Move unknown_field_set.h to protobuf_lite to allow direct usage from the parser without a virtual dispatch.

src/google/protobuf/BUILD.bazel

@@ -712,6 +712,7 @@ cc_library(
         "raw_ptr.cc",
         "repeated_field.cc",
         "repeated_ptr_field.cc",
+        "unknown_field_set_lite.cc",
         "wire_format_lite.cc",
     ],
     # TODO Fix ODR violations across BUILD.bazel files.
@@ -747,6 +748,7 @@ cc_library(
         "runtime_version.h",
         "serial_arena.h",
         "thread_safe_arena.h",
+        "unknown_field_set.h",
         "wire_format_lite.h",
     ],
     copts = COPTS + select({

src/google/protobuf/generated_message_tctable_full.cc

@@ -48,16 +48,10 @@ namespace protobuf {
 namespace internal {
 
 const char* TcParser::GenericFallback(PROTOBUF_TC_PARAM_DECL) {
-  PROTOBUF_MUSTTAIL return GenericFallbackImpl<Message, UnknownFieldSet>(
-      PROTOBUF_TC_PARAM_PASS);
+  PROTOBUF_MUSTTAIL return GenericFallbackImpl<Message>(PROTOBUF_TC_PARAM_PASS);
 }
 
 const char* TcParser::ReflectionFallback(PROTOBUF_TC_PARAM_DECL) {
-  bool must_fallback_to_generic = (ptr == nullptr);
-  if (ABSL_PREDICT_FALSE(must_fallback_to_generic)) {
-    PROTOBUF_MUSTTAIL return GenericFallback(PROTOBUF_TC_PARAM_PASS);
-  }
-
   SyncHasbits(msg, hasbits, table);
   uint32_t tag = data.tag();
   if (tag == 0 || (tag & 7) == WireFormatLite::WIRETYPE_END_GROUP) {

src/google/protobuf/generated_message_tctable_gen.cc

@@ -868,7 +868,10 @@ TailCallTableInfo::TailCallTableInfo(
       // Reflection and weak messages have the reflection fallback
       : !message_options.uses_codegen || HasWeakFields(descriptor)
           ? TcParseFunction::kReflectionFallback
-      // Codegen messages have lite and non-lite version
+      // Messages without extensions fallback directly to MpUnknownFields
+      : descriptor->extension_range_count() == 0
+          ? TcParseFunction::kMpUnknownFields
+      // Codegen messages with extensions have lite and non-lite version
       : message_options.is_lite ? TcParseFunction::kGenericFallbackLite
                                 : TcParseFunction::kGenericFallback;