Unify lifetime elision for `impl Trait` arguments between async and non-async functions

[LukasKalbertodt]

This compiles:

async fn foo(_: impl Iterator<Item = &u32>) {}

But this doesn't:

fn foo(_: impl Iterator<Item = &u32>) {}

error[E0106]: missing lifetime specifier
 --> src/lib.rs:1:32
  |
1 | fn foo(_: impl Iterator<Item = &u32>) {}
  |                                ^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 | fn foo<'a>(_: impl Iterator<Item = &'a u32>) {}
  |       ^^^^                         ^^^

This difference strikes me as very odd. Is this intended? If so, why?

This is formulated as a question, but if I had to guess, I would classify this as oversight or bug -- that's why I created an issue instead of askig in the forum.

_{(originally asked here)}

[tmandry]

We discussed this in our meeting today. The difference in behavior comes from this code:

rust/compiler/rustc_ast_lowering/src/lib.rs

Lines 1800 to 1807 in af2ed1b

	let lt_mode = if make_ret_async.is_some() {
	// In `async fn`, argument-position elided lifetimes
	// must be transformed into fresh generic parameters so that
	// they can be applied to the opaque `impl Trait` return type.
	AnonymousLifetimeMode::CreateParameter
	} else {
	self.anonymous_lifetime_mode
	};

While we decided that async fn probably exhibits the right behavior, it's possible this could affect other things too which could lead to bugs. Before declaring this fixed we should account for anything that might appear in arguments and be affected by this code.

We also noticed that in sync code, we aren't consistent in the behavior of impl Trait and dyn Trait. For example:

trait Something { type Item; }
fn foo(x: &dyn Something<Item = &u32>) { }   // works fine
fn foo(x: &impl Something<Item = &u32>) { }  // error

We should be more consistent and document the elision rules for all these cases in the reference. Since dyn already accepts this, we decided we should go ahead and accept this for impl Trait too.

[eholk]

This seems sort of related to #74256 and #74597.

[estebank]

The output of this is regressing in 1.64.

1.63:

error[[E0106]](https://doc.rust-lang.org/stable/error-index.html#E0106): missing lifetime specifier
 --> src/lib.rs:1:32
  |
1 | fn foo(_: impl Iterator<Item = &u32>) {}
  |                                ^ expected named lifetime parameter
  |
help: consider introducing a named lifetime parameter
  |
1 | fn foo<'a>(_: impl Iterator<Item = &'a u32>) {}
  |       ++++                          ++

1.64:

error[[E0658]](https://doc.rust-lang.org/beta/error-index.html#E0658): anonymous lifetimes in `impl Trait` are unstable
 --> src/lib.rs:1:32
  |
1 | fn foo(_: impl Iterator<Item = &u32>) {}
  |                                ^

[cjgillot]

#97720 originally suggested to remove the difference between the async and regular fns. In order to avoid and insta-stable change, the choice was made to introduce the feature gate instead in #97720 (comment).

Removing this feature gate is trivial: revert 5a20834.
Re-introducing suggestions there is more complicated: the feature gate happens after the suggestion code. (AST-based resolution accepts the code and does not suggest anything, and HIR-based lifetime resolution issues the feature gate later.)