Warnings for issue #32330
This is an extension of the previous PR that issues warnings in more situations than before. It does not handle *all* cases of #32330 but I believe it issues warnings for all cases I've seen in practice.
Before merging I'd like to address:
- open a good issue explaining the problem and how to fix it (I have a [draft writeup][])
- work on the error message, which I think is not as clear as it could/should be (suggestions welcome)
r? @aturon
[draft writeup]: https://gist.github.com/nikomatsakis/631ec8b4af9a18b5d062d9d9b7d3d967
75e23e1b03