Fortunately, `futures::join!` isn't provided by Rust- it's provided by library code. I am not aware (off the top of my head) of an existing equivalent to your `

Fortunately, `futures::join!` isn't provided by Rust- it's provided by library code. I am not aware (off the top of my head) of an existing equivalent to your `Promise.map` example, but it is also implementable in userspace in Rust. The primitive operation provided by a Rust `Future` is `poll`. Calling `some_future.poll(waker)` advances it forward if possible, and stashes `waker` somewhere for it to be signaled when `some_future` is ready to run again. So the implementation of `join` is fairly straightforward: It constructs a new future wrapping its arguments, which when polled itself, re-polls each of them with the same waker it was passed. There are also more elaborate schemes- e.g. `FuturesUnordered` uses a separate waker for each sub-future, so it can handle larger numbers of them at some coordination cost.