Rust's interface at the lowest level is just a single method, which in C++ could roughly look like:
optional<T> poll(*waker_t)
When polled it either returns the final result or that it's pending. If it's pending, it keeps the reference to the waker arg, and uses it to notify when it's ready to be polled again.
This design is very composable, because a Future can trivially forward the poll calls to other futures, and the wakers can be plugged into all kinds of existing callback-based APIs.
async/await is a syntax sugar that auto-generates the poll methods from async function's bodies (where each poll() call advances state to the next .await point).
There's no built-in runtime in the language that does the polling. You make your own event loop, and that gives you freedom to make it simple, or fancy multithreaded, or deterministic for test environments, etc.
This design is very composable, because a Future can trivially forward the poll calls to other futures, and the wakers can be plugged into all kinds of existing callback-based APIs.
async/await is a syntax sugar that auto-generates the poll methods from async function's bodies (where each poll() call advances state to the next .await point).
There's no built-in runtime in the language that does the polling. You make your own event loop, and that gives you freedom to make it simple, or fancy multithreaded, or deterministic for test environments, etc.