Implementation of trait is not general enough when boxing future returning <T as Foo<'a>>::Foo

[ibraheemdev]

Reproduction:

use std::any::Any;
use std::future::Future;

trait Foo<'a>: Sized {
    type Error;
    fn foo(x: &'a str) -> Result<Self, Self::Error>;
}

impl<'a> Foo<'a> for &'a str {
    type Error = ();

    fn foo(x: &'a str) -> Result<Self, Self::Error> {
        Ok(x)
    }
}

async fn get_foo<'a, T>(x: &'a str) -> Result<T, <T as Foo<'a>>::Error>
where
    T: Foo<'a>,
{
    Foo::foo(x)
}

fn bar<'a>(x: &'a str) -> Box<dyn Future<Output = Result<&'a str, ()>> + Send + 'a> {
    Box::new(async move { get_foo(x).await })
}

Output on stable and nightly:

error: implementation of `Foo` is not general enough
  --> src/lib.rs:25:5
   |
25 |     Box::new(async move { get_foo(x).await })
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ implementation of `Foo` is not general enough
   |
   = note: `Foo<'1>` would have to be implemented for the type `&'0 str`, for any two lifetimes `'0` and `'1`...
   = note: ...but `Foo<'2>` is actually implemented for the type `&'2 str`, for some specific lifetime `'2

Interestingly, the code compiles without the + Send. It also compiles without the async move block. This may be related to #61949.

@rustbot label +A-async-await

[compiler-errors]

This is actually probably related to #60658 (and other issues) instead of #61949, and seems to be fixed under #92449.

[eholk]

This does sound similar to several other issues. Glad to see #92449 seems to fix it.

@rustbot label +AsyncAwait-Triaged +AsyncAwait-Polish

[vincenzopalazzo]

@rustbot claim

OK this needs to be fixed in some way, I will try to see what it is going on

[vincenzopalazzo]

Ah ok this looks like a well-known issue with lifetimes, ok I think this is related to #110338 (comment)

Putting this in our board btw