BTreeMap: fix pointer provenance rules in underfullness

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

@@ -89,20 +89,15 @@ impl<K, V> Root<K, V> {
  let mut cur_node = self.node_as_mut();
  while let Internal(internal) = cur_node.force() {
  // Check if right-most child is underfull.
- let mut last_edge = internal.last_edge();
- let right_child_len = last_edge.reborrow().descend().len();
+ let mut last_kv = internal.last_kv().consider_for_balancing();
+ let right_child_len = last_kv.right_child_len();
  if right_child_len < MIN_LEN {
  // We need to steal.
- let mut last_kv = match last_edge.left_kv() {
- Ok(left) => left,
- Err(_) => unreachable!(),
- };
  last_kv.bulk_steal_left(MIN_LEN - right_child_len);
- last_edge = last_kv.right_edge();
  }
 
  // Go further down.
- cur_node = last_edge.descend();
+ cur_node = last_kv.into_right_child();
  }
  }
 }

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

@@ -6,6 +6,7 @@ use core::ptr;
 /// relevant function.
 ///
 /// If a panic occurs in the `change` closure, the entire process will be aborted.
+#[allow(dead_code)] // keep as illustration and for future use
 #[inline]
 pub fn take_mut<T>(v: &mut T, change: impl FnOnce(T) -> T) {
  replace(v, |value| (change(value), ()))

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

@@ -362,20 +362,6 @@ impl<'a, K, V> Handle<NodeRef<marker::ValMut<'a>, K, V, marker::Leaf>, marker::E
  }
 }
 
-impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge> {
- /// Moves the leaf edge handle to the next leaf edge.
- ///
- /// # Safety
- /// There must be another KV in the direction travelled.
- pub unsafe fn move_next_unchecked(&mut self) {
- super::mem::take_mut(self, |leaf_edge| {
- let kv = leaf_edge.next_kv();
- let kv = unsafe { unwrap_unchecked(kv.ok()) };
- kv.next_leaf_edge()
- })
- }
-}
-
 impl<K, V> Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge> {
  /// Moves the leaf edge handle to the next leaf edge and returns the key and value
  /// in between, deallocating any node left behind while leaving the corresponding