confusing diagnostic for incorrect type parameter on BoxFuture

[qutesy]

I tried this code:

use futures::{future::{BoxFuture, Future, FutureExt}};

async fn f() -> bool {
    true
}

#[allow(dead_code)]
struct Foo<'a, F>
where F: Future<Output=bool> + Send {
    f: Option<BoxFuture<'a, F>>,
}

impl<'a, F> Foo<'a, F>
where F: Future<Output=bool> + Send + 'a {
    fn new(f: F) -> Self {
        Self {
            f: Some(f.boxed())
        }
    }
}

fn main() {
    Foo::new(f());
}

I expected to see this happen:
I wish the compiler had suggested that I should change f: Option<BoxFuture<'a, F>> to f: Option<BoxFuture<'a, F::Output>>,

Instead, this happened:

27 | impl<'a, F> Foo<'a, F>
   |          - this type parameter
...
33 |             f: Some(f.boxed())
   |                     ^^^^^^^^^
   |                     |
   |                     expected type parameter `F`, found `bool`
   |                     help: you need to pin and box this expression: `Box::pin(f.boxed())`
*/

Meta

Rust compiler version (stable version on play.rust-lang.org).

1.43.1

Playground link

Backtrace

N/A (compile error)

[tmandry]

cc @estebank, I think the suggestion shouldn't be applied here. Is there a way we can target it more precisely?

[tmandry]

Triage: wg-async-foundations will track this until the incorrect suggestion is removed, other improvements to the suggestion are out of scope for us.

[estebank]

Some extra filtering in the following will be needed, but without further checking I can't tell for sure what the check should be:

https://github.com/rust-lang/rust/pull/69082/files#diff-1d1b0d29a2e8da97c6bfb6e364d920c7R5070-R5089

[bugaevc]

Simple reproducer:

let f: BoxFuture<'static, bool> = async { }.boxed();

playground

[tmandry]

@rustbot claim