Rollup merge of rust-lang#102200 - ink-feather-org:const_default_impls, r=fee1-dead

Constify Default impl's for Arrays and Tuples.

Allows to create arrays and tuples in const Context using the ~const Default implementation of the inner type.

Author: fee1-dead

library/core/src/array/mod.rs

@@ -434,7 +434,8 @@ impl<T: Copy> SpecArrayClone for T {
 macro_rules! array_impl_default {
     {$n:expr, $t:ident $($ts:ident)*} => {
         #[stable(since = "1.4.0", feature = "array_default")]
-        impl<T> Default for [T; $n] where T: Default {
+        #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+        impl<T> const Default for [T; $n] where T: ~const Default {
             fn default() -> [T; $n] {
                 [$t::default(), $($ts::default()),*]
             }

library/core/src/tuple.rs

@@ -93,7 +93,8 @@ macro_rules! tuple_impls {
         maybe_tuple_doc! {
             $($T)+ @
             #[stable(feature = "rust1", since = "1.0.0")]
-            impl<$($T:Default),+> Default for ($($T,)+) {
+            #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
+            impl<$($T: ~const Default),+> const Default for ($($T,)+) {
                 #[inline]
                 fn default() -> ($($T,)+) {
                     ($({ let x: $T = Default::default(); x},)+)