Auto merge of #75257 - ssomers:btree_74762_again, r=Mark-Simulacrum

BTreeMap: better way to postpone root access in DrainFilter

A slightly more elegant (in my opinion) adaptation of #74762. Benchmarks seem irrationally pleased to:
```
benchcmp old new --threshold 5
 name                                           old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 btree::map::clone_fat_100_and_remove_all       215,182      185,052           -30,130  -14.00%   x 1.16
 btree::map::clone_fat_100_and_remove_half      139,667      127,945           -11,722   -8.39%   x 1.09
 btree::map::clone_fat_val_100_and_remove_all   96,755       81,279            -15,476  -16.00%   x 1.19
 btree::map::clone_fat_val_100_and_remove_half  64,678       56,911             -7,767  -12.01%   x 1.14
 btree::map::find_rand_100                      18           17                     -1   -5.56%   x 1.06
 btree::map::first_and_last_0                   33           35                      2    6.06%   x 0.94
 btree::map::first_and_last_100                 40           54                     14   35.00%   x 0.74
 btree::map::insert_rand_100                    45           42                     -3   -6.67%   x 1.07
 btree::map::insert_rand_10_000                 45           41                     -4   -8.89%   x 1.10
 btree::map::iter_0                             2,010        1,759                -251  -12.49%   x 1.14
 btree::map::iter_100                           3,514        2,764                -750  -21.34%   x 1.27
 btree::map::iter_10k                           4,018        3,768                -250   -6.22%   x 1.07
 btree::map::range_unbounded_unbounded          37,269       28,929             -8,340  -22.38%   x 1.29
 btree::map::range_unbounded_vs_iter            31,518       28,814             -2,704   -8.58%   x 1.09
```

r? @Mark-Simulacrum

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

@@ -1697,10 +1697,9 @@ impl<'a, K: 'a, V: 'a> DrainFilterInner<'a, K, V> {
             let (k, v) = kv.kv_mut();
             if pred(k, v) {
                 *self.length -= 1;
-                let RemoveResult { old_kv, pos, emptied_internal_root } = kv.remove_kv_tracking();
+                let (kv, pos) = kv.remove_kv_tracking(|_| self.emptied_internal_root = true);
                 self.cur_leaf_edge = Some(pos);
-                self.emptied_internal_root |= emptied_internal_root;
-                return Some(old_kv);
+                return Some(kv);
             }
             self.cur_leaf_edge = Some(kv.next_leaf_edge());
         }
@@ -2645,35 +2644,28 @@ impl<'a, K: Ord, V> OccupiedEntry<'a, K, V> {
     fn remove_kv(self) -> (K, V) {
         *self.length -= 1;
 
-        let RemoveResult { old_kv, pos, emptied_internal_root } = self.handle.remove_kv_tracking();
-        let root = pos.into_node().into_root_mut();
-        if emptied_internal_root {
-            root.pop_internal_level();
-        }
+        let (old_kv, _) =
+            self.handle.remove_kv_tracking(|root| root.into_root_mut().pop_internal_level());
         old_kv
     }
 }
 
-struct RemoveResult<'a, K, V> {
-    // Key and value removed.
-    old_kv: (K, V),
-    // Unique location at the leaf level that the removed KV lopgically collapsed into.
-    pos: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    // Whether the remove left behind and empty internal root node, that should be removed
-    // using `pop_internal_level`.
-    emptied_internal_root: bool,
-}
-
 impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal>, marker::KV> {
     /// Removes a key/value-pair from the tree, and returns that pair, as well as
     /// the leaf edge corresponding to that former pair. It's possible this leaves
     /// an empty internal root node, which the caller should subsequently pop from
     /// the map holding the tree. The caller should also decrement the map's length.
-    fn remove_kv_tracking(self) -> RemoveResult<'a, K, V> {
-        let (mut pos, old_key, old_val, was_internal) = match self.force() {
+    fn remove_kv_tracking<F>(
+        self,
+        handle_emptied_internal_root: F,
+    ) -> ((K, V), Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>)
+    where
+        F: FnOnce(NodeRef<marker::Mut<'a>, K, V, marker::Internal>),
+    {
+        let (old_kv, mut pos, was_internal) = match self.force() {
             Leaf(leaf) => {
-                let (hole, old_key, old_val) = leaf.remove();
-                (hole, old_key, old_val, false)
+                let (old_kv, pos) = leaf.remove();
+                (old_kv, pos, false)
             }
             Internal(mut internal) => {
                 // Replace the location freed in the internal node with the next KV,
@@ -2688,17 +2680,16 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
                 let to_remove = internal.left_edge().descend().last_leaf_edge().left_kv().ok();
                 let to_remove = unsafe { unwrap_unchecked(to_remove) };
 
-                let (hole, key, val) = to_remove.remove();
+                let (kv, pos) = to_remove.remove();
 
-                let old_key = unsafe { mem::replace(&mut *key_loc, key) };
-                let old_val = unsafe { mem::replace(&mut *val_loc, val) };
+                let old_key = unsafe { mem::replace(&mut *key_loc, kv.0) };
+                let old_val = unsafe { mem::replace(&mut *val_loc, kv.1) };
 
-                (hole, old_key, old_val, true)
+                ((old_key, old_val), pos, true)
             }
         };
 
         // Handle underflow
-        let mut emptied_internal_root = false;
         let mut cur_node = unsafe { ptr::read(&pos).into_node().forget_type() };
         let mut at_leaf = true;
         while cur_node.len() < node::MIN_LEN {
@@ -2719,8 +2710,10 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
 
                     let parent = edge.into_node();
                     if parent.len() == 0 {
-                        // This empty parent must be the root, and should be popped off the tree.
-                        emptied_internal_root = true;
+                        // The parent that was just emptied must be the root,
+                        // because nodes on a lower level would not have been
+                        // left underfull. It has to be popped off the tree soon.
+                        handle_emptied_internal_root(parent);
                         break;
                     } else {
                         cur_node = parent.forget_type();
@@ -2747,7 +2740,7 @@ impl<'a, K: 'a, V: 'a> Handle<NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInter
             pos = unsafe { unwrap_unchecked(pos.next_kv().ok()).next_leaf_edge() };
         }
 
-        RemoveResult { old_kv: (old_key, old_val), pos, emptied_internal_root }
+        (old_kv, pos)
     }
 }
 

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

@@ -1083,12 +1083,12 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>
     /// between the now adjacent key/value pairs (if any) to the left and right of this handle.
     pub fn remove(
         mut self,
-    ) -> (Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>, K, V) {
+    ) -> ((K, V), Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>) {
         unsafe {
             let k = slice_remove(self.node.keys_mut(), self.idx);
             let v = slice_remove(self.node.vals_mut(), self.idx);
             (*self.node.as_leaf_mut()).len -= 1;
-            (self.left_edge(), k, v)
+            ((k, v), self.left_edge())
         }
     }
 }