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nothing about how it works is built into the language, its up to user code/libraries to decide how to do it.

the popular libraries like tokio are using something akin to green threads, though people picky about definitions may not agree to that nomenclature.

> nothing about how it works is built into the language

I think the state machine required to yield, async & await is baked into the language.

But yes, one can implement the Future trait to take finer control.