Rollup merge of #75195 - ssomers:btree_split_up_into_kv_mut, r=Mark-Simulacrum

BTreeMap: purge innocent use of into_kv_mut

Replace the use of `into_kv_mut` into more precise calls. This makes more sense if you know that the single remaining use of `into_kv_mut` is in fact evil and can be trialled in court (#75200) and sent to a correction facility (#73971).

No real performance difference reported (but functions that might benefit a tiny constant bit like `BTreeMap::get_mut` aren't benchmarked):
```
benchcmp old new --threshold 5
 name                       old ns/iter  new ns/iter  diff ns/iter  diff %  speedup
 btree::map::clone_fat_100  63,073       59,256             -3,817  -6.05%   x 1.06
 btree::map::iter_100       3,514        3,235                -279  -7.94%   x 1.09
```

2 files changed, 19 insertions, 9 deletions

diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index e7d243bfcb0..8e800f48c69 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -245,7 +245,7 @@ where
     fn replace(&mut self, key: K) -> Option<K> {
         let root = Self::ensure_is_owned(&mut self.root);
         match search::search_tree::<marker::Mut<'_>, K, (), K>(root.node_as_mut(), &key) {
-            Found(handle) => Some(mem::replace(handle.into_kv_mut().0, key)),
+            Found(handle) => Some(mem::replace(handle.into_key_mut(), key)),
             GoDown(handle) => {
                 VacantEntry { key, handle, length: &mut self.length, _marker: PhantomData }
                     .insert(());
@@ -811,7 +811,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     {
         let root_node = self.root.as_mut()?.node_as_mut();
         match search::search_tree(root_node, key) {
-            Found(handle) => Some(handle.into_kv_mut().1),
+            Found(handle) => Some(handle.into_val_mut()),
             GoDown(_) => None,
         }
     }
@@ -2748,7 +2748,7 @@ impl<'a, K: Ord, V> OccupiedEntry<'a, K, V> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn into_mut(self) -> &'a mut V {
-        self.handle.into_kv_mut().1
+        self.handle.into_val_mut()
     }
 
     /// Sets the value of the entry with the `OccupiedEntry`'s key,
diff --git a/library/alloc/src/collections/btree/node.rs b/library/alloc/src/collections/btree/node.rs
index c0b75fd5eac..b0741a2c00d 100644
--- a/library/alloc/src/collections/btree/node.rs
+++ b/library/alloc/src/collections/btree/node.rs
@@ -1067,6 +1067,16 @@ impl<'a, K: 'a, V: 'a, NodeType> Handle<NodeRef<marker::Immut<'a>, K, V, NodeTyp
 }
 
 impl<'a, K: 'a, V: 'a, NodeType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, marker::KV> {
+    pub fn into_key_mut(self) -> &'a mut K {
+        let keys = self.node.into_key_slice_mut();
+        unsafe { keys.get_unchecked_mut(self.idx) }
+    }
+
+    pub fn into_val_mut(self) -> &'a mut V {
+        let vals = self.node.into_val_slice_mut();
+        unsafe { vals.get_unchecked_mut(self.idx) }
+    }
+
     pub fn into_kv_mut(self) -> (&'a mut K, &'a mut V) {
         unsafe {
             let (keys, vals) = self.node.into_slices_mut();
@@ -1261,8 +1271,8 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
         unsafe {
             let (k, v, edge) = self.reborrow_mut().left_edge().descend().pop();
 
-            let k = mem::replace(self.reborrow_mut().into_kv_mut().0, k);
-            let v = mem::replace(self.reborrow_mut().into_kv_mut().1, v);
+            let k = mem::replace(self.kv_mut().0, k);
+            let v = mem::replace(self.kv_mut().1, v);
 
             match self.reborrow_mut().right_edge().descend().force() {
                 ForceResult::Leaf(mut leaf) => leaf.push_front(k, v),
@@ -1278,8 +1288,8 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
         unsafe {
             let (k, v, edge) = self.reborrow_mut().right_edge().descend().pop_front();
 
-            let k = mem::replace(self.reborrow_mut().into_kv_mut().0, k);
-            let v = mem::replace(self.reborrow_mut().into_kv_mut().1, v);
+            let k = mem::replace(self.kv_mut().0, k);
+            let v = mem::replace(self.kv_mut().1, v);
 
             match self.reborrow_mut().left_edge().descend().force() {
                 ForceResult::Leaf(mut leaf) => leaf.push(k, v),
@@ -1307,7 +1317,7 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
                 let left_kv = left_node.reborrow_mut().into_kv_pointers_mut();
                 let right_kv = right_node.reborrow_mut().into_kv_pointers_mut();
                 let parent_kv = {
-                    let kv = self.reborrow_mut().into_kv_mut();
+                    let kv = self.kv_mut();
                     (kv.0 as *mut K, kv.1 as *mut V)
                 };
 
@@ -1364,7 +1374,7 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
                 let left_kv = left_node.reborrow_mut().into_kv_pointers_mut();
                 let right_kv = right_node.reborrow_mut().into_kv_pointers_mut();
                 let parent_kv = {
-                    let kv = self.reborrow_mut().into_kv_mut();
+                    let kv = self.kv_mut();
                     (kv.0 as *mut K, kv.1 as *mut V)
                 };