I would like to use tcx.def_path_str instead of span_to_snippet, but that does not preserve aliases, and causes regressions in src/test/ui/type-alias-enum-variants/incorrect-variant-form-through-alias-caught.rs by changing Alias::Unit to Enum::Unit 😢

I could possibly find a way to format qpath into a string by joining all of its segments, but that's a bit overkill and this PR does not represent a regression in the status quo.

It wouldn't work well with improperly formatted code, right? If the path is a macro expansion what would happen..? To me I don't think it should preserve the alias path.

Hm, you're right. If we had like

path::
split::
up::
over_many_lines

then we shouldn't use that in diagnostics.

I just used rustc_hir_pretty which does hir -> string for us. Avoids weirdness like:

enum Enum { Braced {} }
type Alias = Enum;

fn main() {
    Alias::
    Braced;
}

error[[E0533]](https://doc.rust-lang.org/stable/error-index.html#E0533): expected unit struct, unit variant or constant, found struct variant `Alias::
                  Braced`
 --> src/main.rs:5:5
  |
5 | /     Alias::
6 | |     Braced;
  | |__________^

Nice, please add that as a test.

Done