThis(ParseNode* pn);
-    MOZ_MUST_USE bool emitGetFunctionThis(const mozilla::Maybe<uint32_t>& offset);
-    MOZ_MUST_USE bool emitGetThisForSuperBase(UnaryNode* superBase);
-    MOZ_MUST_USE bool emitSetThis(BinaryNode* setThisNode);
-    MOZ_MUST_USE bool emitCheckDerivedClassConstructorReturn();
+    [[nodiscard]] bool emitThisLiteral(ThisLiteral* pn);
+    [[nodiscard]] bool emitGetFunctionThis(ParseNode* pn);
+    [[nodiscard]] bool emitGetFunctionThis(const mozilla::Maybe<uint32_t>& offset);
+    [[nodiscard]] bool emitGetThisForSuperBase(UnaryNode* superBase);
+    [[nodiscard]] bool emitSetThis(BinaryNode* setThisNode);
+    [[nodiscard]] bool emitCheckDerivedClassConstructorReturn();
 
     // Handle jump opcodes and jump targets.
-    MOZ_MUST_USE bool emitJumpTarget(JumpTarget* target);
-    MOZ_MUST_USE bool emitJumpNoFallthrough(JSOp op, JumpList* jump);
-    MOZ_MUST_USE bool emitJump(JSOp op, JumpList* jump);
-    MOZ_MUST_USE bool emitBackwardJump(JSOp op, JumpTarget target, JumpList* jump,
+    [[nodiscard]] bool emitJumpTarget(JumpTarget* target);
+    [[nodiscard]] bool emitJumpNoFallthrough(JSOp op, JumpList* jump);
+    [[nodiscard]] bool emitJump(JSOp op, JumpList* jump);
+    [[nodiscard]] bool emitBackwardJump(JSOp op, JumpTarget target, JumpList* jump,
                                        JumpTarget* fallthrough);
     void patchJumpsToTarget(JumpList jump, JumpTarget target);
-    MOZ_MUST_USE bool emitJumpTargetAndPatch(JumpList jump);
+    [[nodiscard]] bool emitJumpTargetAndPatch(JumpList jump);
 
-    MOZ_MUST_USE bool emitCall(JSOp op, uint16_t argc,
+    [[nodiscard]] bool emitCall(JSOp op, uint16_t argc,
                                const mozilla::Maybe<uint32_t>& sourceCoordOffset);
-    MOZ_MUST_USE bool emitCall(JSOp op, uint16_t argc, ParseNode* pn = nullptr);
-    MOZ_MUST_USE bool emitCallIncDec(UnaryNode* incDec);
+    [[nodiscard]] bool emitCall(JSOp op, uint16_t argc, ParseNode* pn = nullptr);
+    [[nodiscard]] bool emitCallIncDec(UnaryNode* incDec);
 
-    MOZ_MUST_USE bool emitLoopHead(ParseNode* nextpn, JumpTarget* top);
-    MOZ_MUST_USE bool emitLoopEntry(ParseNode* nextpn, JumpList entryJump);
+    [[nodiscard]] bool emitLoopHead(ParseNode* nextpn, JumpTarget* top);
+    [[nodiscard]] bool emitLoopEntry(ParseNode* nextpn, JumpList entryJump);
 
-    MOZ_MUST_USE bool emitGoto(NestableControl* target, JumpList* jumplist,
+    [[nodiscard]] bool emitGoto(NestableControl* target, JumpList* jumplist,
                                SrcNoteType noteType = SRC_NULL);
 
-    MOZ_MUST_USE bool emitIndex32(JSOp op, uint32_t index);
-    MOZ_MUST_USE bool emitIndexOp(JSOp op, uint32_t index);
+    [[nodiscard]] bool emitIndex32(JSOp op, uint32_t index);
+    [[nodiscard]] bool emitIndexOp(JSOp op, uint32_t index);
 
-    MOZ_MUST_USE bool emitAtomOp(JSAtom* atom, JSOp op);
-    MOZ_MUST_USE bool emitAtomOp(uint32_t atomIndex, JSOp op);
+    [[nodiscard]] bool emitAtomOp(JSAtom* atom, JSOp op);
+    [[nodiscard]] bool emitAtomOp(uint32_t atomIndex, JSOp op);
 
