Attempting to run an async fn after completion mentions generators

[Nemo157]

If a user attempts to do something like this (playground including trivial executor):

async fn foo() {
}

fn main() {
    let mut future = Box::pin(foo());
    executor::block_on(future.as_mut());
    executor::block_on(future.as_mut());
}

they get a panic talking about generators:

thread 'main' panicked at 'generator resumed after completion', src/main.rs:1:16

This issue has been assigned to @ninjasource via this comment.

[nikomatsakis]

Hey @Zoxc -- where does this error come from, anyway? A quick ripgrep of the source didn't find an obvious source.

[nikomatsakis]

Is it from the Display impl for PanicInfo?

[Centril]

Has to be https://doc.rust-lang.org/nightly/nightly-rustc/rustc/mir/interpret/enum.PanicInfo.html#method.description.

[Zoxc]

Yeah that's it.

[nikomatsakis]

OK, yes, that's what I meant. So to fix this we presumably want to:

• Add a new PanicInfo variant with a different description
• Use that PanicInfo variant for generators that arose from async fn

We would need to get access to the generator_kind for this generator in this function here;

rust/src/librustc_mir/transform/generator.rs

Lines 989 to 995 in 437ca55

	fn create_generator_resume_function<'tcx>(
	tcx: TyCtxt<'tcx>,
	transform: TransformVisitor<'tcx>,
	def_id: DefId,
	source: MirSource<'tcx>,
	body: &mut Body<'tcx>,
	) {

Right now, I don't think that is stored in the MIR, but we could easily do do (I'd rather not retrieve it from the HIR).

Sound about right?

[ninjasource]

@rustbot claim

[ninjasource]

Hi @nikomatsakis, that makes sense. I'll lean on @gilescope for initial guidance on this and I'll add more comments after I dig a little deeper. Cheers, David

[gilescope]

@tmandry we can close this one - @ninjasource has fixed this.