Add message type traits that can be declared in duplicate so that .pr…

…oto.h can

add the traits for their dependencies.

Move StrongPointerToType to the new traits and remove the old codegen for it.

We will extend these traits to contain more information.

PiperOrigin-RevId: 676575099

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

@@ -1460,20 +1460,35 @@ class FileGenerator::ForwardDeclarations {
     if (ShouldGenerateExternSpecializations(options)) {
       for (const auto& c : classes_) {
         if (!ShouldGenerateClass(c.second, options)) continue;
+        auto vars = p->WithVars(
+            {{"class", QualifiedClassName(c.second, options)},
+             {"default_name", QualifiedDefaultInstanceName(c.second, options,
+                                                           /*split=*/false)}});
         // To reduce total linker input size in large binaries we make these
         // functions extern and define then in the pb.cc file. This avoids bloat
         // in callers by having duplicate definitions of the template.
         // However, it increases the size of the pb.cc translation units so it
         // is a tradeoff.
-        p->Emit({{"class", QualifiedClassName(c.second, options)}}, R"cc(
+        p->Emit(R"cc(
           extern template void* Arena::DefaultConstruct<$class$>(Arena*);
         )cc");
         if (!IsMapEntryMessage(c.second)) {
-          p->Emit({{"class", QualifiedClassName(c.second, options)}}, R"cc(
+          p->Emit(R"cc(
             extern template void* Arena::CopyConstruct<$class$>(Arena*,
                                                                 const void*);
           )cc");
         }
+        // We can't make a constexpr pointer to the global if we have DLL
+        // linkage so skip this.
+        // The fallback traits are slower, but correct.
+        if (options.dllexport_decl.empty()) {
+          p->Emit(R"cc(
+            template <>
+            internal::GeneratedMessageTraitsT<decltype($default_name$),
+                                              &$default_name$>
+                internal::MessageTraitsImpl::value<$class$>;
+          )cc");
+        }
       }
     }
   }

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

@@ -2041,18 +2041,6 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
                                                $pbi$::ParseContext* ctx);
           )cc");
         }},
-       {"raw_default_instance",
-        [&] {
-          // We can't make a constexpr pointer to the global if we have DLL
-          // linkage so skip this. The fallback in
-          // `MessageLite::GetStrongPointerForType` will do the right thing in
-          // those platforms.
-          if (!options_.dllexport_decl.empty()) return;
-          p->Emit(R"cc(
-            static constexpr const void* _raw_default_instance_ =
-                &_$classname$_default_instance_;
-          )cc");
-        }},
        {"decl_impl", [&] { GenerateImplDefinition(p); }},
        {"classdata_type",
         HasDescriptorMethods(descriptor_->file(), options_)
@@ -2210,8 +2198,6 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
           $decl_data$;
           $post_loop_handler$;
 
-          $raw_default_instance$;
-
           friend class ::$proto_ns$::MessageLite;
           friend class ::$proto_ns$::Arena;
           template <typename T>