Breaking change: lock down visibility for FileDescriptor syntax APIs.

These will become meaningless once we migrate to editions.  In the meantime, we've implemented APIs to expose the differences between proto2 and proto3 in terms of the features we plan to release in edition 2023.

PiperOrigin-RevId: 517433756

src/google/protobuf/descriptor.h

@@ -752,10 +752,6 @@ class PROTOBUF_EXPORT FieldDescriptor : private internal::SymbolBase {
   bool is_map() const;       // shorthand for type() == TYPE_MESSAGE &&
                              // message_type()->options().map_entry()
 
-  // Returns true if this field was syntactically written with "optional" in the
-  // .proto file. Excludes singular proto3 fields that do not have a label.
-  bool has_optional_keyword() const;
-
   // Returns true if this field tracks presence, ie. does the field
   // distinguish between "unset" and "present with default value."
   // This includes required, optional, and oneof fields. It excludes maps,
@@ -920,6 +916,11 @@ class PROTOBUF_EXPORT FieldDescriptor : private internal::SymbolBase {
   friend class io::Printer;
   friend class compiler::cpp::Formatter;
   friend class Reflection;
+  friend class FieldDescriptorLegacy;
+
+  // Returns true if this field was syntactically written with "optional" in the
+  // .proto file. Excludes singular proto3 fields that do not have a label.
+  bool has_optional_keyword() const;
 
   // Fill the json_name field of FieldDescriptorProto.
   void CopyJsonNameTo(FieldDescriptorProto* proto) const;
@@ -1040,10 +1041,6 @@ class PROTOBUF_EXPORT OneofDescriptor : private internal::SymbolBase {
   // Index of this oneof within the message's oneof array.
   int index() const;
 
-  // Returns whether this oneof was inserted by the compiler to wrap a proto3
-  // optional field. If this returns true, code generators should *not* emit it.
-  bool is_synthetic() const;
-
   // The .proto file in which this oneof was defined.  Never nullptr.
   const FileDescriptor* file() const;
   // The Descriptor for the message containing this oneof.
@@ -1080,6 +1077,11 @@ class PROTOBUF_EXPORT OneofDescriptor : private internal::SymbolBase {
   // Allows access to GetLocationPath for annotations.
   friend class io::Printer;
   friend class compiler::cpp::Formatter;
+  friend class OneofDescriptorLegacy;
+
+  // Returns whether this oneof was inserted by the compiler to wrap a proto3
+  // optional field. If this returns true, code generators should *not* emit it.
+  bool is_synthetic() const;
 
   // See Descriptor::DebugString().
   void DebugString(int depth, std::string* contents,
@@ -1651,7 +1653,11 @@ class PROTOBUF_EXPORT FileDescriptor : private internal::SymbolBase {
   // descriptor.proto, and any available extensions of that message.
   const FileOptions& options() const;
 
-  // Syntax of this file.
+ private:
+  // With the upcoming release of editions, syntax should not be used for
+  // business logic.  Instead, the various feature helpers defined in this file
+  // should be used to query more targeted behaviors.  For example:
+  // has_presence, is_closed, requires_utf8_validation.
   enum Syntax
 #ifndef SWIG
       : int
@@ -1662,8 +1668,15 @@ class PROTOBUF_EXPORT FileDescriptor : private internal::SymbolBase {
     SYNTAX_PROTO3 = 3,
   };
   Syntax syntax() const;
+
+  // Define a visibility-restricted wrapper for internal use until the migration
+  // is complete.
+  friend class FileDescriptorLegacy;
+
   static const char* SyntaxName(Syntax syntax);
 
+ public:
+
   // Find a top-level message type by name (not full_name).  Returns nullptr if
   // not found.
   const Descriptor* FindMessageTypeByName(absl::string_view name) const;