While going over nominated issues, I encountered two that I had trouble categorizing:
The problem here is mostly that for a type like
[T] to be considered “well-formed”, we do not require that
T: Sized, which permits types like
[[T]] that are not currently resolvable. This is bad because we accept things we probably should not, but on the other hand there isn’t much you can do with these types, so the risk of real problems seems low. This feels related to arielb1’s PR https://github.com/rust-lang/rust/pull/25701 as well, which merits a separate discussion thread. I opted for medium priority since I’m not sure how people will come to depend on this. But we should address the overall issue ASAP nonetheless.
The problem here is a hole in macro forward compatibility. If we are going to fix this, it feels like we should do it ASAP, because I can easily imagine people coming to rely on it. Thus I assigned it P-high (at least to make a decision).