undeclared raw lifetime; "consider introducing non-raw lifetime"

[sodiboo]

Code

fn a(_: dyn Trait + 'r#fn) {
    
}

trait Trait {}

Current output

error[E0261]: use of undeclared lifetime name `'fn`
 --> src/lib.rs:1:21
  |
1 | fn a(_: dyn Trait + 'r#fn) {
  |                     ^^^^^ undeclared lifetime
  |
help: consider introducing lifetime `'fn` here
  |
1 | fn a<'fn>(_: dyn Trait + 'r#fn) {
  |     +++++

Desired output

error[E0261]: use of undeclared lifetime name `'r#fn`
 --> src/lib.rs:1:21
  |
1 | fn a(_: dyn Trait + 'r#fn) {
  |                     ^^^^^ undeclared lifetime
  |
help: consider introducing lifetime `'r#fn` here
  |
1 | fn a<'r#fn>(_: dyn Trait + 'r#fn) {
  |     +++++++

Rationale and extra context

'fn, as the diagnostic currently suggests, is not a valid lifetime.

it is my understanding that for non-reserved lifetimes, something like 'r#a is supposed to be equivalent to 'a, and that the diagnostic would be correct, were it not for the fact that in this case the r# is mandatory or else it's a syntax error.

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]

I test it in the newest nightly rustc (rustc 1.90.0-nightly (11ad40bb8 2025-06-28)), but it outputs:

error: expected one of `!` or `[`, found keyword `fn`
 --> src/main.rs:1:24
  |
1 | fn a(_: dyn Trait + 'r#fn) {
  |                        ^^ expected one of `!` or `[`

error: expected one of `)`, `+`, or `,`, found `#`
 --> src/main.rs:1:23
  |
1 | fn a(_: dyn Trait + 'r#fn) {
  |                       ^
  |                       |
  |                       expected one of `)`, `+`, or `,`
  |                       help: missing `,`

error[E0261]: use of undeclared lifetime name `'r`
 --> src/main.rs:1:21
  |
1 | fn a(_: dyn Trait + 'r#fn) {
  |     -               ^^ undeclared lifetime
  |     |
  |     help: consider introducing lifetime `'r` here: `<'r>`

Have you turned on any features?

[fmease]

@xizheyin You are using Rust 2015 or 2018, the author is using Rust 2024 or Rust 2021.

[xizheyin]

Oh! Thanks.

[fmease]

(we should improve the error message for attempted raw lifetimes in Rust <2021 if we're not macro matching (i.e., if self.may_recover() in the parser), you couldn't've known that. we do sth. similar for C-string literals)