Tracking issue for `use $crate;` compatibility warning

[jseyfried]

The warning was introduced in #37361 to avoid breakage from #37213.

[brson]

Presumably the next step is to make this an error by default.

[steveklabnik]

Triage: Digging through all the linked issues is tough, and it's not clear to me exactly what this warning is or does. @jseyfried can you maybe elaborate a little?

[petrochenkov]

This needs to be turned into an error, i.e. behave like use crate; basically:

fn main() {
    use crate; // ERROR crate root imports need to be explicitly named: `use crate as name;`
}

[Centril]

Relevant code is here:

rust/src/librustc_resolve/build_reduced_graph.rs

Line 467 in 488381c

self.r.session.struct_span_warn(item.span, "`$crate` may not be imported")

Should be easy to turn that into .struct_span_err and then do a crater run.