async-closure.rs

//@ run-pass

//@ revisions: default nomiropt
//@[nomiropt]compile-flags: -Z mir-opt-level=0

//@ edition:2018
//@ aux-build:arc_wake.rs

extern crate arc_wake;

use std::pin::Pin;
use std::future::Future;
use std::sync::{
    Arc,
    atomic::{self, AtomicUsize},
};
use std::task::{Context, Poll};
use arc_wake::ArcWake;

struct Counter {
    wakes: AtomicUsize,
}

impl ArcWake for Counter {
    fn wake(self: Arc<Self>) {
        Self::wake_by_ref(&self)
    }
    fn wake_by_ref(arc_self: &Arc<Self>) {
        arc_self.wakes.fetch_add(1, atomic::Ordering::SeqCst);
    }
}

struct WakeOnceThenComplete(bool);

fn wake_and_yield_once() -> WakeOnceThenComplete { WakeOnceThenComplete(false) }

impl Future for WakeOnceThenComplete {
    type Output = ();
    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
        if self.0 {
            Poll::Ready(())
        } else {
            cx.waker().wake_by_ref();
            self.0 = true;
            Poll::Pending
        }
    }
}

fn async_closure(x: u8) -> impl Future<Output = u8> {
    (async move |x: u8| -> u8 {
        wake_and_yield_once().await;
        x
    })(x)
}

fn async_closure_in_unsafe_block(x: u8) -> impl Future<Output = u8> {
    (unsafe {
        async move |x: u8| unsafe_fn(unsafe_async_fn(x).await)
    })(x)
}

async unsafe fn unsafe_async_fn(x: u8) -> u8 {
    wake_and_yield_once().await;
    x
}

unsafe fn unsafe_fn(x: u8) -> u8 {
    x
}

fn test_future_yields_once_then_returns<F, Fut>(f: F)
where
    F: FnOnce(u8) -> Fut,
    Fut: Future<Output = u8>,
{
    let mut fut = Box::pin(f(9));
    let counter = Arc::new(Counter { wakes: AtomicUsize::new(0) });
    let waker = ArcWake::into_waker(counter.clone());
    let mut cx = Context::from_waker(&waker);
    assert_eq!(0, counter.wakes.load(atomic::Ordering::SeqCst));
    assert_eq!(Poll::Pending, fut.as_mut().poll(&mut cx));
    assert_eq!(1, counter.wakes.load(atomic::Ordering::SeqCst));
    assert_eq!(Poll::Ready(9), fut.as_mut().poll(&mut cx));
}

fn main() {
    macro_rules! test {
        ($($fn_name:expr,)*) => { $(
            test_future_yields_once_then_returns($fn_name);
        )* }
    }

    test! {
        async_closure,
        async_closure_in_unsafe_block,
    }
}