Detect proto.Message types when failing to export a field (#370)
While we do not provide 1st class handling of protobuf messages,
we can at least detect whether the type is a proto.Message
and refer the user to the correct package to use to handle
protobuf messages with cmp.Equal.
diff --git a/cmp/options.go b/cmp/options.go
index 754496f..ba3fce8 100644
--- a/cmp/options.go
+++ b/cmp/options.go
@@ -232,7 +232,15 @@
if t := s.curPath.Index(-2).Type(); t.Name() != "" {
// Named type with unexported fields.
name = fmt.Sprintf("%q.%v", t.PkgPath(), t.Name()) // e.g., "path/to/package".MyType
- if _, ok := reflect.New(t).Interface().(error); ok {
+ isProtoMessage := func(t reflect.Type) bool {
+ m, ok := reflect.PointerTo(t).MethodByName("ProtoReflect")
+ return ok && m.Type.NumIn() == 1 && m.Type.NumOut() == 1 &&
+ m.Type.Out(0).PkgPath() == "google.golang.org/protobuf/reflect/protoreflect" &&
+ m.Type.Out(0).Name() == "Message"
+ }
+ if isProtoMessage(t) {
+ help = `consider using "google.golang.org/protobuf/testing/protocmp".Transform to compare proto.Message types`
+ } else if _, ok := reflect.New(t).Interface().(error); ok {
help = "consider using cmpopts.EquateErrors to compare error values"
} else if t.Comparable() {
help = "consider using cmpopts.EquateComparable to compare comparable Go types"
