Moved ui test

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

@@ -1786,13 +1786,6 @@ fn test_ord_absence() {
     }
 }
 
-#[allow(dead_code)]
-fn test_const() {
-    const MAP: &'static BTreeMap<(), ()> = &BTreeMap::new();
-    const LEN: usize = MAP.len();
-    const IS_EMPTY: bool = MAP.is_empty();
-}
-
 #[test]
 fn test_occupied_entry_key() {
     let mut a = BTreeMap::new();

library/alloc/src/collections/btree/set/tests.rs

@@ -16,13 +16,6 @@ fn test_clone_eq() {
     assert_eq!(m.clone(), m);
 }
 
-#[allow(dead_code)]
-fn test_const() {
-    const SET: &'static BTreeSet<()> = &BTreeSet::new();
-    const LEN: usize = SET.len();
-    const IS_EMPTY: bool = SET.is_empty();
-}
-
 #[test]
 fn test_iter_min_max() {
     let mut a = BTreeSet::new();

library/alloc/tests/const_fns.rs

@@ -0,0 +1,53 @@
+// Test several functions can be used for constants
+// 1. Vec::new()
+// 2. String::new()
+// 3. BTreeMap::new()
+// 4. BTreeSet::new()
+
+#[allow(dead_code)]
+pub const MY_VEC: Vec<usize> = Vec::new();
+
+#[allow(dead_code)]
+pub const MY_STRING: String = String::new();
+
+// FIXME remove this struct once we put `K: ?const Ord` on BTreeMap::new.
+#[derive(PartialEq, Eq, PartialOrd)]
+pub struct MyType;
+
+impl const Ord for MyType {
+    fn cmp(&self, _: &Self) -> Ordering {
+        Ordering::Equal
+    }
+
+    fn max(self, _: Self) -> Self {
+        Self
+    }
+
+    fn min(self, _: Self) -> Self {
+        Self
+    }
+
+    fn clamp(self, _: Self, _: Self) -> Self {
+        Self
+    }
+}
+
+use core::cmp::Ordering;
+use std::collections::{BTreeMap, BTreeSet};
+
+pub const MY_BTREEMAP: BTreeMap<MyType, MyType> = BTreeMap::new();
+pub const MAP: &'static BTreeMap<MyType, MyType> = &MY_BTREEMAP;
+pub const MAP_LEN: usize = MAP.len();
+pub const MAP_IS_EMPTY: bool = MAP.is_empty();
+
+pub const MY_BTREESET: BTreeSet<MyType> = BTreeSet::new();
+pub const SET: &'static BTreeSet<MyType> = &MY_BTREESET;
+pub const SET_LEN: usize = SET.len();
+pub const SET_IS_EMPTY: bool = SET.is_empty();
+
+#[test]
+fn test_const() {
+    assert_eq!(MAP_LEN, 0);
+    assert_eq!(SET_LEN, 0);
+    assert!(MAP_IS_EMPTY && SET_IS_EMPTY)
+}

library/alloc/tests/lib.rs

@@ -21,6 +21,8 @@
 #![feature(slice_partition_dedup)]
 #![feature(vec_spare_capacity)]
 #![feature(string_remove_matches)]
+#![feature(const_btree_new)]
+#![feature(const_trait_impl)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};
@@ -30,6 +32,7 @@ mod binary_heap;
 mod borrow;
 mod boxed;
 mod btree_set_hash;
+mod const_fns;
 mod cow_str;
 mod fmt;
 mod heap;