-    MOZ_MUST_USE bool emitArrayLiteral(ListNode* array);
-    MOZ_MUST_USE bool emitArray(ParseNode* arrayHead, uint32_t count, JSOp op);
-    MOZ_MUST_USE bool emitArrayComp(ListNode* pn);
+    [[nodiscard]] bool emitArrayLiteral(ListNode* array);
+    [[nodiscard]] bool emitArray(ParseNode* arrayHead, uint32_t count, JSOp op);
+    [[nodiscard]] bool emitArrayComp(ListNode* pn);
 
-    MOZ_MUST_USE bool emitInternedScopeOp(uint32_t index, JSOp op);
-    MOZ_MUST_USE bool emitInternedObjectOp(uint32_t index, JSOp op);
-    MOZ_MUST_USE bool emitObjectOp(ObjectBox* objbox, JSOp op);
-    MOZ_MUST_USE bool emitObjectPairOp(ObjectBox* objbox1, ObjectBox* objbox2, JSOp op);
-    MOZ_MUST_USE bool emitRegExp(uint32_t index);
+    [[nodiscard]] bool emitInternedScopeOp(uint32_t index, JSOp op);
+    [[nodiscard]] bool emitInternedObjectOp(uint32_t index, JSOp op);
+    [[nodiscard]] bool emitObjectOp(ObjectBox* objbox, JSOp op);
+    [[nodiscard]] bool emitObjectPairOp(ObjectBox* objbox1, ObjectBox* objbox2, JSOp op);
+    [[nodiscard]] bool emitRegExp(uint32_t index);
 
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitFunction(FunctionNode* funNode,
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitFunction(FunctionNode* funNode,
                                                     bool needsProto = false,
                                                     ListNode* classContentsIfConstructor = nullptr);
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitObject(ListNode* objNode);
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitObject(ListNode* objNode);
 
-    MOZ_MUST_USE bool replaceNewInitWithNewObject(JSObject* obj, ptrdiff_t offset);
+    [[nodiscard]] bool replaceNewInitWithNewObject(JSObject* obj, ptrdiff_t offset);
 
-    MOZ_MUST_USE bool emitHoistedFunctionsInList(ListNode* stmtList);
+    [[nodiscard]] bool emitHoistedFunctionsInList(ListNode* stmtList);
 
-    MOZ_MUST_USE bool emitPropertyList(ListNode* obj, PropertyEmitter& pe,
+    [[nodiscard]] bool emitPropertyList(ListNode* obj, PropertyEmitter& pe,
                                        PropListType type);
 
     enum class FieldPlacement { Instance, Static };
     FieldInitializers setupFieldInitializers(ListNode* classMembers, FieldPlacement placement);
-    MOZ_MUST_USE bool emitCreateFieldKeys(ListNode* obj, FieldPlacement placement);
-    MOZ_MUST_USE bool emitCreateFieldInitializers(ClassEmitter& ce, ListNode* obj, FieldPlacement placement);
+    [[nodiscard]] bool emitCreateFieldKeys(ListNode* obj, FieldPlacement placement);
+    [[nodiscard]] bool emitCreateFieldInitializers(ClassEmitter& ce, ListNode* obj, FieldPlacement placement);
     const FieldInitializers& findFieldInitializersForCall();
-    MOZ_MUST_USE bool emitInitializeInstanceFields();
-    MOZ_MUST_USE bool emitInitializeStaticFields(ListNode* classMembers);
+    [[nodiscard]] bool emitInitializeInstanceFields();
+    [[nodiscard]] bool emitInitializeStaticFields(ListNode* classMembers);
 
     // To catch accidental misuse, emitUint16Operand/emit3 assert that they are
     // not used to unconditionally emit JSOP_GETLOCAL. Variable access should
     // instead be emitted using EmitVarOp. In special cases, when the caller
     // definitely knows that a given local slot is unaliased, this function may be
     // used as a non-asserting version of emitUint16Operand.
-    MOZ_MUST_USE bool emitLocalOp(JSOp op, uint32_t slot);
+    [[nodiscard]] bool emitLocalOp(JSOp op, uint32_t slot);
 
-    MOZ_MUST_USE bool emitArgOp(JSOp op, uint16_t slot);
-    MOZ_MUST_USE bool emitEnvCoordOp(JSOp op, EnvironmentCoordinate ec);
+    [[nodiscard]] bool emitArgOp(JSOp op, uint16_t slot);
+    [[nodiscard]] bool emitEnvCoordOp(JSOp op, EnvironmentCoordinate ec);
 
