Don't suggest awaiting in closure patterns

[SparkyPotato]

Fixes #126903.

For

async fn do_async() {}

fn main() {
    Some(do_async()).map(|()| {});
}

the error is now

error[E0308]: mismatched types
 --> src/main.rs:4:27
  |
4 |     Some(do_async()).map(|()| {});
  |                           ^^
  |                           |
  |                           expected future, found `()`
  |                           expected due to this
  |
  = note: expected opaque type `impl Future<Output = ()>`
               found unit type `()`

Ideally, if main were to be async, it should be

error[E0308]: mismatched types
 --> src/main.rs:4:27
  |
4 |     Some(do_async()).map(|()| {});
  |                           ^^
  |                           |
  |                           expected future, found `()`
  |                           expected due to this
  |
  = note: expected opaque type `impl Future<Output = ()>`
               found unit type `()`
help: consider `await`ing on the `Future`
  |
4 |     Some(do_async().await).map(|()| {});
  |                    ++++++

However, this would mean FnCtx::check_pat_top would have to be called with an origin_expr in rustc_hir_typeck::check::check_fn, and that expr would have to be somehow plumbed through FnCtxt::check_expr_closure and closure signature deduction. I'm willing to work on the plumbing but unsure how to start.

[rustbot]

r? @oli-obk

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[compiler-errors]

Is there any test that exercises the case when origin_expr is Some?

[SparkyPotato]

yeah, most of suggest-missing-await.rs does

[compiler-errors]

Please squash this into one commit

[compiler-errors]

thanks

@bors r+ rollup

[bors]

📌 Commit 26677eb has been approved by compiler-errors

It is now in the queue for this repository.