Feature request: `drain_filter` method about priority-queue HOT 1 OPEN

It makes sense to me to wait for the std API to be stabilized, to avoid the need for breaking changes in this lib or to have a divergent API with respect to the std.

Some comments on your assumptions though. For the elements to be returned in order, the algorithm needs to be O(n log(n)).
Moreover, being able to iterate in order and to skip elements (i.e. don't remove the elements from the queue) is more challenging then you are prospecting, because of the way the heap algorithm works.

In my opinion, also to be consistent with other iterators (like iter and iter_mut) also the drain_filter implementation should return the elements in arbitrary order.

To have a drain_filter iterator in sorted order, one could convert the priority queue into_sorted_vec, then drain_filter it and finally convert it back into a PriorityQueue.

The complexity of unsorted drain_filter is O(n), while converting the queue into a sorted Vec is O(n log(n)), drain filtering it is O(n) and getting a PriorityQueue back would be O(m log(m)), where m is the number of remaining elements. So the total operation would be O(n log(n)) with one additional allocation for the sorted Vec

from priority-queue.