-    MOZ_MUST_USE bool emitGetNameAtLocation(JSAtom* name, const NameLocation& loc);
-    MOZ_MUST_USE bool emitGetName(JSAtom* name) {
+    [[nodiscard]] bool emitGetNameAtLocation(JSAtom* name, const NameLocation& loc);
+    [[nodiscard]] bool emitGetName(JSAtom* name) {
         return emitGetNameAtLocation(name, lookupName(name));
     }
-    MOZ_MUST_USE bool emitGetName(ParseNode* pn);
+    [[nodiscard]] bool emitGetName(ParseNode* pn);
 
-    MOZ_MUST_USE bool emitTDZCheckIfNeeded(JSAtom* name, const NameLocation& loc);
+    [[nodiscard]] bool emitTDZCheckIfNeeded(JSAtom* name, const NameLocation& loc);
 
-    MOZ_MUST_USE bool emitNameIncDec(UnaryNode* incDec);
+    [[nodiscard]] bool emitNameIncDec(UnaryNode* incDec);
 
-    MOZ_MUST_USE bool emitDeclarationList(ListNode* declList);
-    MOZ_MUST_USE bool emitSingleDeclaration(ParseNode* declList, ParseNode* decl,
+    [[nodiscard]] bool emitDeclarationList(ListNode* declList);
+    [[nodiscard]] bool emitSingleDeclaration(ParseNode* declList, ParseNode* decl,
                                             ParseNode* initializer);
 
-    MOZ_MUST_USE bool emitNewInit(JSProtoKey key);
-    MOZ_MUST_USE bool emitSingletonInitialiser(ParseNode* pn);
+    [[nodiscard]] bool emitNewInit(JSProtoKey key);
+    [[nodiscard]] bool emitSingletonInitialiser(ParseNode* pn);
 
-    MOZ_MUST_USE bool emitPrepareIteratorResult();
-    MOZ_MUST_USE bool emitFinishIteratorResult(bool done);
-    MOZ_MUST_USE bool iteratorResultShape(unsigned* shape);
-    MOZ_MUST_USE bool emitToIteratorResult(bool done);
+    [[nodiscard]] bool emitPrepareIteratorResult();
+    [[nodiscard]] bool emitFinishIteratorResult(bool done);
+    [[nodiscard]] bool iteratorResultShape(unsigned* shape);
+    [[nodiscard]] bool emitToIteratorResult(bool done);
 
-    MOZ_MUST_USE bool emitGetDotGeneratorInInnermostScope() {
+    [[nodiscard]] bool emitGetDotGeneratorInInnermostScope() {
         return emitGetDotGeneratorInScope(*innermostEmitterScope());
     }
-    MOZ_MUST_USE bool emitGetDotGeneratorInScope(EmitterScope& currentScope);
+    [[nodiscard]] bool emitGetDotGeneratorInScope(EmitterScope& currentScope);
 
-    MOZ_MUST_USE bool emitInitialYield(UnaryNode* yieldNode);
-    MOZ_MUST_USE bool emitYield(UnaryNode* yieldNode);
-    MOZ_MUST_USE bool emitYieldOp(JSOp op);
-    MOZ_MUST_USE bool emitYieldStar(ParseNode* iter);
-    MOZ_MUST_USE bool emitAwaitInInnermostScope() {
+    [[nodiscard]] bool emitInitialYield(UnaryNode* yieldNode);
+    [[nodiscard]] bool emitYield(UnaryNode* yieldNode);
+    [[nodiscard]] bool emitYieldOp(JSOp op);
+    [[nodiscard]] bool emitYieldStar(ParseNode* iter);
+    [[nodiscard]] bool emitAwaitInInnermostScope() {
         return emitAwaitInScope(*innermostEmitterScope());
     }
-    MOZ_MUST_USE bool emitAwaitInInnermostScope(UnaryNode* awaitNode);
-    MOZ_MUST_USE bool emitAwaitInScope(EmitterScope& currentScope);
-    MOZ_MUST_USE bool emitPropLHS(PropertyAccess* prop);
-    MOZ_MUST_USE bool emitPropIncDec(UnaryNode* incDec);
+    [[nodiscard]] bool emitAwaitInInnermostScope(UnaryNode* awaitNode);
+    [[nodiscard]] bool emitAwaitInScope(EmitterScope& currentScope);
+    [[nodiscard]] bool emitPropLHS(PropertyAccess* prop);
+    [[nodiscard]] bool emitPropIncDec(UnaryNode* incDec);
 
-    MOZ_MUST_USE bool emitAsyncWrapperLambda(unsigned index, bool isArrow);
-    MOZ_MUST_USE bool emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isArrow,
+    [[nodiscard]] bool emitAsyncWrapperLambda(unsigned index, bool isArrow);
+    [[nodiscard]] bool emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isArrow,
                                        bool isStarGenerator);
 
