Golang の go-cmp で protobuf の struct を Diff する時、 cannot handle unexported field のような panic になる場合は protocmp.Transform() を試してみると良い

https://pkg.go.dev/google.golang.org/protobuf/testing/protocmp より:

The primary feature is the Transform option, which transform proto.Message types into a Message map that is suitable for cmp to introspect upon. All other options in this package must be used in conjunction with Transform.

before

	if diff := cmp.Diff(want, got); diff != "" {
		t.Errorf("mismatch (-want +got):\n%s", diff)
	}

after

	if diff := cmp.Diff(want, got, protocmp.Transform()); diff != "" {
		t.Errorf("mismatch (-want +got):\n%s", diff)
	}

サンプルコードは https://pkg.go.dev/github.com/google/go-cmp/cmp?tab=doc#example-Diff-Testing を参照。