From f46f05bae8d81246f1ad93ccee051a13c9be393e Mon Sep 17 00:00:00 2001
From: Nicholas Nethercote <nnethercote@mozilla.com>
Date: Fri, 29 Jun 2018 17:02:38 +1000
Subject: [PATCH] Make `BTreeMap::clone()` not allocate when cloning an empty
 tree.

---
 src/liballoc/collections/btree/map.rs | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index e6e454446e232..2aad3149bb211 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -213,7 +213,16 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
             }
         }
 
-        clone_subtree(self.root.as_ref())
+        if self.len() == 0 {
+            // Ideally we'd call `BTreeMap::new` here, but that has the `K:
+            // Ord` constraint, which this method lacks.
+            BTreeMap {
+                root: node::Root::shared_empty_root(),
+                length: 0,
+            }
+        } else {
+            clone_subtree(self.root.as_ref())
+        }
     }
 }