supress niches in coroutines

tests/pass/async-niche-aliasing.rs

@@ -0,0 +1,66 @@
+//@revisions: stack tree
+//@[tree]compile-flags: -Zmiri-tree-borrows
+
+use std::{
+    future::Future,
+    pin::Pin,
+    sync::Arc,
+    task::{Context, Poll, Wake},
+    mem::MaybeUninit,
+};
+
+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!(),
+            }
+        }
+    }
+}