Adding short-circuit if there's no children, to save a compare

src/k_smallest.rs

@@ -152,6 +152,10 @@ where
         let (left_idx, right_idx) = children_of(origin);
         let (left_item, right_item) = (self.get(left_idx), self.get(right_idx));
 
+        if left_item.is_none() { // the left is the earlier child, so if it doesn't exist there's nothing to swap with
+            return;
+        }
+
         let cmp = self
             .compare(left_item, right_item)
             .unwrap_or(Ordering::Greater); // The right item may not exist, so default to picking the left, i.e. lower