If your language has macros, you don't need to wait for it to gain async/await (and it was quite a wait even in Python and JS, while Java got its virtual threads very recently); you can just[1] implement a CPS transform as a macro:

https://github.com/nim-works/cps/blob/master/docs/cps.svg

[1] Yeah, I know, this word is extremely overworked in that sentence. Sorry, that's an exaggeration for effect. In reality, it's not that simple, and not always possible: it depends on the language and macro system. It's still more likely to be implementable in library code if there is a macro system, though.