-    MOZ_MUST_USE bool emitComputedPropertyName(UnaryNode* computedPropName);
+    [[nodiscard]] bool emitComputedPropertyName(UnaryNode* computedPropName);
 
     // Emit bytecode to put operands for a JSOP_GETELEM/CALLELEM/SETELEM/DELELEM
     // opcode onto the stack in the right order. In the case of SETELEM, the
     // value to be assigned must already be pushed.
     enum class EmitElemOption { Get, Set, Call, IncDec, CompoundAssign, Ref };
-    MOZ_MUST_USE bool emitElemOperands(PropertyByValue* elem, EmitElemOption opts);
+    [[nodiscard]] bool emitElemOperands(PropertyByValue* elem, EmitElemOption opts);
 
-    MOZ_MUST_USE bool emitElemObjAndKey(PropertyByValue* elem, bool isSuper, ElemOpEmitter& eoe);
-    MOZ_MUST_USE bool emitElemOpBase(JSOp op);
-    MOZ_MUST_USE bool emitElemIncDec(UnaryNode* incDec);
+    [[nodiscard]] bool emitElemObjAndKey(PropertyByValue* elem, bool isSuper, ElemOpEmitter& eoe);
+    [[nodiscard]] bool emitElemOpBase(JSOp op);
+    [[nodiscard]] bool emitElemIncDec(UnaryNode* incDec);
 
-    MOZ_MUST_USE bool emitCatch(TernaryNode* catchNode);
-    MOZ_MUST_USE bool emitIf(TernaryNode* ifNode);
-    MOZ_MUST_USE bool emitWith(BinaryNode* withNode);
+    [[nodiscard]] bool emitCatch(TernaryNode* catchNode);
+    [[nodiscard]] bool emitIf(TernaryNode* ifNode);
+    [[nodiscard]] bool emitWith(BinaryNode* withNode);
 
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitLabeledStatement(const LabeledStatement* pn);
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitLexicalScope(LexicalScopeNode* lexicalScope);
-    MOZ_MUST_USE bool emitLexicalScopeBody(ParseNode* body,
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitLabeledStatement(const LabeledStatement* pn);
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitLexicalScope(LexicalScopeNode* lexicalScope);
+    [[nodiscard]] bool emitLexicalScopeBody(ParseNode* body,
                                            EmitLineNumberNote emitLineNote = EMIT_LINENOTE);
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitSwitch(SwitchStatement* switchStmt);
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitTry(TryNode* tryNode);
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitSwitch(SwitchStatement* switchStmt);
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitTry(TryNode* tryNode);
 
     // emitDestructuringLHSRef emits the lhs expression's reference.
     // If the lhs expression is object property |OBJ.prop|, it emits |OBJ|.
     // If it's object element |OBJ[ELEM]|, it emits |OBJ| and |ELEM|.
     // If there's nothing to evaluate for the reference, it emits nothing.
     // |emitted| parameter receives the number of values pushed onto the stack.
-    MOZ_MUST_USE bool emitDestructuringLHSRef(ParseNode* target, size_t* emitted);
+    [[nodiscard]] bool emitDestructuringLHSRef(ParseNode* target, size_t* emitted);
 
     // emitSetOrInitializeDestructuring assumes the lhs expression's reference
     // and the to-be-destructured value has been pushed on the stack.  It emits
     // code to destructure a single lhs expression (either a name or a compound
     // []/{} expression).
-    MOZ_MUST_USE bool emitSetOrInitializeDestructuring(ParseNode* target, DestructuringFlavor flav);
+    [[nodiscard]] bool emitSetOrInitializeDestructuring(ParseNode* target, DestructuringFlavor flav);
 
     // emitDestructuringObjRestExclusionSet emits the property exclusion set
     // for the rest-property in an object pattern.
-    MOZ_MUST_USE bool emitDestructuringObjRestExclusionSet(ListNode* pattern);
+    [[nodiscard]] bool emitDestructuringObjRestExclusionSet(ListNode* pattern);
 
