higher-ranked lifetime error when using `AsyncFnMut`

Nobody has claimed this yet.

I tried this code:

use std::future::Future;

async fn spawn<F: Future + Send + 'static>(_: F)
where
    <F as Future>::Output: Send + 'static,
{
}

async fn a(mut input: String) {
    b(async |val| {
        input.pop();
        val + 1
    })
    .await;
}

async fn b(mut f: impl AsyncFnMut(i32) -> i32) {
    let out = f(6).await;
    dbg!(out);
}

async fn c() {
    spawn(a("xx".into())).await;
}

fn main() {}

I expected to see this happen: It compiles.

Instead, this happened: It fails to compile:

error: higher-ranked lifetime error
  --> src/main.rs:23:5
   |
23 |     spawn(a("xx".into())).await;
   |     ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: could not prove `impl Future<Output = ()>: Send`

error: higher-ranked lifetime error
  --> src/main.rs:23:5
   |
23 |     spawn(a("xx".into())).await;
   |     ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: could not prove `impl Future<Output = ()> well-formed`

error: higher-ranked lifetime error
  --> src/main.rs:23:27
   |
23 |     spawn(a("xx".into())).await;
   |                           ^^^^^
   |
   = note: could not prove `impl Future<Output = ()> well-formed`

error: higher-ranked lifetime error
  --> src/main.rs:23:27
   |
23 |     spawn(a("xx".into())).await;
   |                           ^^^^^
   |
   = note: could not prove `<impl Future<Output = ()> as IntoFuture>::IntoFuture well-formed`

error: higher-ranked lifetime error
  --> src/main.rs:23:27
   |
23 |     spawn(a("xx".into())).await;
   |                           ^^^^^
   |
   = note: could not prove `&mut impl Future<Output = ()> well-formed`

error: higher-ranked lifetime error
  --> src/main.rs:23:27
   |
23 |     spawn(a("xx".into())).await;
   |                           ^^^^^
   |
   = note: could not prove `Pin<&mut impl Future<Output = ()>> well-formed`

error: higher-ranked lifetime error
  --> src/main.rs:23:27
   |
23 |     spawn(a("xx".into())).await;
   |                           ^^^^^
   |
   = note: could not prove `Poll<<impl Future<Output = ()> as Future>::Output> well-formed`

error: could not compile `playground` (bin "playground") due to 7 previous errors

Meta

rustc --version --verbose:

rustc 1.92.0 (ded5c06cf 2025-12-08)
rustc 1.94.0-nightly (31cd367b9 2026-01-08)

I looked for prior art but didn't find it. Curiously I encountered this while working on a larger project and the code was more complex, the error I got was also in the invocation of spawn but it said "std::marker::Send would have to be implemented for the type &'0 bool, for any lifetime '0". So I'm not sure I fully reproduced the issue.

Start by compiling the minimal reproducer with rustc 1.92.0 and nightly 1.94.0, focusing on the spawn(a("xx".into())).await call and its AsyncFnMut bound. Investigate the higher-ranked lifetime diagnostics and Send well-formedness notes; done means the reproducer compiles as expected or produces a correct, actionable diagnostic.

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higher-ranked lifetime error when using `AsyncFnMut`

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Contributor guide

First steps

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Assessment

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