Rollup merge of rust-lang#128261 - clarfonthey:iter-default, r=dtolnay

impl `Default` for collection iterators that don't already have it

There is a pretty strong precedent for implementing `Default` for collection iterators, and this does so for some where this implementation was missed.

I don't think this needs a separate ACP (since this precedent already exists, and these feel like they were just missed), however, it *will* need an FCP since these implementations are instantly stable.

alloc/src/collections/binary_heap/mod.rs

@@ -1433,6 +1433,20 @@ pub struct Iter<'a, T: 'a> {
     iter: slice::Iter<'a, T>,
 }
 
+#[stable(feature = "default_iters_sequel", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Iter<'_, T> {
+    /// Creates an empty `binary_heap::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::binary_heap;
+    /// let iter: binary_heap::Iter<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { iter: Default::default() }
+    }
+}
+
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

alloc/src/collections/btree/map.rs

@@ -2016,6 +2016,20 @@ impl<K, V> Default for Range<'_, K, V> {
     }
 }
 
+#[stable(feature = "default_iters_sequel", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for RangeMut<'_, K, V> {
+    /// Creates an empty `btree_map::RangeMut`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::RangeMut<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        RangeMut { inner: Default::default(), _marker: PhantomData }
+    }
+}
+
 #[stable(feature = "map_values_mut", since = "1.10.0")]
 impl<'a, K, V> Iterator for ValuesMut<'a, K, V> {
     type Item = &'a mut V;
@@ -2050,6 +2064,20 @@ impl<K, V> ExactSizeIterator for ValuesMut<'_, K, V> {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<K, V> FusedIterator for ValuesMut<'_, K, V> {}
 
+#[stable(feature = "default_iters_sequel", since = "CURRENT_RUSTC_VERSION")]
+impl<K, V> Default for ValuesMut<'_, K, V> {
+    /// Creates an empty `btree_map::ValuesMut`.
+    ///
+    /// ```
+    /// # use std::collections::btree_map;
+    /// let iter: btree_map::ValuesMut<'_, u8, u8> = Default::default();
+    /// assert_eq!(iter.count(), 0);
+    /// ```
+    fn default() -> Self {
+        ValuesMut { inner: Default::default() }
+    }
+}
+
 #[stable(feature = "map_into_keys_values", since = "1.54.0")]
 impl<K, V, A: Allocator + Clone> Iterator for IntoKeys<K, V, A> {
     type Item = K;

alloc/src/collections/vec_deque/iter.rs

@@ -28,6 +28,20 @@ impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     }
 }
 
+#[stable(feature = "default_iters_sequel", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for Iter<'_, T> {
+    /// Creates an empty `vec_deque::Iter`.
+    ///
+    /// ```
+    /// # use std::collections::vec_deque;
+    /// let iter: vec_deque::Iter<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        Iter { i1: Default::default(), i2: Default::default() }
+    }
+}
+
 // FIXME(#26925) Remove in favor of `#[derive(Clone)]`
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Clone for Iter<'_, T> {

alloc/src/collections/vec_deque/iter_mut.rs

@@ -28,6 +28,20 @@ impl<T: fmt::Debug> fmt::Debug for IterMut<'_, T> {
     }
 }
 
+#[stable(feature = "default_iters_sequel", since = "CURRENT_RUSTC_VERSION")]
+impl<T> Default for IterMut<'_, T> {
+    /// Creates an empty `vec_deque::IterMut`.
+    ///
+    /// ```
+    /// # use std::collections::vec_deque;
+    /// let iter: vec_deque::IterMut<'_, u8> = Default::default();
+    /// assert_eq!(iter.len(), 0);
+    /// ```
+    fn default() -> Self {
+        IterMut { i1: Default::default(), i2: Default::default() }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'a, T> Iterator for IterMut<'a, T> {
     type Item = &'a mut T;