Auto merge of #78631 - ssomers:btree-alias_for_underfull, r=Mark-Simulacrum

BTreeMap: fix pointer provenance rules in underfullness

Continuing on #78480, and for readability, and possibly for performance: avoid aliasing when handling underfull nodes, and consolidate the code doing that. In particular:
- Avoid the rather explicit aliasing for internal nodes in `remove_kv_tracking`.
- Climb down to the root to handle underfull nodes using a reborrowed handle, rather than one copied with `ptr::read`, before resuming on the leaf level.
- Integrate the code tracking leaf edge position into the functions performing changes, rather than bolting it on.

r? `@Mark-Simulacrum`

Author: bors

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