Better diagnostics with mismatched types due to implicit static lifetime
Fixes#78113
I think this is my first diagnostics PR...definitely happy to hear thoughts on the direction/implementation here.
I was originally just trying to solve the error above, where the lifetime on a GAT was causing a cryptic "mismatched types" error. But as I was writing this, I realized that this (unintentionally) also applied to a different case: `wf-in-foreign-fn-decls-issue-80468.rs`. I'm not sure if this diagnostic should get a new error code, or even reuse an existing one. And, there might be some ways to make this even more generalized. Also, the error is a bit more lengthy and verbose than probably needed. So thoughts there are welcome too.
This PR essentially ended up adding a new nice region error pass that triggers if a type doesn't match the self type of an impl which is selected because of a predicate because of an implicit static bound on that self type.
r? `@estebank`
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