Cannot define async fn with no_std

[Nemo157]

#![feature(async_await, futures_api)]

#![no_std]

pub async fn foo() {
}

(playground) currently fails with

error[E0433]: failed to resolve: could not find `from_generator` in `future`

This should also work with whatever the syntax for await is, currently it fails earlier than this error because of await! only being defined in std, see #56767.

[oli-obk]

cc @phil-opp

[nikomatsakis]

Closing as duplicate of #56974.

[nikomatsakis]

Actually, perhaps this is not entirely a duplicate. Regardless, marking as non-blocking.

[nikomatsakis]

Marking as not blocking stabilization -- obviously we want things to work from core eventually, but it's ok if that doesn't work to start, and I don't believe there are any major design questions at play here (right @cramertj?)

[cramertj]

Yup-- this is an unfortunate restriction, but I think it's not a blocker for an MVP and resolving it requires some more complex investigation into generator arguments + some special-sauce :)

[rubdos]

Could someone indicate how difficult this would be to tackle as a new Rust contributor? I fear like this might be a bit over my head, but I'd love to know.

[cramertj]

It's not easy. There needs to be a way to pass an argument into the generator resume function which can be accessed by any .await call. Neither generator arguments nor any mechanism for accessing such a bizarrely scoped local variable exist today.