Add proto2::VisitFields() API.

PiperOrigin-RevId: 597398441

src/google/protobuf/extension_set.h

@@ -60,8 +60,10 @@ class Reflection;       // message.h
 class UnknownFieldSet;  // unknown_field_set.h
 class FeatureSet;
 namespace internal {
-class FieldSkipper;  // wire_format_lite.h
+class FieldSkipper;     // wire_format_lite.h
+class ReflectionVisit;  // message_reflection_util.h
 class WireFormat;
+struct DynamicExtensionInfoHelper;
 void InitializeLazyExtensionSet();
 }  // namespace internal
 }  // namespace protobuf
@@ -530,6 +532,8 @@ class PROTOBUF_EXPORT ExtensionSet {
   friend class RepeatedEnumTypeTraits;
 
   friend class google::protobuf::Reflection;
+  friend class google::protobuf::internal::ReflectionVisit;
+  friend struct google::protobuf::internal::DynamicExtensionInfoHelper;
   friend class google::protobuf::internal::WireFormat;
 
   friend void internal::InitializeLazyExtensionSet();

src/google/protobuf/message.h

@@ -138,8 +138,11 @@ class TextFormat;
 namespace internal {
 struct FuzzPeer;
 struct DescriptorTable;
+template <bool is_oneof>
+struct DynamicFieldInfoHelper;
 class MapFieldBase;
 class MessageUtil;
+class ReflectionVisit;
 class SwapFieldHelper;
 class CachedSize;
 struct TailCallTableInfo;
@@ -979,6 +982,7 @@ class PROTOBUF_EXPORT Reflection final {
 
   friend class FastReflectionBase;
   friend class FastReflectionMessageMutator;
+  friend class internal::ReflectionVisit;
   friend bool internal::IsDescendant(Message& root, const Message& message);
 
   const Descriptor* const descriptor_;
@@ -1034,6 +1038,8 @@ class PROTOBUF_EXPORT Reflection final {
   friend class internal::WireFormat;
   friend class internal::ReflectionOps;
   friend class internal::SwapFieldHelper;
+  template <bool is_oneof>
+  friend struct internal::DynamicFieldInfoHelper;
   friend struct internal::FuzzPeer;
   // Needed for implementing text format for map.
   friend class internal::MapFieldPrinterHelper;