Confusing error: Immutable reference to future is not a future

[Darksonn]

Given the following code:

async fn my_async_fn() {}

async fn foo() {
    tokio::select! {
        _ = &my_async_fn() => {},
    }
}

playground

The current output is:

error[E0599]: no method named `poll` found for struct `Pin<&mut &impl Future>` in the current scope
 --> src/lib.rs:5:5
  |
5 | /     tokio::select! {
6 | |         _ = &my_async_fn() => {}
7 | |     }
  | |_____^ method not found in `Pin<&mut &impl Future>`
  |
  = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

---

Given the following code:

async fn my_async_fn() {}

async fn foo() {
    let fut = &my_async_fn();
    tokio::pin!(fut);
    
    fut.await;
}

playground

The current output is:

error[E0277]: `&impl Future` is not a future
 --> src/lib.rs:8:5
  |
8 |     fut.await;
  |     ^^^^^^^^^ `&impl Future` is not a future
  |
  = help: the trait `Future` is not implemented for `&impl Future`
  = note: required because of the requirements on the impl of `Future` for `Pin<&mut &impl Future>`
  = note: required by `poll`

---

Given the following code:

async fn my_async_fn() {}

async fn foo() {
    let fut = &my_async_fn();
    
    fut.await;
}

playground

The current output is:

error[E0277]: `&impl Future` is not a future
 --> src/lib.rs:7:5
  |
7 |     fut.await;
  |     ^^^^^^^^^ `&impl Future` is not a future
  |
  = help: the trait `Future` is not implemented for `&impl Future`
  = note: required by `poll`

In all three cases, it would be very helpful if the compiler could explicitly call out that an immutable reference to a future is not a future. I see this relatively often in the Tokio discord where people put some extra ampersands in random places and get really confused when the error says "impl future is not future". The ampersand in the error is not enough to clue them in to what the issue is. Especially for the first case.

[tmandry]

Agreed that these could all be clearer. Immutable references to impl Future do not implement Future because you need to be able to mutate the future to poll it.

This is a great issue for someone relatively new to the compiler to pick up. Start by finding the source of each error message, then adding custom logic to print a specialized message for this particular case. (The individual error messages could be fixed one at a time.)

[tmandry]

@rustbot label +AsyncAwait-Triaged +E-mentor +E-help-wanted

[chazkiker2]

@rustbot claim