Anonymous lifetimes in impl trait are allowed in async fn, despite being an unstable feature

[jplatte]

I tried this code (playground):

async fn test(args: impl Iterator<Item = &str>) {}

I expected to see this happen: an error because &str contains an anonymous lifetime, and is part of an impl Trait type.

Instead, this happened: Compiles successfully.

Removing async from the function definition gives the following error (on nightly to see the feature gate):

error[E0658]: anonymous lifetimes in `impl Trait` are unstable
 --> src/lib.rs:1:37
  |
1 | fn test(args: impl Iterator<Item = &str>) {}
  |                                     ^ expected named lifetime parameter
  |
  = help: add `#![feature(anonymous_lifetime_in_impl_trait)]` to the crate attributes to enable
help: consider introducing a named lifetime parameter
  |
1 | fn test<'a>(args: impl Iterator<Item = &'a str>) {}
  |        ++++                             ++

This feature seems to have no documentation or tracking issue, only an unstable book entry. Given this, and the fact this behavior has been stable on async accidentally (AFAICT) for many releases now, maybe the best solution will be to just un-featuregate this functionality.

Meta

rustc --version --verbose:

rustc 1.67.1 (d5a82bbd2 2023-02-07)

@rustbot label: +A-impl-trait +T-compiler

[clubby789]

This is explicitly allowed, and the feature is being stabilized soon #107378

rust/compiler/rustc_hir_analysis/src/collect/resolve_bound_vars.rs

Lines 1210 to 1211 in 31448ba

	// Fresh lifetimes in APIT used to be allowed in async fns and forbidden in
	// regular fns.

[jplatte]

That comment says "used to be allowed in async fns and forbidden in regular fns" though?

[traviscross]

@rustbot labels +AsyncAwait-Triaged +T-lang

We discussed this in async triage. This is pending action from lang, and we've renominated #107378.

[ijackson]

I just came across this. The note about #107378 seems out of date. See #137575.

It would be nice if in the meantime the error message said something more true. I've filed #149076 for that.