Remove confusing 'while checking' note from opaque future type mismatches
Maybe I'm just misinterpreting the wording of the note. The only value I can see in this note is that it points out where the async's opaque future is coming from, but the way it's doing it is misleading IMO.
For example:
```rust
note: while checking the return type of the `async fn`
--> $DIR/dont-suggest-missing-await.rs:7:24
|
LL | async fn make_u32() -> u32 {
| ^^^ checked the `Output` of this `async fn`, found opaque type
```
We point at the type `u32` in the HIR, but then say "found opaque type". We also say "while checking"... but we're typechecking a totally different function when we get this type mismatch!
r? ``@estebank`` but feel free to reassign and/or take your time reviewing this. I'd be inclined to also discuss reworking the presentation of this type mismatch to restore some of these labels in a way that makes it more clear what it's trying to point out.
Don't treat closures from other crates as local
fixes#104817
r? `@lcnr`
Specialization can prefer an impl for an opaque type over a blanket impls that also matches. If the blanket impl only applies if an auto-trait applies, we look at the hidden type of the opaque type to see if that implements the auto trait. The hidden type can be a closure or generator, and thus we will end up seeing these types in coherence and have to handle them properly.
