Prevent niche-optimization on futures to avoid triggering miri

tokio/src/future/maybe_done.rs

@@ -10,6 +10,7 @@ pin_project! {
     #[derive(Debug)]
     #[project = MaybeDoneProj]
     #[project_replace = MaybeDoneProjReplace]
+    #[repr(C)] // https://github.com/rust-lang/miri/issues/3780
     pub enum MaybeDone<Fut: Future> {
         /// A not-yet-completed future.
         Future { #[pin] future: Fut },
@@ -69,3 +70,54 @@ impl<Fut: Future> Future for MaybeDone<Fut> {
         Poll::Ready(())
     }
 }
+
+// Test for https://github.com/tokio-rs/tokio/issues/6729
+#[cfg(test)]
+mod miri_tests {
+    use super::maybe_done;
+
+    use std::{
+        future::Future,
+        pin::Pin,
+        sync::Arc,
+        task::{Context, Poll, Wake},
+    };
+
+    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
+        }
+    }
+
+    #[test]
+    fn maybe_done_miri() {
+        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 = maybe_done(fut);
+        let mut fut = unsafe { Pin::new_unchecked(&mut fut) };
+
+        let waker = Arc::new(DummyWaker).into();
+        let mut ctx = Context::from_waker(&waker);
+        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>) {}
+    }
+}

tokio/src/runtime/task/core.rs

@@ -196,6 +196,7 @@ generate_addr_of_methods! {
 }
 
 /// Either the future or the output.
+#[repr(C)] // https://github.com/rust-lang/miri/issues/3780
 pub(super) enum Stage<T: Future> {
     Running(T),
     Finished(super::Result<T::Output>),

tokio/src/runtime/tests/task.rs

@@ -223,6 +223,100 @@ fn shutdown_immediately() {
     })
 }
 
+// Test for https://github.com/tokio-rs/tokio/issues/6729
+#[test]
+fn spawn_niche_in_task() {
+    use crate::future::poll_fn;
+    use std::task::{Context, Poll, Waker};
+
+    with(|rt| {
+        let state = Arc::new(Mutex::new(State::new()));
+
+        let mut subscriber = Subscriber::new(Arc::clone(&state), 1);
+        rt.spawn(async move {
+            subscriber.wait().await;
+            subscriber.wait().await;
+        });
+
+        rt.spawn(async move {
+            state.lock().unwrap().set_version(2);
+            state.lock().unwrap().set_version(0);
+        });
+
+        rt.tick_max(10);
+        assert!(rt.is_empty());
+        rt.shutdown();
+    });
+
+    pub(crate) struct Subscriber {
+        state: Arc<Mutex<State>>,
+        observed_version: u64,
+        waker_key: Option<usize>,
+    }
+
+    impl Subscriber {
+        pub(crate) fn new(state: Arc<Mutex<State>>, version: u64) -> Self {
+            Self {
+                state,
+                observed_version: version,
+                waker_key: None,
+            }
+        }
+
+        pub(crate) async fn wait(&mut self) {
+            poll_fn(|cx| {
+                self.state
+                    .lock()
+                    .unwrap()
+                    .poll_update(&mut self.observed_version, &mut self.waker_key, cx)
+                    .map(|_| ())
+            })
+            .await;
+        }
+    }
+
+    struct State {
+        version: u64,
+        wakers: Vec<Waker>,
+    }
+
+    impl State {
+        pub(crate) fn new() -> Self {
+            Self {
+                version: 1,
+                wakers: Vec::new(),
+            }
+        }
+
+        pub(crate) fn poll_update(
+            &mut self,
+            observed_version: &mut u64,
+            waker_key: &mut Option<usize>,
+            cx: &Context<'_>,
+        ) -> Poll<Option<()>> {
+            if self.version == 0 {
+                *waker_key = None;
+                Poll::Ready(None)
+            } else if *observed_version < self.version {
+                *waker_key = None;
+                *observed_version = self.version;
+                Poll::Ready(Some(()))
+            } else {
+                self.wakers.push(cx.waker().clone());
+                *waker_key = Some(self.wakers.len());
+                Poll::Pending
+            }
+        }
+
+        pub(crate) fn set_version(&mut self, version: u64) {
+            self.version = version;
+            for waker in self.wakers.drain(..) {
+                waker.wake();
+            }
+        }
+    }
+}
+
 #[test]
 fn spawn_during_shutdown() {
     static DID_SPAWN: AtomicBool = AtomicBool::new(false);

tokio/tests/macros_join.rs

@@ -81,7 +81,7 @@ fn join_size() {
         let ready2 = future::ready(0i32);
         tokio::join!(ready1, ready2)
     };
-    assert_eq!(mem::size_of_val(&fut), 40);
+    assert_eq!(mem::size_of_val(&fut), 48);
 }
 
 async fn non_cooperative_task(permits: Arc<Semaphore>) -> usize {