Implement Default for iterators

This matches Default implementations introduced for HashMap and
HashSet in Rust 1.70.

src/map/iter.rs

@@ -89,6 +89,12 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Iter<'_, K, V> {
     }
 }
 
+impl<K, V> Default for Iter<'_, K, V> {
+    fn default() -> Self {
+        Self { iter: [].iter() }
+    }
+}
+
 /// A mutable iterator over the entries of a `IndexMap`.
 ///
 /// This `struct` is created by the [`iter_mut`] method on [`IndexMap`]. See its
@@ -145,6 +151,14 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IterMut<'_, K, V> {
     }
 }
 
+impl<K, V> Default for IterMut<'_, K, V> {
+    fn default() -> Self {
+        Self {
+            iter: [].iter_mut(),
+        }
+    }
+}
+
 /// An owning iterator over the entries of a `IndexMap`.
 ///
 /// This `struct` is created by the [`into_iter`] method on [`IndexMap`]
@@ -199,6 +213,14 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IntoIter<K, V> {
     }
 }
 
+impl<K, V> Default for IntoIter<K, V> {
+    fn default() -> Self {
+        Self {
+            iter: Vec::new().into_iter(),
+        }
+    }
+}
+
 /// A draining iterator over the entries of a `IndexMap`.
 ///
 /// This `struct` is created by the [`drain`] method on [`IndexMap`]. See its
@@ -298,6 +320,12 @@ impl<K: fmt::Debug, V> fmt::Debug for Keys<'_, K, V> {
     }
 }
 
+impl<K, V> Default for Keys<'_, K, V> {
+    fn default() -> Self {
+        Self { iter: [].iter() }
+    }
+}
+
 /// An owning iterator over the keys of a `IndexMap`.
 ///
 /// This `struct` is created by the [`into_keys`] method on [`IndexMap`].
@@ -342,6 +370,14 @@ impl<K: fmt::Debug, V> fmt::Debug for IntoKeys<K, V> {
     }
 }
 
+impl<K, V> Default for IntoKeys<K, V> {
+    fn default() -> Self {
+        Self {
+            iter: Vec::new().into_iter(),
+        }
+    }
+}
+
 /// An iterator over the values of a `IndexMap`.
 ///
 /// This `struct` is created by the [`values`] method on [`IndexMap`]. See its
@@ -394,6 +430,12 @@ impl<K, V: fmt::Debug> fmt::Debug for Values<'_, K, V> {
     }
 }
 
+impl<K, V> Default for Values<'_, K, V> {
+    fn default() -> Self {
+        Self { iter: [].iter() }
+    }
+}
+
 /// A mutable iterator over the values of a `IndexMap`.
 ///
 /// This `struct` is created by the [`values_mut`] method on [`IndexMap`]. See its
@@ -438,6 +480,14 @@ impl<K, V: fmt::Debug> fmt::Debug for ValuesMut<'_, K, V> {
     }
 }
 
+impl<K, V> Default for ValuesMut<'_, K, V> {
+    fn default() -> Self {
+        Self {
+            iter: [].iter_mut(),
+        }
+    }
+}
+
 /// An owning iterator over the values of a `IndexMap`.
 ///
 /// This `struct` is created by the [`into_values`] method on [`IndexMap`].
@@ -481,3 +531,11 @@ impl<K, V: fmt::Debug> fmt::Debug for IntoValues<K, V> {
         f.debug_list().entries(iter).finish()
     }
 }
+
+impl<K, V> Default for IntoValues<K, V> {
+    fn default() -> Self {
+        Self {
+            iter: Vec::new().into_iter(),
+        }
+    }
+}

src/map/tests.rs

@@ -427,3 +427,20 @@ fn from_array() {
 
     assert_eq!(map, expected)
 }
+
+#[test]
+fn iter_default() {
+    struct K;
+    struct V;
+    fn assert_default<T: Default + Iterator>() {
+        assert!(T::default().next().is_none());
+    }
+    assert_default::<Iter<'static, K, V>>();
+    assert_default::<IterMut<'static, K, V>>();
+    assert_default::<IntoIter<K, V>>();
+    assert_default::<Keys<'static, K, V>>();
+    assert_default::<IntoKeys<K, V>>();
+    assert_default::<Values<'static, K, V>>();
+    assert_default::<ValuesMut<'static, K, V>>();
+    assert_default::<IntoValues<K, V>>();
+}

src/set/iter.rs

@@ -80,6 +80,12 @@ impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     }
 }
 
+impl<T> Default for Iter<'_, T> {
+    fn default() -> Self {
+        Self { iter: [].iter() }
+    }
+}
+
 /// An owning iterator over the items of a `IndexSet`.
 ///
 /// This `struct` is created by the [`into_iter`] method on [`IndexSet`]
@@ -129,6 +135,14 @@ impl<T: fmt::Debug> fmt::Debug for IntoIter<T> {
     }
 }
 
+impl<T> Default for IntoIter<T> {
+    fn default() -> Self {
+        Self {
+            iter: Vec::new().into_iter(),
+        }
+    }
+}
+
 /// A draining iterator over the items of a `IndexSet`.
 ///
 /// This `struct` is created by the [`drain`] method on [`IndexSet`].

src/set/tests.rs

@@ -530,3 +530,16 @@ fn from_array() {
 
     assert_eq!(set1, set2);
 }
+
+#[test]
+fn iter_default() {
+    struct Item;
+    fn assert_default<T>()
+    where
+        T: Default + Iterator,
+    {
+        assert!(T::default().next().is_none());
+    }
+    assert_default::<Iter<'static, Item>>();
+    assert_default::<IntoIter<Item>>();
+}