FuturesUnordered and yield_now()

[duarten]

FuturesUnordered detects whether the futures it is managing intend to yield by checking whether the waker has been notified when the poll result was Pending. If there are two instances of this, FuturesUnordered will itself yield.

However, Tokio's yield_now utility doesn't follow this pattern, as it defers so the waker is only notified after the scheduler runs the I/O loop (from what I could tell). In this playground you can see how these two behaviors of yielding change the schedule order.

It also seems that Tokio has a similar detection, so I was wondering if it makes sense to change Tokio's yield_now to immediately wake the waker?

[Darksonn]

We used to do that in the past, but we changed it in #5223.

[duarten]

Is there a different way to implement #5223, or is the situation just hopeless until something like this proposal comes to be?