anonymous lifetimes in `impl Trait` are unstable *except in async*

Code

pub fn f(
    _: impl Iterator<Item=&str>,
) {}

Current output

error[E0658]: anonymous lifetimes in `impl Trait` are unstable
 --> src/lib.rs:2:28
  |
2 |     _: impl Iterator<Item=&str>,
  |                            ^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 ~ pub fn f<'a>(
2 ~     _: impl Iterator<Item=&'a str>,
  |

Desired output

error[E0658]: anonymous lifetimes in `impl Trait` are unstable
 --> src/lib.rs:2:28
  |
2 |     _: impl Iterator<Item=&str>,
  |                            ^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 ~ pub fn f<'a>(
2 ~     _: impl Iterator<Item=&'a str>,
  |
help: note that this is allowed in async fn, due to a ??bug or something??
see ticket https://github.com/rust-lang/rust/issues/108468

Rationale and extra context

I was just refactoring some code and broke out a sync function from an async one, with many of the same arguments. The compiler tells me that the anonymous lifetimes aren't allowed - but of course they were.

Other cases

This one compiles.

pub async fn f(
    _: impl Iterator<Item=&str>,
) {}

Rust Version

Today's playground, stable.

Anything else?

This anomaly has persisted for two and a half years. Presumably it's not going to be fixed soon.

In the meantime, we could avoid gaslighting the poor user..

Start by running the sync and async snippets from the issue in the stable Playground and compare their diagnostics. Then trace the compiler diagnostic path for anonymous lifetimes in impl Trait and its async exception. Done means the sync diagnostic explains the async behavior with the proposed note while preserving the existing lifetime suggestion.