constify Index trait and its slice impls

Author: oli-obk

core/src/array/mod.rs

@@ -374,9 +374,10 @@ impl<'a, T, const N: usize> IntoIterator for &'a mut [T; N] {
 }
 
 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
-impl<T, I, const N: usize> Index<I> for [T; N]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T, I, const N: usize> const Index<I> for [T; N]
 where
-    [T]: Index<I>,
+    [T]: ~const Index<I>,
 {
     type Output = <[T] as Index<I>>::Output;
 
@@ -387,9 +388,10 @@ where
 }
 
 #[stable(feature = "index_trait_on_arrays", since = "1.50.0")]
-impl<T, I, const N: usize> IndexMut<I> for [T; N]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T, I, const N: usize> const IndexMut<I> for [T; N]
 where
-    [T]: IndexMut<I>,
+    [T]: ~const IndexMut<I>,
 {
     #[inline]
     fn index_mut(&mut self, index: I) -> &mut Self::Output {

core/src/ops/index.rs

@@ -55,6 +55,8 @@
 #[doc(alias = "]")]
 #[doc(alias = "[")]
 #[doc(alias = "[]")]
+#[const_trait]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
 pub trait Index<Idx: ?Sized> {
     /// The returned type after indexing.
     #[stable(feature = "rust1", since = "1.0.0")]
@@ -165,7 +167,9 @@ see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#ind
 #[doc(alias = "[")]
 #[doc(alias = "]")]
 #[doc(alias = "[]")]
-pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+#[const_trait]
+pub trait IndexMut<Idx: ?Sized>: ~const Index<Idx> {
     /// Performs the mutable indexing (`container[index]`) operation.
     ///
     /// # Panics

core/src/slice/index.rs

@@ -6,9 +6,10 @@ use crate::ub_checks::assert_unsafe_precondition;
 use crate::{ops, range};
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, I> ops::Index<I> for [T]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T, I> const ops::Index<I> for [T]
 where
-    I: SliceIndex<[T]>,
+    I: ~const SliceIndex<[T]>,
 {
     type Output = I::Output;
 
@@ -19,9 +20,10 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, I> ops::IndexMut<I> for [T]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T, I> const ops::IndexMut<I> for [T]
 where
-    I: SliceIndex<[T]>,
+    I: ~const SliceIndex<[T]>,
 {
     #[inline(always)]
     fn index_mut(&mut self, index: I) -> &mut I::Output {

core/src/str/traits.rs

@@ -49,9 +49,10 @@ impl PartialOrd for str {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> ops::Index<I> for str
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<I> const ops::Index<I> for str
 where
-    I: SliceIndex<str>,
+    I: ~const SliceIndex<str>,
 {
     type Output = I::Output;
 
@@ -62,9 +63,10 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> ops::IndexMut<I> for str
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<I> const ops::IndexMut<I> for str
 where
-    I: SliceIndex<str>,
+    I: ~const SliceIndex<str>,
 {
     #[inline]
     fn index_mut(&mut self, index: I) -> &mut I::Output {