Raw identifier keywords are sometimes silently accepted in paths

[petrochenkov]

Example:

type A = crate::r#break; // No error (?!)

fn main() {}

Found by crater run (https://crater-reports.s3.amazonaws.com/beta-1.38-1/beta-2019-08-13/gh/xi-frontend.xi-term/log.txt) when visibility resolution switched to a different algorithm (early resolution), which doesn't have this problem.

[petrochenkov]

This is a regression from Rust 1.33.

[petrochenkov]

Diagnostic improvements strike again:

rust/src/librustc_resolve/lib.rs

Lines 2186 to 2191 in 42ce9b4

	} else if !ident.is_reserved() {
	(format!("maybe a missing crate `{}`?", ident), None)
	} else {
	// the parser will already have complained about the keyword being used
	return PathResult::NonModule(PartialRes::new(Res::Err));
	}

[petrochenkov]

Fixed in #70000.