     // emitDestructuringOps assumes the to-be-destructured value has been
     // pushed on the stack and emits code to destructure each part of a [] or
     // {} lhs expression.
-    MOZ_MUST_USE bool emitDestructuringOps(ListNode* pattern, DestructuringFlavor flav);
-    MOZ_MUST_USE bool emitDestructuringOpsArray(ListNode* pattern, DestructuringFlavor flav);
-    MOZ_MUST_USE bool emitDestructuringOpsObject(ListNode* pattern, DestructuringFlavor flav);
+    [[nodiscard]] bool emitDestructuringOps(ListNode* pattern, DestructuringFlavor flav);
+    [[nodiscard]] bool emitDestructuringOpsArray(ListNode* pattern, DestructuringFlavor flav);
+    [[nodiscard]] bool emitDestructuringOpsObject(ListNode* pattern, DestructuringFlavor flav);
 
     typedef bool
     (*DestructuringDeclEmitter)(BytecodeEmitter* bce, ParseNode* pn);
 
     // Throw a TypeError if the value atop the stack isn't convertible to an
     // object, with no overall effect on the stack.
-    MOZ_MUST_USE bool emitRequireObjectCoercible();
+    [[nodiscard]] bool emitRequireObjectCoercible();
 
     enum class CopyOption {
         Filtered, Unfiltered
@@ -658,27 +658,27 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     // Calls either the |CopyDataProperties| or the
     // |CopyDataPropertiesUnfiltered| intrinsic function, consumes three (or
     // two in the latter case) elements from the stack.
-    MOZ_MUST_USE bool emitCopyDataProperties(CopyOption option);
+    [[nodiscard]] bool emitCopyDataProperties(CopyOption option);
 
     // emitIterator expects the iterable to already be on the stack.
     // It will replace that stack value with the corresponding iterator
-    MOZ_MUST_USE bool emitIterator();
+    [[nodiscard]] bool emitIterator();
 
-    MOZ_MUST_USE bool emitAsyncIterator();
+    [[nodiscard]] bool emitAsyncIterator();
 
     // XXX currently used only by OptionalEmitter, research still required
     // to identify when this was introduced in m-c.
-    MOZ_MUST_USE bool emitPushNotUndefinedOrNull();
+    [[nodiscard]] bool emitPushNotUndefinedOrNull();
 
     // Pops iterator from the top of the stack. Pushes the result of |.next()|
     // onto the stack.
-    MOZ_MUST_USE bool emitIteratorNext(ParseNode* pn, IteratorKind kind = IteratorKind::Sync,
+    [[nodiscard]] bool emitIteratorNext(ParseNode* pn, IteratorKind kind = IteratorKind::Sync,
                                        bool allowSelfHosted = false);
-    MOZ_MUST_USE bool emitIteratorCloseInScope(EmitterScope& currentScope,
+    [[nodiscard]] bool emitIteratorCloseInScope(EmitterScope& currentScope,
                                                IteratorKind iterKind = IteratorKind::Sync,
                                                CompletionKind completionKind = CompletionKind::Normal,
                                                bool allowSelfHosted = false);
-    MOZ_MUST_USE bool emitIteratorCloseInInnermostScope(IteratorKind iterKind = IteratorKind::Sync,
+    [[nodiscard]] bool emitIteratorCloseInInnermostScope(IteratorKind iterKind = IteratorKind::Sync,
                                                         CompletionKind completionKind = CompletionKind::Normal,
                                                         bool allowSelfHosted = false) {
         return emitIteratorCloseInScope(*innermostEmitterScope(), iterKind, completionKind,
@@ -686,7 +686,7 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     }
 
     template <typename InnerEmitter>
-    MOZ_MUST_USE bool wrapWithDestructuringIteratorCloseTryNote(int32_t iterDepth,
+    [[nodiscard]] bool wrapWithDestructuringIteratorCloseTryNote(int32_t iterDepth,
                                                                 InnerEmitter emitter);
 
     // Check if the value on top of the stack is "undefined". If so, replace
@@ -694,114 +694,114 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     // |pattern| is a lhs node of the default expression.  If it's an
     // identifier and |defaultExpr| is an anonymous function, |SetFunctionName|
     // is called at compile time.
-    MOZ_MUST_USE bool emitDefault(ParseNode* defaultExpr, ParseNode* pattern);
+    [[nodiscard]] bool emitDefault(ParseNode* defaultExpr, ParseNode* pattern);
 
-    MOZ_MUST_USE bool setOrEmitSetFunName(ParseNode* maybeFun, HandleAtom name);
-    MOZ_MUST_USE bool setFunName(JSFunction* fun, JSAtom* name);
-    MOZ_MUST_USE bool emitSetClassConstructorName(JSAtom* name);
-    MOZ_MUST_USE bool emitSetFunctionNameFromStack(uint8_t offset);
+    [[nodiscard]] bool setOrEmitSetFunName(ParseNode* maybeFun, HandleAtom name);
+    [[nodiscard]] bool setFunName(JSFunction* fun, JSAtom* name);
+    [[nodiscard]] bool emitSetClassConstructorName(JSAtom* name);
+    [[nodiscard]] bool emitSetFunctionNameFromStack(uint8_t offset);
 
