Generally the way async Rust works is that you have a Future trait with a poll method, and if you call it the future will attempt to make progress if it can — e.g. if the task is a timer it will check the time and complete the task if so and otherwise return Pending.
However, async Rust includes an additional concept: Wakers. When your runtime (Tokio) calls poll on a future, it gives the future a waker object, and when the future is ready to continue work, something needs to call wake on the waker. Once this happens, Tokio will poll the task again soon, and Tokio wont poll tasks that have not been woken.
For example, for timers, Tokio includes a timer wheel (a sort of priority queue) with all the registered timers, and the timer wheel calls wake on the appropriate waker whenever a timer expires. Similarly with a message passing channel, the sender calls wake on the receiver's waker when a message is sent.
However, async Rust includes an additional concept: Wakers. When your runtime (Tokio) calls poll on a future, it gives the future a waker object, and when the future is ready to continue work, something needs to call wake on the waker. Once this happens, Tokio will poll the task again soon, and Tokio wont poll tasks that have not been woken.
For example, for timers, Tokio includes a timer wheel (a sort of priority queue) with all the registered timers, and the timer wheel calls wake on the appropriate waker whenever a timer expires. Similarly with a message passing channel, the sender calls wake on the receiver's waker when a message is sent.