Check two invariants

src/k_smallest.rs

@@ -57,6 +57,7 @@ where
     }
 
     iter.for_each(|val| {
+        debug_assert_eq!(storage.len(), k);
         if is_less_than(&val, &storage[0]) {
             // Treating this as an push-and-pop saves having to write a sift-up implementation.
             // https://en.wikipedia.org/wiki/Binary_heap#Insert_then_extract

src/lib.rs

@@ -3190,6 +3190,7 @@ pub trait Itertools: Iterator {
                 let mut data: Vec<_> = iter.by_ref().take(n).collect();
                 // Update `data` cyclically.
                 let idx = iter.fold(0, |i, val| {
+                    debug_assert_eq!(data.len(), n);
                     data[i] = val;
                     if i + 1 == n {
                         0