-    MOZ_MUST_USE bool emitInitializer(ParseNode* initializer, ParseNode* pattern);
+    [[nodiscard]] bool emitInitializer(ParseNode* initializer, ParseNode* pattern);
 
-    MOZ_MUST_USE bool emitCallSiteObject(CallSiteNode* callSiteObj);
-    MOZ_MUST_USE bool emitTemplateString(ListNode* templateString);
-    MOZ_MUST_USE bool emitAssignmentOrInit(ParseNodeKind kind, JSOp compoundOp,
+    [[nodiscard]] bool emitCallSiteObject(CallSiteNode* callSiteObj);
+    [[nodiscard]] bool emitTemplateString(ListNode* templateString);
+    [[nodiscard]] bool emitAssignmentOrInit(ParseNodeKind kind, JSOp compoundOp,
                                            ParseNode* lhs, ParseNode* rhs);
-    MOZ_MUST_USE bool emitShortCircuitAssignment(ParseNodeKind kind, JSOp op,
+    [[nodiscard]] bool emitShortCircuitAssignment(ParseNodeKind kind, JSOp op,
                                                  ParseNode* lhs, ParseNode* rhs);
 
-    MOZ_MUST_USE bool emitReturn(UnaryNode* returnNode);
-    MOZ_MUST_USE bool emitStatement(UnaryNode* exprStmt);
-    MOZ_MUST_USE bool emitStatementList(ListNode* stmtList);
+    [[nodiscard]] bool emitReturn(UnaryNode* returnNode);
+    [[nodiscard]] bool emitStatement(UnaryNode* exprStmt);
+    [[nodiscard]] bool emitStatementList(ListNode* stmtList);
 
-    MOZ_MUST_USE bool emitDeleteName(UnaryNode* deleteNode);
-    MOZ_MUST_USE bool emitDeleteProperty(UnaryNode* deleteNode);
-    MOZ_MUST_USE bool emitDeleteElement(UnaryNode* deleteNode);
-    MOZ_MUST_USE bool emitDeleteExpression(UnaryNode* deleteNode);
+    [[nodiscard]] bool emitDeleteName(UnaryNode* deleteNode);
+    [[nodiscard]] bool emitDeleteProperty(UnaryNode* deleteNode);
+    [[nodiscard]] bool emitDeleteElement(UnaryNode* deleteNode);
+    [[nodiscard]] bool emitDeleteExpression(UnaryNode* deleteNode);
 
     // Optional methods which emit Optional Jump Target
-    MOZ_MUST_USE bool emitOptionalChain(UnaryNode* optionalChain,
+    [[nodiscard]] bool emitOptionalChain(UnaryNode* optionalChain,
                                         ValueUsage valueUsage);
-    MOZ_MUST_USE bool emitCalleeAndThisForOptionalChain(UnaryNode* optionalChain,
+    [[nodiscard]] bool emitCalleeAndThisForOptionalChain(UnaryNode* optionalChain,
                                                         ParseNode* callNode,
                                                         CallOrNewEmitter& cone);
-    MOZ_MUST_USE bool emitDeleteOptionalChain(UnaryNode* deleteNode);
+    [[nodiscard]] bool emitDeleteOptionalChain(UnaryNode* deleteNode);
 
     // Optional methods which emit a shortCircuit jump. They need to be called by
     // a method which emits an Optional Jump Target, see below.
-    MOZ_MUST_USE bool emitOptionalDotExpression(PropertyAccessBase* prop,
+    [[nodiscard]] bool emitOptionalDotExpression(PropertyAccessBase* prop,
                                                 PropOpEmitter& poe, bool isSuper,
                                                 OptionalEmitter& oe);
-    MOZ_MUST_USE bool emitOptionalElemExpression(PropertyByValueBase* elem,
+    [[nodiscard]] bool emitOptionalElemExpression(PropertyByValueBase* elem,
                                                  ElemOpEmitter& eoe, bool isSuper,
                                                  OptionalEmitter& oe);
-    MOZ_MUST_USE bool emitOptionalCall(BinaryNode* callNode,
+    [[nodiscard]] bool emitOptionalCall(BinaryNode* callNode,
                                        OptionalEmitter& oe,
                                        ValueUsage valueUsage);
-    MOZ_MUST_USE bool emitDeletePropertyInOptChain(PropertyAccessBase* propExpr,
+    [[nodiscard]] bool emitDeletePropertyInOptChain(PropertyAccessBase* propExpr,
                                                    OptionalEmitter& oe);
-    MOZ_MUST_USE bool emitDeleteElementInOptChain(PropertyByValueBase* elemExpr,
+    [[nodiscard]] bool emitDeleteElementInOptChain(PropertyByValueBase* elemExpr,
                                                   OptionalEmitter& oe);
 
