State machines aren't a general solution — you have to write a state machine by hand for every application.

Stackless coroutines are — the compiler (or a clever library) transforms your ordinary function into a state machine.

Yes, I know. That's exactly what I mean by syntactic sugar: it is an automated way to write something that could already be written, but is annoying to do by hand.

Oh, OK. When I hear syntactic sugar, I think of like a macro. Thanks though, I'm glad this is on the core team's radar.