Manually implement Debug for BTreeMap::ValuesMut struct

Deriving debug prints all the values including keys. But ValuesMut
struct should only print the values.

library/alloc/src/collections/btree/map.rs

@@ -361,11 +361,17 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 ///
 /// [`values_mut`]: BTreeMap::values_mut
 #[stable(feature = "map_values_mut", since = "1.10.0")]
-#[derive(Debug)]
 pub struct ValuesMut<'a, K: 'a, V: 'a> {
     inner: IterMut<'a, K, V>,
 }
 
+#[stable(feature = "map_values_mut", since = "1.10.0")]
+impl<K, V: fmt::Debug> fmt::Debug for ValuesMut<'_, K, V> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_list().entries(self.inner.iter().map(|(_, val)| val)).finish()
+    }
+}
+
 /// An owning iterator over the keys of a `BTreeMap`.
 ///
 /// This `struct` is created by the [`into_keys`] method on [`BTreeMap`].
@@ -1519,6 +1525,14 @@ fn len(&self) -> usize {
 #[stable(feature = "fused", since = "1.26.0")]
 impl<K, V> FusedIterator for IterMut<'_, K, V> {}
 
+impl<'a, K, V> IterMut<'a, K, V> {
+    /// Returns an iterator of references over the remaining items.
+    #[inline]
+    pub(super) fn iter(&self) -> Iter<'_, K, V> {
+        Iter { range: self.range.iter(), length: self.length }
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V> IntoIterator for BTreeMap<K, V> {
     type Item = (K, V);
@@ -2006,6 +2020,15 @@ fn is_empty(&self) -> bool {
     unsafe fn next_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
         unsafe { unwrap_unchecked(self.front.as_mut()).next_unchecked() }
     }
+
+    /// Returns an iterator of references over the remaining items.
+    #[inline]
+    pub(super) fn iter(&self) -> Range<'_, K, V> {
+        Range {
+            front: self.front.as_ref().map(|f| f.reborrow()),
+            back: self.back.as_ref().map(|b| b.reborrow()),
+        }
+    }
 }
 
 #[stable(feature = "btree_range", since = "1.17.0")]