Bug 1572205 - Use a single-member union as the storage for Maybe r=jwalden

My personal motivation for doing this is to stop the compiler from inserting stack protectors into tons of functions that wouldn't otherwise have them (arrays on the stack are one of the triggers for stack protection). But others see different benefits, such as being able to remove the `MOZ_NON_PARAM` annotation. Also this is the approach recommended by https://github.com/CppCon/CppCon2019/tree/master/Presentations/how_to_hold_a_t.

Differential Revision: https://phabricator.services.mozilla.com/D58995

--HG--
extra : moz-landing-system : lando

mfbt/Maybe.h

@@ -154,16 +154,16 @@ struct MaybePoisoner {
  *     functions |Some()| and |Nothing()|.
  */
 template <class T>
-class MOZ_NON_PARAM MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Maybe {
-  MOZ_ALIGNAS_IN_STRUCT(T) unsigned char mStorage[sizeof(T)];
+class MOZ_INHERIT_TYPE_ANNOTATIONS_FROM_TEMPLATE_ARGS Maybe {
+  using NonConstT = typename RemoveConst<T>::Type;
+  union Union {
+    Union() {}
+    ~Union() {}
+    NonConstT val;
+  } mStorage;
   char mIsSome;  // not bool -- guarantees minimal space consumption
 
-  // GCC fails due to -Werror=strict-aliasing if |mStorage| is directly cast to
-  // T*.  Indirecting through these functions addresses the problem.
-  void* data() { return mStorage; }
-  const void* data() const { return mStorage; }
-
-  void poisonData() { detail::MaybePoisoner<T>::poison(data()); }
+  void poisonData() { detail::MaybePoisoner<T>::poison(&mStorage.val); }
 
  public:
   using ValueType = T;
@@ -498,13 +498,13 @@ const T* Maybe<T>::operator->() const {
 template <typename T>
 T& Maybe<T>::ref() {
   MOZ_DIAGNOSTIC_ASSERT(mIsSome);
-  return *static_cast<T*>(data());
+  return mStorage.val;
 }
 
 template <typename T>
 const T& Maybe<T>::ref() const {
   MOZ_DIAGNOSTIC_ASSERT(mIsSome);
-  return *static_cast<const T*>(data());
+  return mStorage.val;
 }
 
 template <typename T>
@@ -523,7 +523,7 @@ template <typename T>
 template <typename... Args>
 void Maybe<T>::emplace(Args&&... aArgs) {
   MOZ_DIAGNOSTIC_ASSERT(!mIsSome);
-  ::new (KnownNotNull, data()) T(std::forward<Args>(aArgs)...);
+  ::new (KnownNotNull, &mStorage.val) T(std::forward<Args>(aArgs)...);
   mIsSome = true;
 }