Deref coercion does not work for some await returned values

[EFanZh]

I tried this code:

fn consume_reference<T: ?Sized>(_: &T) {}

async fn foo() {
    consume_reference::<i32>(&Box::new(7_i32)); // OK.
    consume_reference::<i32>(&async { Box::new(7_i32) }.await); // Error.
    consume_reference::<[i32]>(&vec![7_i32]); // OK.
    consume_reference::<[i32]>(&async { vec![7_i32] }.await); // OK.
}

I expected to see this happen: The code compiles.

Instead, this happened: This line does not compile while the others lines do:

consume_reference::<i32>(&async { Box::new(7_i32) }.await);

The same thing also happened to awaiting futures::lock::Mutex::lock function:

fn consume_reference(_: &mut i32) {}

async fn foo() {
    let mutex = futures::lock::Mutex::new(7_i32);

    consume_reference(&mut mutex.lock().await);
}

Meta

rustc --version --verbose:

rustc 1.51.0 (2fd73fabe 2021-03-23)
binary: rustc
commit-hash: 2fd73fabe469357a12c2c974c140f67e7cdd76d0
commit-date: 2021-03-23
host: x86_64-pc-windows-msvc
release: 1.51.0
LLVM version: 11.0.1

[jyn514]

error[E0308]: mismatched types
 --> src/lib.rs:5:31
  |
5 |     consume_reference::<i32>(&async { Box::new(7_i32) }.await);
  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |                               |
  |                               expected `i32`, found struct `Box`
  |                               help: consider dereferencing the type: `*async { Box::new(7_i32) }.await`
  |
  = note: expected type `i32`
           found struct `Box<i32>`

error: aborting due to previous error

The error also happens on nightly.

[tmandry]

@rustbot label +E-medium

Not sure what exactly is going wrong here. Doing some code spelunking with -Ztreat-err-as-bug (and maybe some added debug logs) should make it possible to trace the issue.

[ldm0]

This issue has nothing to do with async or await, this is an old coercion issue: #57749.

[pnkfelix]

@rustbot claim