Extremely un-informative error when a future that capture the environment is used as Send

[Ciel-MC]

Code

use futures::prelude::future::{join_all, FutureExt};

const THING: () = ();
async fn foo() {
    let updates = [async {}.then(|()| async move {
        &THING
    })];
    join_all(updates).await;
}

fn trouble() -> impl Send {
    foo()
}

Current output

error[E0308]: mismatched types
  --> src/main.rs:12:5
   |
5  |       let updates = [async {}.then(|()| async move {
   |  _______________________________________-
6  | |         &THING
7  | |     })];
   | |     -
   | |     |
   | |_____the expected `async` block
   |       the found `async` block
...
12 |       foo()
   |       ^^^^^ one type is more general than the other
   |
   = note: expected `async` block `{async block@src/main.rs:5:39: 7:6}`
              found `async` block `{async block@src/main.rs:5:39: 7:6}`

For more information about this error, try `rustc --explain E0308`.
error: could not compile `test_project` (bin "test_project") due to 1 previous error

Desired output

No response

Rationale and extra context

No response

Other cases

No response

Rust Version

rustc 1.79.0 (129f3b996 2024-06-10)
binary: rustc
commit-hash: 129f3b9964af4d4a709d1383930ade12dfe7c081
commit-date: 2024-06-10
host: x86_64-unknown-linux-gnu
release: 1.79.0
LLVM version: 18.1.7

(Happens on nightly too)

Anything else?

The code compiles without the reference, so I'm guessing it's to do with the Future being made "unique" by that even though it doesn't actually contain any "real" captures that should make the closure different.

[traviscross]

@rustbot labels +AsyncAwait-Triaged

We discussed this in the async meeting today. We think this is probably related to:

• https://github.com/rust-lang/rust/issues/110338

[theemathas]

Possibly related to / duplicate of #112985