Problem: these seem to be the wrong substs.  Maybe I'm getting the formal paramters of the enum instead of the actual parameters of the use?  Do I need to actually resolve the type here?

Alternately, I could just allow any non-C-like enums (ones where any variant has a non-zero number of fields), because those are less likely to be used accidentally, and file an issue to tighten up the warning in the future.  Thoughts?