Streamline creation of new elements in RepeatedPtrField

The logic to create new elements in `RepeatedPtrField` was spread across an inconsistent patchwork of specializations of individual `GenericTypeHandler` static functions as well as external member and non-member functions, all living in multiple sources. Now all that logic is concentrated in three `GenericTypeHandler` full class specializations.

One change that to carefully verify in postsubmit is the now-removed old workaround for an MSVC bug #254 (MSVC compatibility is not checked in presubmit).

PiperOrigin-RevId: 670689695

src/google/protobuf/arena.h

@@ -87,6 +87,17 @@ template <typename T>
 void arena_delete_object(void* object) {
   delete reinterpret_cast<T*>(object);
 }
+
+inline bool CanUseInternalSwap(Arena* lhs, Arena* rhs) {
+  if (DebugHardenForceCopyInSwap()) {
+    // We force copy in swap when we are not using an arena.
+    // If we did with an arena we would grow arena usage too much.
+    return lhs != nullptr && lhs == rhs;
+  } else {
+    return lhs == rhs;
+  }
+}
+
 }  // namespace internal
 
 // ArenaOptions provides optional additional parameters to arena construction

src/google/protobuf/arenastring.h

@@ -503,22 +503,22 @@ inline PROTOBUF_NDEBUG_INLINE void ArenaStringPtr::InternalSwap(
   // Silence unused variable warnings in release buildls.
   (void)arena;
   std::swap(lhs->tagged_ptr_, rhs->tagged_ptr_);
-#ifdef PROTOBUF_FORCE_COPY_IN_SWAP
-  for (auto* p : {lhs, rhs}) {
-    if (p->IsDefault()) continue;
-    std::string* old_value = p->tagged_ptr_.Get();
-    std::string* new_value =
-        p->IsFixedSizeArena()
-            ? Arena::Create<std::string>(arena, *old_value)
-            : Arena::Create<std::string>(arena, std::move(*old_value));
-    if (arena == nullptr) {
-      delete old_value;
-      p->tagged_ptr_.SetAllocated(new_value);
-    } else {
-      p->tagged_ptr_.SetMutableArena(new_value);
+  if (internal::DebugHardenForceCopyInSwap()) {
+    for (auto* p : {lhs, rhs}) {
+      if (p->IsDefault()) continue;
+      std::string* old_value = p->tagged_ptr_.Get();
+      std::string* new_value =
+          p->IsFixedSizeArena()
+              ? Arena::Create<std::string>(arena, *old_value)
+              : Arena::Create<std::string>(arena, std::move(*old_value));
+      if (arena == nullptr) {
+        delete old_value;
+        p->tagged_ptr_.SetAllocated(new_value);
+      } else {
+        p->tagged_ptr_.SetMutableArena(new_value);
+      }
     }
   }
-#endif  // PROTOBUF_FORCE_COPY_IN_SWAP
 }
 
 inline void ArenaStringPtr::ClearNonDefaultToEmpty() {

src/google/protobuf/compiler/cpp/message.cc

@@ -2126,11 +2126,7 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
           friend void swap($classname$& a, $classname$& b) { a.Swap(&b); }
           inline void Swap($classname$* other) {
             if (other == this) return;
-#ifdef PROTOBUF_FORCE_COPY_IN_SWAP
-            if (GetArena() != nullptr && GetArena() == other->GetArena()) {
-#else   // PROTOBUF_FORCE_COPY_IN_SWAP
-            if (GetArena() == other->GetArena()) {
-#endif  // !PROTOBUF_FORCE_COPY_IN_SWAP
+            if ($pbi$::CanUseInternalSwap(GetArena(), other->GetArena())) {
               InternalSwap(other);
             } else {
               $pbi$::GenericSwap(this, other);