reflect/protodesc: fix packed field conditional

Before this change, calling dynamicpb.NewMessage() on a
protoreflect.MessageDescriptor that uses Editions would fail with:

message field "[…].relatedqueries" is not packable

(While Go Protobuf does not yet support Editions, C++ programs
can save descriptors to a file that is later loaded by a Go program.)

This was an oversight in commit e8baad6b6c.

Change-Id: I45d2b07a85ee40cb7c028098b81c4e76bf2ff555
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/554896
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cassondra Foesch <cfoesch@gmail.com>
Reviewed-by: Christian Höppner <hoeppi@google.com>
Auto-Submit: Michael Stapelberg <stapelberg@google.com>

reflect/protodesc/desc_init.go

@@ -114,6 +114,27 @@ func (r descsByName) initMessagesDeclarations(mds []*descriptorpb.DescriptorProt
 	return ms, nil
 }
 
+// canBePacked returns whether the field can use packed encoding:
+// https://protobuf.dev/programming-guides/encoding/#packed
+func canBePacked(fd *descriptorpb.FieldDescriptorProto) bool {
+	if fd.GetLabel() != descriptorpb.FieldDescriptorProto_LABEL_REPEATED {
+		return false // not a repeated field
+	}
+
+	switch protoreflect.Kind(fd.GetType()) {
+	case protoreflect.MessageKind, protoreflect.GroupKind:
+		return false // not a scalar type field
+
+	case protoreflect.StringKind, protoreflect.BytesKind:
+		// string and bytes can explicitly not be declared as packed,
+		// see https://protobuf.dev/programming-guides/encoding/#packed
+		return false
+
+	default:
+		return true
+	}
+}
+
 func (r descsByName) initFieldsFromDescriptorProto(fds []*descriptorpb.FieldDescriptorProto, parent protoreflect.Descriptor, sb *strs.Builder) (fs []filedesc.Field, err error) {
 	fs = make([]filedesc.Field, len(fds)) // allocate up-front to ensure stable pointers
 	for i, fd := range fds {
@@ -142,7 +163,7 @@ func (r descsByName) initFieldsFromDescriptorProto(fds []*descriptorpb.FieldDesc
 			f.L1.Presence = resolveFeatureHasFieldPresence(f.Base.L0.ParentFile, fd)
 			// We reuse the existing field because the old option `[packed =
 			// true]` is mutually exclusive with the editions feature.
-			if fd.GetLabel() == descriptorpb.FieldDescriptorProto_LABEL_REPEATED {
+			if canBePacked(fd) {
 				f.L1.HasPacked = true
 				f.L1.IsPacked = resolveFeatureRepeatedFieldEncodingPacked(f.Base.L0.ParentFile, fd)
 			}