     // |op| must be JSOP_TYPEOF or JSOP_TYPEOFEXPR.
-    MOZ_MUST_USE bool emitTypeof(UnaryNode* typeofNode, JSOp op);
+    [[nodiscard]] bool emitTypeof(UnaryNode* typeofNode, JSOp op);
 
-    MOZ_MUST_USE bool emitUnary(UnaryNode* unaryNode);
-    MOZ_MUST_USE bool emitRightAssociative(ListNode* node);
-    MOZ_MUST_USE bool emitLeftAssociative(ListNode* node);
-    MOZ_MUST_USE bool emitLogical(ListNode* node);
-    MOZ_MUST_USE bool emitSequenceExpr(ListNode* node,
+    [[nodiscard]] bool emitUnary(UnaryNode* unaryNode);
+    [[nodiscard]] bool emitRightAssociative(ListNode* node);
+    [[nodiscard]] bool emitLeftAssociative(ListNode* node);
+    [[nodiscard]] bool emitLogical(ListNode* node);
+    [[nodiscard]] bool emitSequenceExpr(ListNode* node,
                                        ValueUsage valueUsage = ValueUsage::WantValue);
 
-    MOZ_NEVER_INLINE MOZ_MUST_USE bool emitIncOrDec(UnaryNode* incDec);
+    [[nodiscard]] MOZ_NEVER_INLINE bool emitIncOrDec(UnaryNode* incDec);
 
-    MOZ_MUST_USE bool emitConditionalExpression(ConditionalExpression& conditional,
+    [[nodiscard]] bool emitConditionalExpression(ConditionalExpression& conditional,
                                                 ValueUsage valueUsage = ValueUsage::WantValue);
 
-    MOZ_MUST_USE bool isRestParameter(ParseNode* pn);
-    MOZ_MUST_USE bool emitOptimizeSpread(ParseNode* arg0, JumpList* jmp, bool* emitted);
+    [[nodiscard]] bool isRestParameter(ParseNode* pn);
+    [[nodiscard]] bool emitOptimizeSpread(ParseNode* arg0, JumpList* jmp, bool* emitted);
 
-    MOZ_MUST_USE ParseNode* getCoordNode(ParseNode* callNode, ParseNode* calleeNode,
+    [[nodiscard]] ParseNode* getCoordNode(ParseNode* callNode, ParseNode* calleeNode,
                                          ListNode* argsList);
-    MOZ_MUST_USE bool emitArguments(ListNode* argsList, bool isCall, bool isSpread,
+    [[nodiscard]] bool emitArguments(ListNode* argsList, bool isCall, bool isSpread,
                                     CallOrNewEmitter& cone);
-    MOZ_MUST_USE bool emitCallOrNew(BinaryNode* pn,
+    [[nodiscard]] bool emitCallOrNew(BinaryNode* pn,
                                     ValueUsage valueUsage = ValueUsage::WantValue);
-    MOZ_MUST_USE bool emitCalleeAndThis(ParseNode* callNode,
+    [[nodiscard]] bool emitCalleeAndThis(ParseNode* callNode,
                                         ParseNode* calleeNode,
                                         CallOrNewEmitter& cone);
-    MOZ_MUST_USE bool emitOptionalCalleeAndThis(ParseNode* callNode,
+    [[nodiscard]] bool emitOptionalCalleeAndThis(ParseNode* callNode,
                                                 ParseNode* calleeNode,
                                                 CallOrNewEmitter& cone,
                                                 OptionalEmitter& oe);
 
