min_type_alias_impl_trait: poor diagnostics when TAIT is unconstrained

[nikomatsakis]

The minimized example (playground:

#![feature(min_type_alias_impl_trait)]

type Foo = impl Copy;

enum Wrapper<T> {
    First(T),
    Second
}

fn produce() -> Wrapper<Foo> {
    Wrapper::Second
}

gives what appears to me to be the correct error:

error: could not find defining uses
 --> src/lib.rs:3:12
  |
3 | type Foo = impl Copy;
  |            ^^^^^^^^^

but the diagnostics are not very good.

Originally posted by @nikomatsakis in #66426 (comment)

[voidc]

I'd be interested in taking a stab at this. However, I'm not quite sure what the desired behavior is. I looked at PR #66431 which implemented the fallback to a non-defining use for unconstrained opaque types. One possibility would be to somewhere store every location at which such a fallback occurs, to later show a warning/note when no other defining use is found. I would also appreciate some mentoring instructions on how to approach this.

[oli-obk]

Yea, I think the only way to fix this is to add a new field to https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/struct.BorrowCheckResult.html that maps from opaque type def ids to spans and use that when reporting the unconstrained diagnostic.