Do not emit gencode for map fields

They are currently unsupported. Up until now we've been generating gencode
assuming that map fields are standard message fields, and the gencode was
expecting thunks such as _serialize to exist, but they don't.

In some toolchains this becomes a linking problem.

PiperOrigin-RevId: 541572265

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

@@ -219,6 +219,10 @@ MessageGenerator::MessageGenerator(Context<Descriptor> msg) {
 }
 
 void MessageGenerator::GenerateRs(Context<Descriptor> msg) {
+  if (msg.desc().map_key() != nullptr) {
+    ABSL_LOG(WARNING) << "unsupported map field: " << msg.desc().full_name();
+    return;
+  }
   msg.Emit(
       {
           {"Msg", msg.desc().name()},
@@ -338,6 +342,10 @@ void MessageGenerator::GenerateRs(Context<Descriptor> msg) {
 // Generates code for a particular message in `.pb.thunk.cc`.
 void MessageGenerator::GenerateThunksCc(Context<Descriptor> msg) {
   ABSL_CHECK(msg.is_cpp());
+  if (msg.desc().map_key() != nullptr) {
+    ABSL_LOG(WARNING) << "unsupported map field: " << msg.desc().full_name();
+    return;
+  }
 
   msg.Emit(
       {