I think I understand what you mean. Lets say you have two task, the Main and the Worker1, in this case you are polling the worker1 to wait for an answer; BUT there is a better way, and this is to wait for competition of the Worker1; and this can be done without having any Future, you simply call from Main the Worker1 function, when the worker is over the Main will go on. You need no future, you are simply calling a function, and the division Main and Worker1 is just an over-complication.

Now, I think your question became relevant in the moment you add at least another worker, last add Worker2, and you want the Main to resume the computation as soon as one of the two task complete; and you don't want those task to be executed in another thread/process, maybe because you are using asynchronous call (which simply mean the threading is done somewhere else, or you are low level enough that you receive Hardware Interrupt).

Since your Worker1 and Worker2 have to share the same thread, you need a way to save the current execution Main, create the one for one of the worker, and after a certain amount of work, time or other even (Scheduler), switch to the other worker, and so on. This is a Multi-Tasking system, and there are various software implementation for it in Rust; but with HW support you could do things that in software only you could not do (like have the hardware prevent one Task to access the resource from the other), plus you can have the CPU take care of the task switching and all... Well, this is what Thread and Process are.

Future are not what you are looking for, they are higher level and you can find some software scheduler that support them.