k_smollest: Add debug_assert for invariant

Author: nicoonoclaste

src/k_smallest.rs

@@ -7,6 +7,7 @@ pub(crate) fn k_smallest<T: Ord, I: Iterator<Item = T>>(mut iter: I, k: usize) -
     let mut heap = iter.by_ref().take(k).collect::<BinaryHeap<_>>();
 
     for i in iter {
+        debug_assert_eq!(heap.len(), k);
         // Guaranteed not-None, since we keep exactly k>0 elements in the heap.
         let mut lorgest = heap.peek_mut().unwrap();
         if *lorgest > i { *lorgest = i; }