What is a future?

A future is a representation of some operation that will be completed in the future.

Async in Rust uses a poll-based approach in which an asynchronous task will have three phases:

1. The poll phase: A future is polled, which results in the task progressing until a point where it can no longer make progress. We often refer to the part of the runtime that polls a future as an executor.
2. The wait phase: An event source, most often referred to as a reactor, registers that a future is waiting for an event to happen and makes sure that it will wake the future when that event is ready.
3. The wake phase: The event happens and the future is woken up. It’s now up to the executor that polled the future in step 1 to schedule the future to be polled again and make further progress until it completes or reaches a new point where it can’t make further progress and the cycle repeats.

Now, when we talk about futures, I find it useful to make a distinction...