`miri` detect UB for local task.

[nurmohammed840]

More precisely Checked<Fut: Future>

async-task/src/runnable.rs

Lines 449 to 452 in b4486cd

	struct Checked<F> {
	id: ThreadId,
	inner: ManuallyDrop<F>,
	}

Playground

async fn yield_now() {
    let mut yielded = false;
    let yield_now = poll_fn(|cx| {
        if yielded {
            return Poll::Ready(());
        }
        yielded = true;
        cx.waker().wake_by_ref();
        Poll::Pending
    });
    yield_now.await;
    println!("Done");
}

Miri complain:

   Compiling playground v0.0.1 (/playground)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.60s
     Running `/playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/cargo-miri runner target/miri/x86_64-unknown-linux-gnu/debug/playground`
error: Undefined Behavior: attempting a read access using <2044> at alloc247[0x10], but that tag does not exist in the borrow stack for this location
  --> src/main.rs:19:12
   |
19 |         if yielded {
   |            ^^^^^^^ this error occurs as part of an access at alloc247[0x10..0x11]
   |
   = help: this indicates a potential bug in the program: it performed an invalid operation, but the Stacked Borrows rules it violated are still experimental
   = help: see https://github.com/rust-lang/unsafe-code-guidelines/blob/master/wip/stacked-borrows.md for further information
help: <2044> was created by a Unique retag at offsets [0x10..0x11]
  --> src/main.rs:26:5
   |
26 |     yield_now.await;
   |     ^^^^^^^^^^^^^^^
help: <2044> was later invalidated at offsets [0x0..0x18] by a Unique retag
  --> src/main.rs:11:16
   |
11 |         future.await;

[nurmohammed840]

Miri did not complain, when mutably accessing id via get_unchecked_mut.
Weird! 😕

Playground

impl<F: Future> Future for Checked<F> {
    type Output = F::Output;
    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        unsafe {
            let me = self.get_unchecked_mut();
            assert!(
                me.id == thread_id(),
                "local task polled by a thread that didn't spawn it"
            );
            Pin::new_unchecked(&mut *me.inner).poll(cx)
        }
    }
}