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//@revisions: stack tree
//@[tree]compile-flags: -Zmiri-tree-borrows
use std::future::Future;
use std::mem::MaybeUninit;
use std::pin::Pin;
use std::sync::Arc;
use std::task::{Context, Poll, Wake};
struct ThingAdder<'a> {
// Using `MaybeUninit` to ensure there are no niches here.
thing: MaybeUninit<&'a mut String>,
}
impl Future for ThingAdder<'_> {
type Output = ();
fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
unsafe {
**self.get_unchecked_mut().thing.assume_init_mut() += ", world";
}
Poll::Pending
}
}
fn main() {
let mut thing = "hello".to_owned();
// This future has (at least) two fields, a String (`thing`) and a ThingAdder pointing to that string.
let fut = async move { ThingAdder { thing: MaybeUninit::new(&mut thing) }.await };
let mut fut = MaybeDone::Future(fut);
let mut fut = unsafe { Pin::new_unchecked(&mut fut) };
let waker = Arc::new(DummyWaker).into();
let mut ctx = Context::from_waker(&waker);
// This ends up reading the discriminant of the `MaybeDone`. If that is stored inside the
// `thing: String` as a niche optimization, that causes aliasing conflicts with the reference
// stored in `ThingAdder`.
assert_eq!(fut.as_mut().poll(&mut ctx), Poll::Pending);
assert_eq!(fut.as_mut().poll(&mut ctx), Poll::Pending);
}
struct DummyWaker;
impl Wake for DummyWaker {
fn wake(self: Arc<Self>) {}
}
pub enum MaybeDone<F: Future> {
Future(F),
Done,
}
impl<F: Future<Output = ()>> Future for MaybeDone<F> {
type Output = ();
fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
unsafe {
match *self.as_mut().get_unchecked_mut() {
MaybeDone::Future(ref mut f) => Pin::new_unchecked(f).poll(cx),
MaybeDone::Done => unreachable!(),
}
}
}
}
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