protoadapt: helper functions to convert v1 or v2 message to either v1…

… or v2 message.

Fixes golang/protobuf#1442

Change-Id: I7ec60982be81d5dc060094ccec51d819b17a3a8f
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/449576
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Lasse Folger <lassefolger@google.com>

protoadapt/convert.go

@@ -0,0 +1,31 @@
+// Copyright 2023 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package protoadapt bridges the original and new proto APIs.
+package protoadapt
+
+import (
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/runtime/protoiface"
+	"google.golang.org/protobuf/runtime/protoimpl"
+)
+
+// MessageV1 is the original "github.com/golang/protobuf/proto".Message type.
+type MessageV1 = protoiface.MessageV1
+
+// MessageV2 is the Message type used by the current google.golang.org/protobuf
+// module, adding support for reflection.
+type MessageV2 = proto.Message
+
+// MessageV1Of converts a v2 message to a v1 message.
+// It returns nil if m is nil.
+func MessageV1Of(m MessageV2) protoiface.MessageV1 {
+	return protoimpl.X.ProtoMessageV1Of(m)
+}
+
+// MessageV2Of converts a v1 message to a v2 message.
+// It returns nil if m is nil.
+func MessageV2Of(m MessageV1) proto.Message {
+	return protoimpl.X.ProtoMessageV2Of(m)
+}