Suggestion when encountering assoc types from hrtb

[estebank]

When encountering E0212, detect whether this is a representable case or
not, i.e. if it's happening on an fn or on an ADT. If the former,
provide a structured suggestion, otherwise note that this can't be
represented in Rust.

Fix #69000.

[rust-highfive]

r? @petrochenkov

(rust_highfive has picked a reviewer for you, use r? to override)

[estebank]

cc @nagisa

[nagisa]

Hmm, this says "not representable", but you can still do things like:

pub trait Foo<T> {
    type A;
    fn get(&self, t: T) -> Self::A;
}

struct SomeStruct<'a, I : for<'x> Foo<&'x isize>> {
    field: <I as Foo<&'a isize>>::A
}

And it’ll work fine, likely doing exactly what you’d expect?

[estebank]

I'll have a follow up PR with only the logic for this suggestion, as it will be more involved.

Added a very hacky version of the appropriate suggestion that would require some extra logic to be always accurate, but as a first approach it LGTM.

[nagisa]

Very cool overall! I think we should remove the "not representable" message for now (or implement an alternative suggestion) and r=me then.

[nagisa]

cc @eddyb

[eddyb]

cc @nikomatsakis @pnkfelix

[petrochenkov]

Could someone take this over? I have too many assigned PRs.
r? @nagisa

[nagisa]

@bors r+

[bors]

📌 Commit bde9677 has been approved by nagisa