> Rust has green threads in the early days and abandoned them in favour of async / await. Granted the original green thread implementation needed a bit of refinement - making every low level choose between event driven and blocking on every invocation was a mistake.

That's a mischaraterization. They were abandoned because having green threads introduces non-trivial runtime. It means Rust can't run on egzotic architectures.

> It sounds like Zig with its pluggable I/O interface finally got it right

That remains to be seen. It looks good, with emphasis on looks. Who knows what interesting design constraints and limitation that entails.

Looking at comptime, which is touted as Zig's mega feature, it does come at expense of a more strictly typed system.