AFIT no longer seems to play nice with lifetime GATs

[yoshuawuyts]

Tracking issue: #91611

A few months ago I authored a complete example for a possible AsyncIterator::lend method. This used to compile fine on rustc 1.71.0-nightly (f5559e338 2023-04-24), but it no longer does on the latest nightly. Here is a minimized repro:

#![allow(incomplete_features)]
#![feature(async_fn_in_trait)]

trait AsyncLendingIterator {
    type Item<'a>
    where
        Self: 'a;

    async fn next(&mut self) -> Option<Self::Item<'_>>;
}

struct Lend<I>(I);
impl<I> AsyncLendingIterator for Lend<I> {
    type Item<'a> = &'a I
    where
        Self: 'a;

    async fn next(&mut self) -> Option<Self::Item<'_>> {
        todo!()
    }
}

This gives the following error:

Compiling playground v0.0.1 (/playground)
error[[E0311]](https://doc.rust-lang.org/nightly/error_codes/E0311.html): the parameter type `T` may not live long enough
  --> src/main.rs:29:5
   |
29 |     async fn next(&mut self) -> Option<Self::Item<'_>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
note: the parameter type `T` must be valid for the anonymous lifetime as defined here...
  --> src/main.rs:9:19
   |
9  |     async fn next(&mut self) -> Option<Self::Item<'_>>;
   |                   ^
note: ...so that the type `LendingIter<T>` will meet its required lifetime bounds
  --> src/main.rs:29:5
   |
29 |     async fn next(&mut self) -> Option<Self::Item<'_>> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0311`.
error: could not compile `playground` (bin "playground") due to previous error

I have a feeling async traits no longer play nice with lifetime GATs?

Meta

Tested on the playground using the current nightly:

nightly-2023-07-16 0e8e857b11f60a785aea

---

cc/ @rust-lang/wg-async, @compiler-errors

[compiler-errors]

I'll at least bisect this, and check if it's fixed by #113704.

[lqd]

• bisected to nightly-2023-07-11: 8ca44ef unsurprisingly Replace RPITIT current impl with new strategy that lowers as a GAT  #112988.
• is the issue fixed by Make RPITITs inherit the assumed_wf_types of their parent method #113704: I don't think so

[compiler-errors]

it should be fixed by a slight modification of #113704 though.

[lqd]

Great! (I just tested the PR as-is)