future miscompiling to ud2

[ambr-e]

Using rustc 1.36.0-nightly (3991285 2019-04-25), the following test compiles to ud2, when using the following command: rustc --edition=2018 -Copt-level=z -Cdebuginfo=2 --test test.rs -o test. Both the opt-level and debuginfo are necessary to get ud2.

#![feature(async_await, await_macro)]
#![allow(unused)]

#[cfg(test)]
mod tests {
    use std::future::Future;
    use std::task::Poll;
    use std::task::Context;
    use std::pin::Pin;
    use std::rc::Rc;
    
    struct Never();
    impl Future for Never {
        type Output = ();
        fn poll(self: Pin<&mut Self>, _cx: &mut Context<'_>) -> Poll<Self::Output> {
            Poll::Pending
        }
    }

    #[test]
    fn crashing_test() {
        let fut = async {
            let _rc = Rc::new(()); // Also crashes with Arc
            await!(Never());
        };
        let _bla = fut; // Moving the future is required.
    }
}

@cramertj @tmandry

[cramertj]

This does not reproduce for me on current nightly (rustc 1.36.0-nightly (a784a8022 2019-05-09)). I wonder if some of @tmandry's generator representation work has affected this.

[Centril]

@cramertj In that case, add it as a regression test?

[tmandry]

This was fixed by #59897, specifically 5a7af54. I'm not sure why, though.

[cramertj]

@tmandry possibly the inhabitedness fix?

[tmandry]

@cramertj I was thinking that, but Never in this example is very much inhabited.

[cramertj]

@tmandry yeah- the stack slot that it returns to is never going to be hit, though, since it's always pending. Not sure if that matters...

[nikomatsakis]

Marking as deferred, not blocking stabilization.