False positive for needless_lifetimes (possibly due to async?)

[tmccombs]

Output of cargo clippy -V:

clippy 0.0.212 (4e7e71b 2019-10-11)

I have a method that looks like this:

    pub async fn wait<'a, T>(&self, guard: MutexGuard<'a, T>) -> MutexGuard<'a, T> {
        let mutex = guard_lock(&guard);

        self.await_notify(guard).await;

        mutex.lock().await
    }

Clippy flags the signature for needless lifetimes. However, replacing the 'a lifteime with the anonymous '_ as suggested, results in a lifetime mismatch error, because the lifetime of the return value comes from the guard argument, not self. As far as I understand, lifetimes cannot be elided here.

[flip1995]

Reproducer: Playground

use std::sync::MutexGuard;

struct Foo;

impl Foo {
    // doesn't get linted without async
    pub async fn wait<'a, T>(&self, guard: MutexGuard<'a, T>) -> MutexGuard<'a, T> {
        guard
    }
}

[flip1995]

Closing as duplicate of #5787, which has some more examples