There's this example:

// Instead of writing:
fn higher_ranked<F>(callback: F)
where
    F: Fn(&Arg) -> Pin<Box<dyn Future<Output = ()> + '_>>
{ todo!() }

// Write this:
fn higher_ranked<F: async Fn(&Arg)> { todo!() }
// Or this:
fn higher_ranked<F: AsyncFn(&Arg)> { todo!() }

But it seems that's in error and it should have the parameter:

fn higher_ranked<F: async Fn(&Arg)>(callback: F){ todo!() }