Comment by zamalek

It's sans-io at the language level, I like the concept.

So I did a bit of research into how this works in Zig under the hood, in terms of compilation.

First things first, Zig does compile async fns to a state machine: https://github.com/ziglang/zig/issues/23446

The compiler decides at compile time which color to compile the function as (potentially both). That's a neat idea, but... https://github.com/ziglang/zig/issues/23367

> It would be checked illegal behavior to make an indirect call through a pointer to a restricted function type when the value of that pointer is not in the set of possible callees that were analyzed during compilation.

That's... a pretty nasty trade-off. Object safety in Rust is really annoying for async, and this smells a lot like it. The main difference is that it's vaguely late-bound in a magical way; you might get an unexpected runtime error and - even worse - potentially not have the tools to force the compiler to add a fn to the set of callees.

I still think sans-io at the language level might be the future, but this isn't a complete solution. Maybe we should be simply compiling all fns to state machines (with the Rust polling implementation detail, a sans-io interface could be used to make such functions trivially sync - just do the syscall and return a completed future).