> No, just have select!() on a bunch of owned Futures return the futures that weren't selected instead of dropping them. Then you don't lose state.

How does that prevent this kind of deadlock? If the owned future has acquired a mutex, and you return that future from the select so that it might be polled again, and the user assigns it to a variable, then the future that has acquired the mutex but has not completed is still not dropped. This is basically the same as polling an `&mut future`, but with more steps.