-    MOZ_MUST_USE bool emitSelfHostedCallFunction(BinaryNode* callNode);
-    MOZ_MUST_USE bool emitSelfHostedResumeGenerator(BinaryNode* callNode);
-    MOZ_MUST_USE bool emitSelfHostedForceInterpreter(ParseNode* pn);
-    MOZ_MUST_USE bool emitSelfHostedAllowContentIter(BinaryNode* callNode);
+    [[nodiscard]] bool emitSelfHostedCallFunction(BinaryNode* callNode);
+    [[nodiscard]] bool emitSelfHostedResumeGenerator(BinaryNode* callNode);
+    [[nodiscard]] bool emitSelfHostedForceInterpreter(ParseNode* pn);
+    [[nodiscard]] bool emitSelfHostedAllowContentIter(BinaryNode* callNode);
 
-    MOZ_MUST_USE bool emitComprehensionFor(ForNode* forNode);
-    MOZ_MUST_USE bool emitComprehensionForIn(ForNode* forNode);
-    MOZ_MUST_USE bool emitComprehensionForInOrOfVariables(ParseNode* pn, bool* lexicalScope);
-    MOZ_MUST_USE bool emitComprehensionForOf(ForNode* forNode);
+    [[nodiscard]] bool emitComprehensionFor(ForNode* forNode);
+    [[nodiscard]] bool emitComprehensionForIn(ForNode* forNode);
+    [[nodiscard]] bool emitComprehensionForInOrOfVariables(ParseNode* pn, bool* lexicalScope);
+    [[nodiscard]] bool emitComprehensionForOf(ForNode* forNode);
 
-    MOZ_MUST_USE bool emitDo(BinaryNode* doNode);
-    MOZ_MUST_USE bool emitWhile(BinaryNode* whileNode);
+    [[nodiscard]] bool emitDo(BinaryNode* doNode);
+    [[nodiscard]] bool emitWhile(BinaryNode* whileNode);
 
-    MOZ_MUST_USE bool emitFor(ForNode* forNode, const EmitterScope* headLexicalEmitterScope = nullptr);
-    MOZ_MUST_USE bool emitCStyleFor(ForNode* forNode, const EmitterScope* headLexicalEmitterScope);
-    MOZ_MUST_USE bool emitForIn(ForNode* forNode, const EmitterScope* headLexicalEmitterScope);
-    MOZ_MUST_USE bool emitForOf(ForNode* forNode, const EmitterScope* headLexicalEmitterScope);
+    [[nodiscard]] bool emitFor(ForNode* forNode, const EmitterScope* headLexicalEmitterScope = nullptr);
+    [[nodiscard]] bool emitCStyleFor(ForNode* forNode, const EmitterScope* headLexicalEmitterScope);
+    [[nodiscard]] bool emitForIn(ForNode* forNode, const EmitterScope* headLexicalEmitterScope);
+    [[nodiscard]] bool emitForOf(ForNode* forNode, const EmitterScope* headLexicalEmitterScope);
 
-    MOZ_MUST_USE bool emitInitializeForInOrOfTarget(TernaryNode* forHead);
+    [[nodiscard]] bool emitInitializeForInOrOfTarget(TernaryNode* forHead);
 
-    MOZ_MUST_USE bool emitBreak(PropertyName* label);
-    MOZ_MUST_USE bool emitContinue(PropertyName* label);
+    [[nodiscard]] bool emitBreak(PropertyName* label);
+    [[nodiscard]] bool emitContinue(PropertyName* label);
 
-    MOZ_MUST_USE bool emitFunctionFormalParameters(ListNode* paramsBody);
-    MOZ_MUST_USE bool emitInitializeFunctionSpecialNames();
-    MOZ_MUST_USE bool emitLexicalInitialization(NameNode* pn);
-    MOZ_MUST_USE bool emitLexicalInitialization(JSAtom* name);
+    [[nodiscard]] bool emitFunctionFormalParameters(ListNode* paramsBody);
+    [[nodiscard]] bool emitInitializeFunctionSpecialNames();
+    [[nodiscard]] bool emitLexicalInitialization(NameNode* pn);
+    [[nodiscard]] bool emitLexicalInitialization(JSAtom* name);
 
     // Emit bytecode for the spread operator.
     //
@@ -811,9 +811,9 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     // |.next()| and put the results into the I-th element of array with
     // incrementing I, then push the result I (it will be original I +
     // iteration count). The stack after iteration will look like |ARRAY INDEX|.
-    MOZ_MUST_USE bool emitSpread(bool allowSelfHosted = false);
+    [[nodiscard]] bool emitSpread(bool allowSelfHosted = false);
 
-    MOZ_MUST_USE bool emitClass(ClassNode* classNode);
+    [[nodiscard]] bool emitClass(ClassNode* classNode);
 };
 
 class MOZ_RAII AutoCheckUnstableEmitterScope {