Constified `Default` implementations

The libs-api team agrees to allow const_trait_impl to appear in the
standard library as long as stable code cannot be broken (they are
properly gated) this means if the compiler teams thinks it's okay, then
it's okay.

My priority on constifying would be:

	1. Non-generic impls (e.g. Default) or generic impls with no
	   bounds
	2. Generic functions with bounds (that use const impls)
	3. Generic impls with bounds
	4. Impls for traits with associated types

For people opening constification PRs: please cc me and/or oli-obk.

5 files changed, 19 insertions, 15 deletions

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index d2ececaa975..1a387f291cc 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -95,6 +95,7 @@
 #![feature(const_fn_trait_bound)]
 #![feature(cow_is_borrowed)]
 #![feature(const_cow_is_borrowed)]
+#![feature(const_trait_impl)]
 #![feature(destructuring_assignment)]
 #![feature(dispatch_from_dyn)]
 #![feature(core_intrinsics)]
diff --git a/library/alloc/src/string.rs b/library/alloc/src/string.rs
index 9aefd882af4..6568d9f9907 100644
--- a/library/alloc/src/string.rs
+++ b/library/alloc/src/string.rs
@@ -2105,7 +2105,8 @@ impl_eq! { Cow<'a, str>, &'b str }
 impl_eq! { Cow<'a, str>, String }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl Default for String {
+#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+impl const Default for String {
     /// Creates an empty `String`.
     #[inline]
     fn default() -> String {
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index 933361d812c..87a0d371815 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -2758,7 +2758,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for Vec<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T> Default for Vec<T> {
+#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+impl<T> const Default for Vec<T> {
     /// Creates an empty `Vec<T>`.
     fn default() -> Vec<T> {
         Vec::new()
diff --git a/library/alloc/tests/const_fns.rs b/library/alloc/tests/const_fns.rs
index b6ef3eee291..da58ae92e11 100644
--- a/library/alloc/tests/const_fns.rs
+++ b/library/alloc/tests/const_fns.rs
@@ -1,16 +1,8 @@
-// Test several functions can be used for constants
-// 1. Vec::new()
-// 2. String::new()
-// 3. BTreeMap::new()
-// 4. BTreeSet::new()
+// Test const functions in the library
 
-#[allow(dead_code)]
-pub const MY_VEC: Vec<usize> = Vec::new();
-
-#[allow(dead_code)]
-pub const MY_STRING: String = String::new();
+use core::cmp::Ordering;
 
-// FIXME(fee1-dead) remove this struct once we put `K: ?const Ord` on BTreeMap::new.
+// FIXME remove this struct once we put `K: ?const Ord` on BTreeMap::new.
 #[derive(PartialEq, Eq, PartialOrd)]
 pub struct MyType;
 
@@ -32,7 +24,12 @@ impl const Ord for MyType {
     }
 }
 
-use core::cmp::Ordering;
+pub const MY_VEC: Vec<usize> = Vec::new();
+pub const MY_VEC2: Vec<usize> = Default::default();
+
+pub const MY_STRING: String = String::new();
+pub const MY_STRING2: String = Default::default();
+
 use std::collections::{BTreeMap, BTreeSet};
 
 pub const MY_BTREEMAP: BTreeMap<MyType, MyType> = BTreeMap::new();
@@ -47,7 +44,10 @@ pub const SET_IS_EMPTY: bool = SET.is_empty();
 
 #[test]
 fn test_const() {
+    assert_eq!(MY_VEC, MY_VEC2);
+    assert_eq!(MY_STRING, MY_STRING2);
+
     assert_eq!(MAP_LEN, 0);
     assert_eq!(SET_LEN, 0);
-    assert!(MAP_IS_EMPTY && SET_IS_EMPTY)
+    assert!(MAP_IS_EMPTY && SET_IS_EMPTY);
 }
diff --git a/library/alloc/tests/lib.rs b/library/alloc/tests/lib.rs
index 7284c05d5ff..5767108d423 100644
--- a/library/alloc/tests/lib.rs
+++ b/library/alloc/tests/lib.rs
@@ -24,6 +24,7 @@
 #![feature(vec_spare_capacity)]
 #![feature(string_remove_matches)]
 #![feature(const_btree_new)]
+#![feature(const_default_impls)]
 #![feature(const_trait_impl)]
 
 use std::collections::hash_map::DefaultHasher;