Make BTreeMap::new and BTreeSet::new const

src/liballoc/collections/btree/map.rs

@@ -556,7 +556,8 @@ impl<K: Ord, V> BTreeMap<K, V> {
     /// map.insert(1, "a");
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new() -> BTreeMap<K, V> {
+    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    pub const fn new() -> BTreeMap<K, V> {
         BTreeMap { root: None, length: 0 }
     }
 

src/liballoc/collections/btree/set.rs

@@ -309,7 +309,8 @@ impl<T: Ord> BTreeSet<T> {
     /// let mut set: BTreeSet<i32> = BTreeSet::new();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new() -> BTreeSet<T> {
+    #[rustc_const_unstable(feature = "const_btree_new", issue = "71835")]
+    pub const fn new() -> BTreeSet<T> {
         BTreeSet { map: BTreeMap::new() }
     }
 

src/liballoc/lib.rs

@@ -82,6 +82,7 @@
 #![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
+#![feature(const_btree_new)]
 #![feature(const_generic_impls_guard)]
 #![feature(const_generics)]
 #![feature(const_in_array_repeat_expressions)]

src/test/ui/collections-const-new.rs

@@ -3,9 +3,18 @@
 // Test several functions can be used for constants
 // 1. Vec::new()
 // 2. String::new()
+// 3. BTreeMap::new()
+// 4. BTreeSet::new()
+
+#![feature(const_btree_new)]
 
 const MY_VEC: Vec<usize> = Vec::new();
 
 const MY_STRING: String = String::new();
 
+use std::collections::{BTreeMap, BTreeSet};
+const MY_BTREEMAP: BTreeMap<u32, u32> = BTreeMap::new();
+
+const MY_BTREESET: BTreeSet<u32> = BTreeSet::new();
+
 fn main() {}