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.

1 files changed, 2 insertions, 1 deletions

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index 3bc9f71375c..3c638e655dc 100644
--- a/library/core/src/array/mod.rs
+++ b/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] { [] }
         }
     };