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Infer async closure args from `Fn` bound even if there is no corresponding `Future` bound on return In #127482, I implemented the functionality to infer an async closure signature when passed into a function that has `Fn` + `Future` where clauses that look like: ``` fn whatever(callback: F) where F: Fn(Arg) -> Fut, Fut: Future<Output = Out>, ``` However, #127781 demonstrates that this is still incomplete to address the cases users care about. So let's not bail when we fail to find a `Future` bound, and try our best to just use the args from the `Fn` bound if we find it. This is *fine* since most users of closures only really care about the *argument* types for inference guidance, since we require the receiver of a `.` method call to be known in order to probe methods. When I experimented with programmatically rewriting `|| async {}` to `async || {}` in #127827, this also seems to have fixed ~5000 regressions (probably all coming from usages `TryFuture`/`TryStream` from futures-rs): the [before](https://github.com/rust-lang/rust/pull/127827#issuecomment-2254061733) and [after](https://github.com/rust-lang/rust/pull/127827#issuecomment-2255470176) crater runs. Fixes #127781. |
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