impl [const] Default for BTreeMap

Author: estebank

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

@@ -2594,7 +2594,8 @@ impl<K: Hash, V: Hash, A: Allocator + Clone> Hash for BTreeMap<K, V, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<K, V> Default for BTreeMap<K, V> {
+#[rustc_const_unstable(feature = "const_default", issue = "143894")]
+const impl<K, V> Default for BTreeMap<K, V> {
     /// Creates an empty `BTreeMap`.
     fn default() -> BTreeMap<K, V> {
         BTreeMap::new()

tests/ui/traits/const-traits/const-traits-alloc.rs

@@ -5,5 +5,8 @@
 const STRING: String = Default::default();
 // alloc::vec
 const VEC: Vec<()> = Default::default();
+// alloc::collections::btree::map::BTreeMap
+use std::collections::BTreeMap;
+const BTREE: BTreeMap<(), ()> = Default::default();
 
 fn main() {}