The short answer is that an async function in rust (and C++ and C#) is rewritten by the compiler to become a state machine (a struct + an enum to discriminate t

The short answer is that an async function in rust (and C++ and C#) is rewritten by the compiler to become a state machine (a struct + an enum to discriminate the current state + an union store local variables). You could do it by hand, though of course it's tedious and error prone. Hence why almost nobody writes software like that. For the compiler, the state machine code is amenable to optimizations. You can find examples online where a chain of async function is optimized to a single call in the main. In contrast green thread and the like are very similar to a kernel thread. The difference is they are lighter because there is less bagage to carry around. But composing green threads require an arguably light context switch.