[steveklabnik]

It is not possible to implement async/await efficiently and safely with macros, and so doing so would be “contradicting the language’s mission.”

They were considered and even originally built that way, but empirically it was worse.

[erickt]

I’d go further and say it’s not possible to fully implement async/await without compiler help.

I got really far with stateful, back in 2016 [1]. Stateful was an attempt to write a coroutine library in a proc macro, which generated state machines, as opposed to using os primitives like green threading. This was back before the rust community really started working in this space. I ended up extracting the type system from rustc to do much of the analysis, but it ultimately failed due to how difficult it was to output rust code that respected the borrow checker rules. I also didn’t have anything like the pinning system, so I couldn’t catch move issues either.

It was a much better idea to just implement this in the compiler.

[1]: http://erickt.github.io/blog/2016/01/27/stateful-in-progress...

[api]

Hmm… didn’t know they tried.

Personally I just loathe async in general. Go has it right, but I understand why you can’t do that in Rust. Async is an ugly workaround for the inefficiency of OS threads, and I wish they would just fix that so we can stop all this madness.