higher-ranked-assumptions: "unable to satisfy constraints involving placeholders" when proving Send for AFIT/RPITIT future constrained by HRTB

[tmandry]

use std::future::Future;

pub trait Acceptor<'a> {
    fn accept(self) -> impl Future<Output = Self> + Send + 'a;
}

pub trait Stream {
    fn consume<A>(self, acceptor: A) -> impl Future<Output = ()>
    where
        for<'a> A: Acceptor<'a>;
}

fn block_on<F: Send>(_: F) {}

fn check() {
    block_on(async move {
        struct S;
        impl Stream for S {
            async fn consume<A>(self, acceptor: A)
            where
                for<'a> A: Acceptor<'a>,
            {
                acceptor.accept().await;
            }
        }

        struct A;
        impl<'a> Acceptor<'a> for A {
            fn accept(self) -> impl Future<Output = Self> + Send + 'a {
                async { A }
            }
        }

        S.consume(A).await;
    });
}

godbolt

error: unable to satisfy constraints involving placeholders due to unknown implied bounds

error[E0277]: `{coroutine witness@<source>:25:13: 27:14}` cannot be sent between threads safely
  --> <source>:19:5
   |
19 |       block_on(async move {
   |       ^        ---------- within this `{async block@<source>:19:14: 19:24}`
   |  _____|
   | |
20 | |         struct S;
21 | |         impl Stream for S {
22 | |             async fn consume<A>(self, acceptor: A)
...  |
37 | |         S.consume(A).await;
38 | |     });
   | |______^ `{coroutine witness@<source>:25:13: 27:14}` cannot be sent between threads safely
   |
   = help: within `{async block@<source>:19:14: 19:24}`, the trait `Send` is not implemented for `{coroutine witness@<source>:25:13: 27:14}`
note: required because it's used within this `async` fn body
  --> <source>:25:13
   |
25 | /             {
26 | |                 acceptor.accept().await;
27 | |             }
   | |_____________^
note: required because it's used within this `async` block
  --> <source>:19:14
   |
19 |     block_on(async move {
   |              ^^^^^^^^^^
note: required by a bound in `block_on`
  --> <source>:16:16
   |
16 | fn block_on<F: Send>(_: F) {}
   |                ^^^^ required by this bound in `block_on`

[tmandry]

cc @BoxyUwU