test(maitake): Miri test for rust-lang/miri#3780

This reproduces a potential UB where a `task::Cell` gets niche-optimized
in the `Joined` case. This is based on the test added to Tokio in
tokio-rs/tokio#6744

maitake/src/task.rs

@@ -734,21 +734,7 @@ where
         #[cfg(any(feature = "tracing-01", feature = "tracing-02", test))]
         let _span = self.span().enter();
 
-        self.inner.with_mut(|cell| {
-            let cell = unsafe { &mut *cell };
-            let poll = match cell {
-                Cell::Pending(future) => unsafe { Pin::new_unchecked(future).poll(&mut cx) },
-                _ => unreachable!("tried to poll a completed future!"),
-            };
-
-            match poll {
-                Poll::Ready(ready) => {
-                    *cell = Cell::Ready(ready);
-                    Poll::Ready(())
-                }
-                Poll::Pending => Poll::Pending,
-            }
-        })
+        self.inner.with_mut(|cell| unsafe { (*cell).poll(&mut cx) })
     }
 
     /// Wakes the task's [`JoinHandle`], if it has one.
@@ -1427,6 +1413,23 @@ impl<F: Future> fmt::Debug for Cell<F> {
     }
 }
 
+impl<F: Future> Cell<F> {
+    fn poll(&mut self, cx: &mut Context<'_>) -> Poll<()> {
+        let poll = match self {
+            Cell::Pending(future) => unsafe { Pin::new_unchecked(future).poll(cx) },
+            _ => unreachable!("tried to poll a completed future!"),
+        };
+
+        match poll {
+            Poll::Ready(ready) => {
+                *self = Cell::Ready(ready);
+                Poll::Ready(())
+            }
+            Poll::Pending => Poll::Pending,
+        }
+    }
+}
+
 // === impl Vtable ===
 
 impl fmt::Debug for Vtable {

maitake/src/task/tests/alloc_tests.rs

@@ -264,3 +264,53 @@ fn drop_join_handle() {
 
     assert!(COMPLETED.load(Ordering::Relaxed))
 }
+
+// Test for potential UB in `Cell::poll` due to niche optimization.
+// See https://github.com/rust-lang/miri/issues/3780 for details.
+//
+// This is based on the test for analogous types in Tokio added in
+// https://github.com/tokio-rs/tokio/pull/6744
+#[test]
+fn cell_miri() {
+    use super::Cell;
+    use alloc::{string::String, sync::Arc, task::Wake};
+    use core::{
+        future::Future,
+        pin::Pin,
+        task::{Context, Poll},
+    };
+
+    struct DummyWaker;
+
+    impl Wake for DummyWaker {
+        fn wake(self: Arc<Self>) {}
+    }
+
+    struct ThingAdder<'a> {
+        thing: &'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 += ", world";
+            }
+            Poll::Pending
+        }
+    }
+
+    let mut thing = "hello".to_owned();
+
+    // The async block is necessary to trigger the miri failure.
+    #[allow(clippy::redundant_async_block)]
+    let fut = async move { ThingAdder { thing: &mut thing }.await };
+
+    let mut fut = Cell::Pending(fut);
+
+    let waker = Arc::new(DummyWaker).into();
+    let mut ctx = Context::from_waker(&waker);
+    assert_eq!(fut.poll(&mut ctx), Poll::Pending);
+    assert_eq!(fut.poll(&mut ctx), Poll::Pending);
+}