From e6cf6a7bfe98a421cd7304688fa4937764e43bb3 Mon Sep 17 00:00:00 2001
From: Mark Rousskov <mark.simulacrum@gmail.com>
Date: Fri, 24 Apr 2020 13:22:52 -0400
Subject: [PATCH] Take a single root node in range_search

The unsafe code can be justified within range_search, as it makes sure to not
overlap the returned references, but from the callers perspective it's an
entirely safe algorithm and there's no need for the caller to know about the
duplication.
---
 src/liballoc/collections/btree/map.rs | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index 91d93a1be1c98..8d0cd191c2a90 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -1034,9 +1034,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         R: RangeBounds<T>,
     {
         if let Some(root) = &self.root {
-            let root1 = root.as_ref();
-            let root2 = root.as_ref();
-            let (f, b) = range_search(root1, root2, range);
+            let (f, b) = range_search(root.as_ref(), range);
 
             Range { front: Some(f), back: Some(b) }
         } else {
@@ -1082,9 +1080,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         R: RangeBounds<T>,
     {
         if let Some(root) = &mut self.root {
-            let root1 = root.as_mut();
-            let root2 = unsafe { ptr::read(&root1) };
-            let (f, b) = range_search(root1, root2, range);
+            let (f, b) = range_search(root.as_mut(), range);
 
             RangeMut { front: Some(f), back: Some(b), _marker: PhantomData }
         } else {
@@ -2043,8 +2039,7 @@ where
 }
 
 fn range_search<BorrowType, K, V, Q: ?Sized, R: RangeBounds<Q>>(
-    root1: NodeRef<BorrowType, K, V, marker::LeafOrInternal>,
-    root2: NodeRef<BorrowType, K, V, marker::LeafOrInternal>,
+    root: NodeRef<BorrowType, K, V, marker::LeafOrInternal>,
     range: R,
 ) -> (
     Handle<NodeRef<BorrowType, K, V, marker::Leaf>, marker::Edge>,
@@ -2064,8 +2059,10 @@ where
         _ => {}
     };
 
-    let mut min_node = root1;
-    let mut max_node = root2;
+    // We duplicate the root NodeRef here -- we will never access it in a way
+    // that overlaps references obtained from the root.
+    let mut min_node = unsafe { ptr::read(&root) };
+    let mut max_node = root;
     let mut min_found = false;
     let mut max_found = false;