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.

Author: fee1-dead

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)]

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 {

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()

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);
 }

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;

library/core/src/array/mod.rs

@@ -280,7 +280,8 @@ macro_rules! array_impl_default {
     };
     {$n:expr,} => {
         #[stable(since = "1.4.0", feature = "array_default")]
-        impl<T> Default for [T; $n] {
+        #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+        impl<T> const Default for [T; $n] {
             fn default() -> [T; $n] { [] }
         }
     };

library/core/src/default.rs

@@ -171,7 +171,8 @@ pub macro Default($item:item) {
 macro_rules! default_impl {
     ($t:ty, $v:expr, $doc:tt) => {
         #[stable(feature = "rust1", since = "1.0.0")]
-        impl Default for $t {
+        #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+        impl const Default for $t {
             #[inline]
             #[doc = $doc]
             fn default() -> $t {

library/core/src/hash/mod.rs

@@ -599,7 +599,8 @@ impl<H> Clone for BuildHasherDefault<H> {
 }
 
 #[stable(since = "1.7.0", feature = "build_hasher")]
-impl<H> Default for BuildHasherDefault<H> {
+#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+impl<H> const Default for BuildHasherDefault<H> {
     fn default() -> BuildHasherDefault<H> {
         BuildHasherDefault(marker::PhantomData)
     }

library/core/src/iter/sources/empty.rs

@@ -85,7 +85,8 @@ impl<T> Clone for Empty<T> {
 // not #[derive] because that adds a Default bound on T,
 // which isn't necessary.
 #[stable(feature = "iter_empty", since = "1.2.0")]
-impl<T> Default for Empty<T> {
+#[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+impl<T> const Default for Empty<T> {
     fn default() -> Empty<T> {
         Empty(marker::PhantomData)
     }

library/core/src/lib.rs

@@ -103,6 +103,7 @@
 #![feature(const_type_id)]
 #![feature(const_type_name)]
 #![feature(const_unreachable_unchecked)]
+#![feature(const_default_impls)]
 #![feature(duration_consts_2)]
 #![feature(ptr_metadata)]
 #![feature(slice_ptr_get)]