Addressed PR comments.

upb/msg_test.cc

@@ -60,13 +60,13 @@ TEST(MessageTest, Extensions) {
   upb::MessageDefPtr m(upb_test_TestExtensions_getmsgdef(symtab.ptr()));
   EXPECT_TRUE(m.ptr() != nullptr);
 
-  std::string json = R"(
+  std::string json = R"json(
   {
       "[upb_test.TestExtensions.optional_int32_ext]": 123,
       "[upb_test.TestExtensions.Nested.repeated_int32_ext]": [2, 4, 6],
       "[upb_test.optional_msg_ext]": {"optional_int32": 456}
   }
-  )";
+  )json";
   upb::Status status;
   EXPECT_TRUE(upb_json_decode(json.data(), json.size(), ext_msg, m.ptr(),
                               symtab.ptr(), 0, arena.ptr(), status.ptr()))
@@ -98,4 +98,3 @@ TEST(MessageTest, Extensions) {
       << status.error_message();
   VerifyMessage(ext_msg3);
 }
-

upbc/protoc-gen-upb.cc

@@ -1172,18 +1172,16 @@ int WriteExtensions(const protobuf::FileDescriptor* file, Output& output) {
 
   // Order by full name for consistent ordering.
   std::map<std::string, const protobuf::Descriptor*> forward_messages;
-  auto add_forward_decl = [&](const protobuf::Descriptor* d) {
-    forward_messages[d->full_name()] = d;
-  };
 
   for (auto ext : exts) {
-    add_forward_decl(ext->containing_type());
+    forward_messages[ext->containing_type()->full_name()] =
+        ext->containing_type();
     if (ext->message_type()) {
-      add_forward_decl(ext->message_type());
+      forward_messages[ext->message_type()->full_name()] = ext->message_type();
     }
   }
 
-  for (auto decl : forward_messages) {
+  for (const auto& decl : forward_messages) {
     output("extern const upb_msglayout $0;\n", MessageInit(decl.second));
   }