So what then is async? If there is no scheduling/preemption in executing an "async" function, it's not really "async", but rather synchronous. Is async just a new way to say "instruction reordering"? Or is async really just "*defer* until the last possible moment: await"?

edit to match yours: not sure what a "green" thread is.

In any case, if threads are not an option, an async may always be ignorable, after all, a program may not rely on the order of the execution of its threads/its preemption; that would be a race condition. Textbook definition.

As I see it, the fork/join model is very useful for "shingling" code paths that don't have to happen in sequence, but _could_ happen in parallel.

Final edit: nothing is otherwise stopping the implementer of async to add a pre-started threadpool to the runtime, with whatever configurable parameters one would want.

Post final edit: I think fork/join vs send/sync are just implementation details. But I respect Rust for going with send/sync, just that it doesn't change all that much in terms of the programming interface.