Allocate FieldArenaRep type in Arena::DefaultConstruct when using arenas.

This change is prerequisite to supporting split fields with removed arena pointers. Split field types which have removed arena pointers have a larger representation on the arena which holds a copy of `InternalMetadata` from the parent message, since they need to be able to find a nearby `InternalMetadata` within a 32-bit offset. This is exactly how directly arena-allocated field types with arena offsets will work, but this logic is currently only implemented in `DoCreateMessage`. We will do the same trick here to construct this larger type when allocating in `Arena::DefaultConstruct`, which is the API split fields use.

PiperOrigin-RevId: 814896561

Author: ClaytonKnittel

src/google/protobuf/arena.h

@@ -741,18 +741,31 @@ class PROTOBUF_EXPORT PROTOBUF_ALIGNAS(8)
 template <typename T>
 PROTOBUF_NOINLINE void* PROTOBUF_NONNULL
 Arena::DefaultConstruct(Arena* PROTOBUF_NULLABLE arena) {
-  static_assert(is_destructor_skippable<T>::value, "");
-  void* mem;
   if (arena != nullptr) {
-    mem = arena->AllocateAligned(sizeof(T));
+    using ArenaRepT = typename internal::FieldArenaRep<T>::Type;
+    static_assert(is_destructor_skippable<ArenaRepT>::value, "");
+
+    void* mem = arena->AllocateAligned(sizeof(ArenaRepT));
+    ArenaRepT* arena_rep = new (mem) ArenaRepT(arena);
+    return internal::FieldArenaRep<T>::Get(arena_rep);
   } else {
+    static_assert(is_destructor_skippable<T>::value, "");
+
 #if ABSL_HAVE_BUILTIN(__builtin_operator_new)
-    mem = __builtin_operator_new(sizeof(T));
+    void* mem = __builtin_operator_new(sizeof(T));
 #else
-    mem = ::operator new(sizeof(T));
+    void* mem = ::operator new(sizeof(T));
 #endif
+
+    // Fields which use arena offsets don't have constructors that take an arena
+    // pointer. Since the arena is nullptr, it is safe to default construct the
+    // object.
+    if constexpr (internal::FieldHasArenaOffset<T>()) {
+      return new (mem) T();
+    } else {
+      return new (mem) T(nullptr);
+    }
   }
-  return new (mem) T(arena);
 }
 
 template <typename T>