Comment by anonymoushn

Lua comes with this sort of thing. OCaml, Python, and C have libraries providing this sort of thing in decreasing order of adoption.

Python also comes with 2 features that seem to be stackless coroutines with attached syntax ceremonies, but one of those 2 features is commonly used with a hefty runtime instead of being used for control flow. JavaScript comes with 2 features named similarly to those of Python, but only one of them seems to be "runtime-free" stackless coroutines.