Add proto hooks for unknown fields.

PiperOrigin-RevId: 531306472

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

@@ -1302,14 +1302,18 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
       "}\n"
       "\n");
 
-  format(
-      "inline const $unknown_fields_type$& unknown_fields() const {\n"
-      "  return $unknown_fields$;\n"
-      "}\n"
-      "inline $unknown_fields_type$* mutable_unknown_fields() {\n"
-      "  return $mutable_unknown_fields$;\n"
-      "}\n"
-      "\n");
+  p->Emit(R"cc(
+    inline const $unknown_fields_type$& unknown_fields() const {
+      $annotate_unknown_fields$;
+      return $unknown_fields$;
+    }
+    inline $unknown_fields_type$* mutable_unknown_fields() {
+      $annotate_mutable_unknown_fields$;
+      return $mutable_unknown_fields$;
+    }
+  )cc");
+  // Adding a blank line to be consistent with the previous version.
+  p->Emit("\n");
 
   // Only generate this member if it's not disabled.
   if (HasDescriptorMethods(descriptor_->file(), options_) &&

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

@@ -180,6 +180,8 @@ std::vector<Sub> MakeTrackerCalls(const Descriptor* message,
           Call("reflection", "OnGetMetadata").This(absl::nullopt),
           Call("bytesize", "OnByteSize"),
           Call("mergefrom", "OnMergeFrom").This("_this").Arg("&from"),
+          Call("unknown_fields", "OnUnknownFields"),
+          Call("mutable_unknown_fields", "OnMutableUnknownFields"),
 
           // "Has" is here as users calling "has" on a repeated field is a
           // mistake.

src/google/protobuf/field_access_listener.h

@@ -128,6 +128,12 @@ struct NoOpAccessListener {
   template <int kFieldNum>
   static void OnSize(const MessageLite* msg, const void* field) {}
 
+  // unknown_fields()
+  static void OnUnknownFields(const MessageLite* msg) {}
+
+  // mutable_unknown_fields()
+  static void OnMutableUnknownFields(const MessageLite* msg) {}
+
   static void OnHasExtension(const MessageLite* msg, int extension_tag,
                              const void* field) {}
   // TODO(b/190614678): Support clear in the proto compiler.