Be more precise when suggesting removal of parens on unit ctor

[compiler-errors]

• Fixes Diagnostic misclassifies a complex expression as a unit enum variant when flagging an attempt to call a non-function #99240 by only suggesting to remove parens on path exprs, not arbitrary expressions with enum type
• Generalizes by suggesting removal of parens on unit struct, too, because why not?

[rust-highfive]

r? @fee1-dead

(rust-highfive has picked a reviewer for you, use r? to override)

[compiler-errors]

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 😢

[compiler-errors]

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.

[fee1-dead]

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.

[compiler-errors]

Hm, you're right. If we had like

path::
split::
up::
over_many_lines

then we shouldn't use that in diagnostics.

[compiler-errors]

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;
  | |__________^

[fee1-dead]

Nice, please add that as a test.

[compiler-errors]

Done

[fee1-dead]

@bors r+

[bors]

📌 Commit 7a45a60 has been approved by fee1-dead

It is now in the queue for this repository.