Migrate syntax reflection to new feature-based system.

As part of go/protobuf-editions, we will be abandoning `syntax` in place of the new editions framework.  The first edition will turn each difference between proto2 and proto3 into a more targeted *feature* placed on proto descriptors.  In preparation for this, we want to migrate everyone off direct inspection of `syntax`.

PiperOrigin-RevId: 515457103

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

@@ -1051,7 +1051,7 @@ void ParseFunctionGenerator::GenerateLengthDelim(Formatter& format,
           const FieldDescriptor* val = field->message_type()->map_value();
           ABSL_CHECK(val);
           if (val->type() == FieldDescriptor::TYPE_ENUM &&
-              !internal::cpp::HasPreservingUnknownEnumSemantics(field)) {
+              !internal::cpp::HasPreservingUnknownEnumSemantics(val)) {
             format(
                 "auto object = "
                 "::$proto_ns$::internal::InitEnumParseWrapper<"

src/google/protobuf/descriptor.cc

@@ -3521,6 +3521,11 @@ bool FieldDescriptor::is_packed() const {
   }
 }
 
+bool FieldDescriptor::requires_utf8_validation() const {
+  return type() == TYPE_STRING &&
+         file()->syntax() == FileDescriptor::SYNTAX_PROTO3;
+}
+
 bool Descriptor::GetSourceLocation(SourceLocation* out_location) const {
   std::vector<int> path;
   GetLocationPath(&path);
@@ -8415,7 +8420,7 @@ void LazyDescriptor::Once(const ServiceDescriptor* service) {
 
 namespace cpp {
 bool HasPreservingUnknownEnumSemantics(const FieldDescriptor* field) {
-  return field->file()->syntax() == FileDescriptor::SYNTAX_PROTO3;
+  return !field->legacy_enum_field_treated_as_closed();
 }
 
 bool HasHasbit(const FieldDescriptor* field) {

src/google/protobuf/descriptor.h

@@ -2457,11 +2457,6 @@ inline bool FieldDescriptor::has_presence() const {
          file()->syntax() == FileDescriptor::SYNTAX_PROTO2;
 }
 
-inline bool FieldDescriptor::requires_utf8_validation() const {
-  return type() == TYPE_STRING &&
-         file()->syntax() == FileDescriptor::SYNTAX_PROTO3;
-}
-
 inline bool FieldDescriptor::legacy_enum_field_treated_as_closed() const {
   return type() == TYPE_ENUM &&
          file()->syntax() == FileDescriptor::SYNTAX_PROTO2;

src/google/protobuf/generated_message_reflection.cc

@@ -1551,7 +1551,7 @@ void CheckInOrder(const FieldDescriptor* field, uint32_t* last) {
 namespace internal {
 bool CreateUnknownEnumValues(const FieldDescriptor* field) {
   bool open_enum = false;
-  return field->file()->syntax() == FileDescriptor::SYNTAX_PROTO3 || open_enum;
+  return !field->legacy_enum_field_treated_as_closed() || open_enum;
 }
 }  // namespace internal
 using internal::CreateUnknownEnumValues;

src/google/protobuf/wire_format.cc

@@ -411,10 +411,6 @@ bool WireFormat::ParseAndMergeMessageSetField(uint32_t field_number,
   }
 }
 
-static bool StrictUtf8Check(const FieldDescriptor* field) {
-  return field->file()->syntax() == FileDescriptor::SYNTAX_PROTO3;
-}
-
 bool WireFormat::ParseAndMergeField(
     uint32_t tag,
     const FieldDescriptor* field,  // May be nullptr for unknown
@@ -484,8 +480,7 @@ bool WireFormat::ParseAndMergeField(
           if (!WireFormatLite::ReadPrimitive<int, WireFormatLite::TYPE_ENUM>(
                   input, &value))
             return false;
-          if (message->GetDescriptor()->file()->syntax() ==
-              FileDescriptor::SYNTAX_PROTO3) {
+          if (!field->legacy_enum_field_treated_as_closed()) {
             message_reflection->AddEnumValue(message, field, value);
           } else {
             const EnumValueDescriptor* enum_value =
@@ -566,7 +561,7 @@ bool WireFormat::ParseAndMergeField(
 
       // Handle strings separately so that we can optimize the ctype=CORD case.
       case FieldDescriptor::TYPE_STRING: {
-        bool strict_utf8_check = StrictUtf8Check(field);
+        bool strict_utf8_check = field->requires_utf8_validation();
         std::string value;
         if (!WireFormatLite::ReadString(input, &value)) return false;
         if (strict_utf8_check) {
@@ -868,9 +863,7 @@ const char* WireFormat::_InternalParseAndMergeField(
         case FieldDescriptor::TYPE_ENUM: {
           auto rep_enum =
               reflection->MutableRepeatedFieldInternal<int>(msg, field);
-          bool open_enum = false;
-          if (field->file()->syntax() == FileDescriptor::SYNTAX_PROTO3 ||
-              open_enum) {
+          if (!field->legacy_enum_field_treated_as_closed()) {
             ptr = internal::PackedEnumParser(rep_enum, ptr, ctx);
           } else {
             return ctx->ReadPackedVarint(
@@ -982,7 +975,7 @@ const char* WireFormat::_InternalParseAndMergeField(
     // Handle strings separately so that we can optimize the ctype=CORD case.
     case FieldDescriptor::TYPE_STRING:
       utf8_check = true;
-      strict_utf8_check = StrictUtf8Check(field);
+      strict_utf8_check = field->requires_utf8_validation();
       PROTOBUF_FALLTHROUGH_INTENDED;
     case FieldDescriptor::TYPE_BYTES: {
       int size = ReadSize(&ptr);
@@ -1418,7 +1411,7 @@ uint8_t* WireFormat::InternalSerializeField(const FieldDescriptor* field,
       // Handle strings separately so that we can get string references
       // instead of copying.
       case FieldDescriptor::TYPE_STRING: {
-        bool strict_utf8_check = StrictUtf8Check(field);
+        bool strict_utf8_check = field->requires_utf8_validation();
         std::string scratch;
         const std::string& value =
             field->is_repeated()