combinations: count and size_hint

src/combinations.rs

@@ -135,6 +135,11 @@ impl<I> Iterator for Combinations<I>
         // Create result vector based on the indices
         Some(self.indices.iter().map(|i| self.pool[*i].clone()).collect())
     }
+
+    fn count(mut self) -> usize {
+        self.pool.fill();
+        self.remaining_for(self.n()).expect("Iterator count greater than usize::MAX")
+    }
 }
 
 impl<I> FusedIterator for Combinations<I>

tests/test_std.rs

@@ -909,6 +909,20 @@ fn combinations_zero() {
     it::assert_equal((0..0).combinations(0), vec![vec![]]);
 }
 
+#[test]
+fn combinations_range_count() {
+    for n in 0..6 {
+        for k in 0..=n {
+            let len = (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>();
+            let mut it = (0..n).combinations(k);
+            for count in (0..=len).rev() {
+                assert_eq!(it.clone().count(), count);
+                assert_eq!(it.next().is_none(), count == 0);
+            }
+        }
+    }
+}
+
 #[test]
 fn permutations_zero() {
     it::assert_equal((1..3).permutations(0), vec![vec![]]);