Auto merge of #75105 - ssomers:btree_respect_min_len_hard, r=Mark-Sim…

…ulacrum

Hard way to respect BTreeMap's minimum node length

Resolves #74834 the hard way (though not the hardest imaginable).

Benchmarks (which are all biased/realistic, inserting keys in ascending order) say:
```
benchcmp r0 r1 --threshold 10
 name                                        r0 ns/iter  r1 ns/iter  diff ns/iter   diff %  speedup
 btree::map::clone_slim_100_and_clear        2,183       2,723                540   24.74%   x 0.80
 btree::map::clone_slim_100_and_drain_all    3,652       4,173                521   14.27%   x 0.88
 btree::map::clone_slim_100_and_drain_half   3,320       3,940                620   18.67%   x 0.84
 btree::map::clone_slim_100_and_into_iter    2,154       2,717                563   26.14%   x 0.79
 btree::map::clone_slim_100_and_pop_all      3,372       3,870                498   14.77%   x 0.87
 btree::map::clone_slim_100_and_remove_all   5,111       5,647                536   10.49%   x 0.91
 btree::map::clone_slim_100_and_remove_half  3,259       3,821                562   17.24%   x 0.85
 btree::map::iter_0                          1,733       1,509               -224  -12.93%   x 1.15
 btree::map::iter_100                        2,714       3,739              1,025   37.77%   x 0.73
 btree::map::iter_10k                        3,728       4,269                541   14.51%   x 0.87
 btree::map::range_unbounded_unbounded       28,426      36,631             8,205   28.86%   x 0.78
 btree::map::range_unbounded_vs_iter         28,808      34,056             5,248   18.22%   x 0.85
```
This difference is not caused by the `debug_assert`-related code in the function `splitpoint`, it's the same without.

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

@@ -821,6 +821,53 @@ impl<BorrowType, K, V, NodeType> Handle<NodeRef<BorrowType, K, V, NodeType>, mar
     }
 }
 
+enum InsertionPlace {
+    Left(usize),
+    Right(usize),
+}
+
+/// Given an edge index where we want to insert into a node filled to capacity,
+/// computes a sensible KV index of a split point and where to perform the insertion.
+/// The goal of the split point is for its key and value to end up in a parent node;
+/// the keys, values and edges to the left of the split point become the left child;
+/// the keys, values and edges to the right of the split point become the right child.
+fn splitpoint(edge_idx: usize) -> (usize, InsertionPlace) {
+    debug_assert!(edge_idx <= CAPACITY);
+    // Rust issue #74834 tries to explain these symmetric rules.
+    let middle_kv_idx;
+    let insertion;
+    if edge_idx <= B - 2 {
+        middle_kv_idx = B - 2;
+        insertion = InsertionPlace::Left(edge_idx);
+    } else if edge_idx == B - 1 {
+        middle_kv_idx = B - 1;
+        insertion = InsertionPlace::Left(edge_idx);
+    } else if edge_idx == B {
+        middle_kv_idx = B - 1;
+        insertion = InsertionPlace::Right(0);
+    } else {
+        middle_kv_idx = B;
+        let new_edge_idx = edge_idx - (B + 1);
+        insertion = InsertionPlace::Right(new_edge_idx);
+    }
+    let mut left_len = middle_kv_idx;
+    let mut right_len = CAPACITY - middle_kv_idx - 1;
+    match insertion {
+        InsertionPlace::Left(edge_idx) => {
+            debug_assert!(edge_idx <= left_len);
+            left_len += 1;
+        }
+        InsertionPlace::Right(edge_idx) => {
+            debug_assert!(edge_idx <= right_len);
+            right_len += 1;
+        }
+    }
+    debug_assert!(left_len >= MIN_LEN);
+    debug_assert!(right_len >= MIN_LEN);
+    debug_assert!(left_len + right_len == CAPACITY);
+    (middle_kv_idx, insertion)
+}
+
 impl<'a, K, V, NodeType> Handle<NodeRef<marker::Mut<'a>, K, V, NodeType>, marker::Edge> {
     /// Helps implementations of `insert_fit` for a particular `NodeType`,
     /// by taking care of leaf data.
@@ -863,18 +910,20 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge
             let kv = unsafe { Handle::new_kv(self.node, self.idx) };
             (InsertResult::Fit(kv), ptr)
         } else {
-            let middle = unsafe { Handle::new_kv(self.node, B) };
+            let (middle_kv_idx, insertion) = splitpoint(self.idx);
+            let middle = unsafe { Handle::new_kv(self.node, middle_kv_idx) };
             let (mut left, k, v, mut right) = middle.split();
-            let ptr = if self.idx <= B {
-                unsafe { Handle::new_edge(left.reborrow_mut(), self.idx).insert_fit(key, val) }
-            } else {
-                unsafe {
+            let ptr = match insertion {
+                InsertionPlace::Left(insert_idx) => unsafe {
+                    Handle::new_edge(left.reborrow_mut(), insert_idx).insert_fit(key, val)
+                },
+                InsertionPlace::Right(insert_idx) => unsafe {
                     Handle::new_edge(
                         right.node_as_mut().cast_unchecked::<marker::Leaf>(),
-                        self.idx - (B + 1),
+                        insert_idx,
                     )
                     .insert_fit(key, val)
-                }
+                },
             };
             (InsertResult::Split(SplitResult { left: left.forget_type(), k, v, right }), ptr)
         }
@@ -936,20 +985,20 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Internal>, marker::
             let kv = unsafe { Handle::new_kv(self.node, self.idx) };
             InsertResult::Fit(kv)
         } else {
-            let middle = unsafe { Handle::new_kv(self.node, B) };
+            let (middle_kv_idx, insertion) = splitpoint(self.idx);
+            let middle = unsafe { Handle::new_kv(self.node, middle_kv_idx) };
             let (mut left, k, v, mut right) = middle.split();
-            if self.idx <= B {
-                unsafe {
-                    Handle::new_edge(left.reborrow_mut(), self.idx).insert_fit(key, val, edge);
-                }
-            } else {
-                unsafe {
+            match insertion {
+                InsertionPlace::Left(insert_idx) => unsafe {
+                    Handle::new_edge(left.reborrow_mut(), insert_idx).insert_fit(key, val, edge);
+                },
+                InsertionPlace::Right(insert_idx) => unsafe {
                     Handle::new_edge(
                         right.node_as_mut().cast_unchecked::<marker::Internal>(),
-                        self.idx - (B + 1),
+                        insert_idx,
                     )
                     .insert_fit(key, val, edge);
-                }
+                },
             }
             InsertResult::Split(SplitResult { left: left.forget_type(), k, v, right })
         }