Propose updates to closure pretty print

[arora-aman]

Rendered

r? @rust-lang/wg-diagnostics @davidtwco

[estebank]

Have you considered the possible notation mod1::f::{{closure@src/main.rs:8:14: 10:6}} instead? A nice property of the current output is the embedded textual span. I prefer to use labels for spans except for closures, as they are types that might appear very nested in their parent type.

[arora-aman]

I feel like that can be done with/after rust-lang/rust#70334.

From rust-lang/rust#70334 (comment) on that issue

For the user-facing stuff we might want to (but not for this issue, it should probably be separate) replace the #N disambiguator with @ file.rs:line:column instead (which would neatly fit in the {...}) - this could allow us to replace the odd [closure ...] type syntax with "just" a path (IIRC, @arora-aman has been looking at this for a different reason).

[arora-aman]

Because of rust-lang/rust@07e7823,
if the path to the closure is unambiguous, it will be trimmed to minimum required.

So the last example would really print f::{{closure}}#0 in non verbose and f<T>::{{closure}}#0 in verbose unless -Z trim-diagnostic-paths=off is set in which case it will print what's in the document.

If this feels acceptable, I'll update the proposal.

cc @nikomatsakis