Futures involving uninhabited variables are incorrectly considered uninhabited.

[Lymia]

The following code crashes with an illegal instruction, presumably because all the code after contains_never is considered unreachable:

#![feature(futures_api, async_await, await_macro)]

pub enum Uninhabited { }

fn uninhabited_async() -> Uninhabited {
    unreachable!()
}

async fn noop() { }

async fn contains_never() {
    let error = uninhabited_async();
    await!(noop());
    let error2 = error;
}

fn main() {
    contains_never();
}

It seems the root cause is that the struct representing the future contains a field of type Uninhabited as it is preserved across the (unreachable) yield point and thus is considered uninhabited itself, despite being perfectly constructible.

[RalfJung]

This sounds like it is caused by the fact that generators have an "incorrect" layout (claiming to have fields with certain types when really these might not be initialized), and some code not taking that special case into account properly.

Cc @eddyb @cramertj

[cramertj]

cc @tmandry

[tmandry]

This is actually already fixed in #59897 (currently a WIP), around here. I'll make sure to add this as a test to that PR!

[RalfJung]

@tmandry does that mean that generators will no longer "cheat" in their layout, and

rust/src/librustc/ty/layout.rs

Line 1813 in a7cef0b

// Locals variables which live across yields are stored

can be removed? That'd be cool :D

I'd also have to adjust

rust/src/librustc_mir/interpret/visitor.rs

Line 315 in a7cef0b

ty::Generator(..) => {

-- can that be treated like any other type then because the variants have all the right information?

[eddyb]

@RalfJung I don't think it's that simple, we'd likely still need to keep uninitialized variables for now.
Although... I suppose the state variants, for yields across which they're not live, could just omit those variables, despite not using that space for anything else.

[tmandry]

@RalfJung I don't think it's that simple, we'd likely still need to keep uninitialized variables for now.
Although... I suppose the state variants, for yields across which they're not live, could just omit those variables, despite not using that space for anything else.

We could do that, yes, with some further changes to my PR. I would like to get there, and then we can have UB checking for our generators as well :)