Awaiting on Pin<Box<dyn Future>> causes ICE

[LunaBorowska]

The following program causes an ICE when ran with cargo miri:

use std::{
    future::Future,
    pin::Pin,
    task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
};

pub fn block_on<F: Future>(mut future: F) -> F::Output {
    let mut future = unsafe { Pin::new_unchecked(&mut future) };

    static VTABLE: RawWakerVTable = RawWakerVTable::new(
        |_| unimplemented!("clone"),
        |_| unimplemented!("wake"),
        |_| unimplemented!("wake_by_ref"),
        |_| (),
    );
    let waker = unsafe { Waker::from_raw(RawWaker::new(core::ptr::null(), &VTABLE)) };
    let mut context = Context::from_waker(&waker);

    loop {
        if let Poll::Ready(val) = future.as_mut().poll(&mut context) {
            break val;
        }
    }
}

fn main() {
    block_on(async {
        let boxed: Pin<Box<dyn Future<Output = ()>>> = Box::pin(async {});
        boxed.await;
    });
}

Error message:

error: internal compiler error: src/librustc_mir/interpret/place.rs:231: vtable not supported on type std::pin::Pin<&mut dyn std::future::Future<Output = ()>>

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:880:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.44.0-nightly (94d346360 2020-04-09) running on x86_64-unknown-linux-gnu

note: compiler flags: -Z always-encode-mir -Z mir-emit-retag -Z mir-opt-level=0 -C debug-assertions=on -C debuginfo=2 -C incremental --crate-type bin

note: some of the compiler flags provided by cargo are hidden

[RalfJung]

Thanks for the report! I think this is a duplicate of #1038. Could you confirm?

[LunaBorowska]

It's panicking in a different place (place.rs:258:9 and place.rs:231), and the error message is different ("called Result::unwrap() on an Err value" and "vtable not supported on type"), this was tested with the same nightly (2020-04-09). The issues seem very similar, both involve calling a method on Pin<&mut dyn Trait> but I'm not sure whether it's a duplicate.

[RalfJung]

Hm, good point.

I looked a bit more into this, and I think the difference is because builtin_deref still works in your case, but then .vtable() on the result fails. That's probably because there is a Box here but Rc in the other example.

Still, both seem about dyn Trait with pointer types other than Box, &, &mut, so I'll close this as duplicate.