internal/impl: expose MessageInfo

The v2 MessageInfo is needed by v1 to be able to access the OneofWrappers
and Exporter function. The ProtoMessageInfo method can be deleted
once v1 is entirely implemented in terms of v2.

Change-Id: Iabb1b429af5210faffc6477f52b5020b3aa1fb50
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/186577
Reviewed-by: Damien Neil <dneil@google.com>

internal/cmd/generate-types/impl.go

@@ -591,6 +591,15 @@ func (m *{{.}}) ProtoMethods() *protoiface.Methods {
 	return &m.mi.methods
 }
 
+// ProtoMessageInfo is a pseudo-internal API for allowing the v1 code
+// to be able to retrieve a v2 MessageInfo struct.
+//
+// WARNING: This method is exempt from the compatibility promise and
+// may be removed in the future without warning.
+func (m *{{.}}) ProtoMessageInfo() *MessageInfo {
+	return m.mi
+}
+
 func (m *{{.}}) Range(f func(protoreflect.FieldDescriptor, protoreflect.Value) bool) {
 	m.mi.init()
 	for _, fi := range m.mi.fields {

internal/impl/message_reflect_gen.go

@@ -31,6 +31,15 @@ func (m *messageState) ProtoMethods() *protoiface.Methods {
 	return &m.mi.methods
 }
 
+// ProtoMessageInfo is a pseudo-internal API for allowing the v1 code
+// to be able to retrieve a v2 MessageInfo struct.
+//
+// WARNING: This method is exempt from the compatibility promise and
+// may be removed in the future without warning.
+func (m *messageState) ProtoMessageInfo() *MessageInfo {
+	return m.mi
+}
+
 func (m *messageState) Range(f func(protoreflect.FieldDescriptor, protoreflect.Value) bool) {
 	m.mi.init()
 	for _, fi := range m.mi.fields {
@@ -129,6 +138,15 @@ func (m *messageReflectWrapper) ProtoMethods() *protoiface.Methods {
 	return &m.mi.methods
 }
 
+// ProtoMessageInfo is a pseudo-internal API for allowing the v1 code
+// to be able to retrieve a v2 MessageInfo struct.
+//
+// WARNING: This method is exempt from the compatibility promise and
+// may be removed in the future without warning.
+func (m *messageReflectWrapper) ProtoMessageInfo() *MessageInfo {
+	return m.mi
+}
+
 func (m *messageReflectWrapper) Range(f func(protoreflect.FieldDescriptor, protoreflect.Value) bool) {
 	m.mi.init()
 	for _, fi := range m.mi.fields {