Issue #2166 - Part 4: Use alignas/alignof to define Variant's internal raw storage

Based on https://bugzilla.mozilla.org/show_bug.cgi?id=1338374

mfbt/Variant.h

@@ -8,9 +8,9 @@
 #include <new>
 #include <stdint.h>
 
-#include "mozilla/Alignment.h"
 #include "mozilla/Assertions.h"
 #include "mozilla/Move.h"
+#include "mozilla/TemplateLib.h"
 #include "mozilla/TypeTraits.h"
 
 #ifndef mozilla_Variant_h
@@ -23,22 +23,6 @@ class Variant;
 
 namespace detail {
 
-// MaxSizeOf computes the maximum sizeof(T) for each T in Ts.
-
-template<typename T, typename... Ts>
-struct MaxSizeOf
-{
-  static const size_t size = sizeof(T) > MaxSizeOf<Ts...>::size
-    ? sizeof(T)
-    : MaxSizeOf<Ts...>::size;
-};
-
-template<typename T>
-struct MaxSizeOf<T>
-{
-  static const size_t size = sizeof(T);
-};
-
 // The `IsVariant` helper is used in conjunction with static_assert and
 // `mozilla::EnableIf` to catch passing non-variant types to `Variant::is<T>()`
 // and friends at compile time, rather than at runtime. It ensures that the
@@ -429,23 +413,35 @@ struct AsVariantTemporary
  *
  *       ...
  *     };
+ *
+ * Because Variant must be aligned suitable to hold any value stored within it,
+ * and because |alignas| requirements don't affect platform ABI with respect to
+ * how parameters are laid out in memory, Variant can't be used as the type of a
+ * function parameter.  Pass Variant to functions by pointer or reference
+ * instead.
  */
 template<typename... Ts>
-class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Variant
+class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS MOZ_NON_PARAM Variant
 {
   using Tag = typename detail::VariantTag<Ts...>::Type;
   using Impl = detail::VariantImplementation<Tag, 0, Ts...>;
-  using RawData = AlignedStorage<detail::MaxSizeOf<Ts...>::size>;
+
+  static const size_t RawDataAlignment = tl::Max<alignof(Ts)...>::value;
+  static const size_t RawDataSize = tl::Max<sizeof(Ts)...>::value;
 
   // Raw storage for the contained variant value.
-  RawData raw;
+  alignas(RawDataAlignment) unsigned char rawData[RawDataSize];
 
   // Each type is given a unique tag value that lets us keep track of the
   // contained variant value's type.
   Tag tag;
 
   void* ptr() {
-    return reinterpret_cast<void*>(&raw);
+    return rawData;
+  }
+
+  const void* ptr() const {
+    return rawData;
   }
 
 public:
@@ -552,7 +548,7 @@ public:
     static_assert(detail::IsVariant<T, Ts...>::value,
                   "provided a type not found in this Variant's type list");
     MOZ_ASSERT(is<T>());
-    return *reinterpret_cast<T*>(&raw);
+    return *static_cast<T*>(ptr());
   }
 
   /** Immutable const reference. */
@@ -561,7 +557,7 @@ public:
     static_assert(detail::IsVariant<T, Ts...>::value,
                   "provided a type not found in this Variant's type list");
     MOZ_ASSERT(is<T>());
-    return *reinterpret_cast<const T*>(&raw);
+    return *static_cast<const T*>(ptr());
   }
 
   /**