Make BTreeMap::new_in const

Closes rust-lang/wg-allocators#118

1 files changed, 1 insertions, 1 deletions

diff --git a/library/alloc/src/collections/btree/map.rs b/library/alloc/src/collections/btree/map.rs
index 5481b327d69..4bdd9639557 100644
--- a/library/alloc/src/collections/btree/map.rs
+++ b/library/alloc/src/collections/btree/map.rs
@@ -669,7 +669,7 @@ impl<K, V, A: Allocator + Clone> BTreeMap<K, V, A> {
     /// map.insert(1, "a");
     /// ```
     #[unstable(feature = "btreemap_alloc", issue = "32838")]
-    pub fn new_in(alloc: A) -> BTreeMap<K, V, A> {
+    pub const fn new_in(alloc: A) -> BTreeMap<K, V, A> {
         BTreeMap { root: None, length: 0, alloc: ManuallyDrop::new(alloc), _marker: PhantomData }
     }
 }