Don't suggest awaiting in closure patterns
Fixes#126903.
For
```rust
async fn do_async() {}
fn main() {
Some(do_async()).map(|()| {});
}
```
the error is now
```rust
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
```rs
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.
dad39e8840