Well, you can still have select; it "just" has to react to one of the futures becoming ready by cancelling all the other ones and waiting (asynchronously) for the cancellation to be complete. Future doesn't currently have a "cancel" method, but I guess it would just be represented as async drop. So this requires some way of enforcing that async drop is called, which is hard, but I believe it's equally hard as enforcing that futures are polled to completion: either way you're requiring that some method on the future be called, and polled on, before the memory the future refers to can be reused. For the sake of this post I'll assume it's somehow possible.
Having to wait for cancellation does sound expensive, especially if the end goal is to pervasively use APIs like io_uring where cancellation can be slow.
But then, in a typical use of select, you don't actually want to cancel the I/O operations represented by the other futures. Rather, you're running select in a loop in order to handle each completed operation as it comes.
So I think the endgame of this hypothetical world is to encourage having the actual I/O be initiated by a Future or Stream created outside the loop. Then within the loop you would poll on `&mut future` or `stream.next()`. This already exists and is already cheaper in some cases, but it would be significantly cheaper when the backend is io_uring.
Having to wait for cancellation does sound expensive, especially if the end goal is to pervasively use APIs like io_uring where cancellation can be slow.
But then, in a typical use of select, you don't actually want to cancel the I/O operations represented by the other futures. Rather, you're running select in a loop in order to handle each completed operation as it comes.
So I think the endgame of this hypothetical world is to encourage having the actual I/O be initiated by a Future or Stream created outside the loop. Then within the loop you would poll on `&mut future` or `stream.next()`. This already exists and is already cheaper in some cases, but it would be significantly cheaper when the backend is io_uring.