Doesn't that assume there are no intrinsic differences between single-threaded, multi-threaded and async execution? Plenty of code works fine single-threaded but breaks once it's run multi-threaded or async, and some works fine multi-threaded but breaks under async. With explicit async you have precise control over where you yield back to the event loop. With Zig's Io you don't get that unless you deliberately design for it, and at that point you've just got an implicitly coloured function.

> it allows the async implementation to do the "prepare operation, suspend until completed, wake up" dance, where the threaded implementation would just run the blocking syscall directly. There are plans to add stackless coroutines

Yeah so that's a lot of hidden magic and implicit logic. ¯\_(ツ)_/¯