`'r#_` gives two confusingly similar errors

[sodiboo]

Code

fn f<'r#_>(){}

Current output

error: `_` cannot be a raw lifetime
 --> src/lib.rs:1:6
  |
1 | fn f<'r#_>(){}
  |      ^^^^

error[E0637]: `'_` cannot be used here
 --> src/lib.rs:1:6
  |
1 | fn f<'r#_>(){}
  |      ^^^^ `'_` is a reserved lifetime name

Desired output

error: `_` cannot be a raw lifetime
 --> src/lib.rs:1:6
  |
1 | fn f<'r#_>(){}
  |      ^^^^

Rationale and extra context

I believe the first diagnostic is from the lexer, and the second diagnostic occurs in parsing.

These diagnostics technically don't communicate the same information:

• the first diagnostic states that there is something wrong with the token's shape
• the second diagnostic states that if 'r#_ was a valid token (in which case it would be a synonym of '_, similar to 'r#static), it would not be allowed in this position.

But both boil down to "this isn't allowed" without any suggestion for how to change it. And the second diagnostic is really about a hypothetical synonym to 'r#_, but because that token is invalid, it has no synonyms.

So, i think the best alternative would be to only display the error saying that "`_` cannot be a raw lifetime"

Rust Version

rustc 1.90.0-nightly (bdaba05a9 2025-06-27)
binary: rustc
commit-hash: bdaba05a953eb5abeba0011cdda2560d157aed2e
commit-date: 2025-06-27
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.7

[xizheyin]

This doesn't seem to be a duplicate error, the first one points out that underscore lifetime can't be expressed as a raw string, and the second one points out that we should not use underscore lifetime in a generic parameter list.

[fee1-dead]

Nope, this is definitely a duplicated message that should be deduplicated.

[xizheyin]

To de-dupe it, we need to check if '_ is raw when the second error is issued. I'll open another PR to fix it.