Rollup merge of rust-lang#143921 - oli-obk:const-index, r=fee1-dead

Constify `Index` traits

tracking issue: rust-lang#143775

the `SliceIndex` trait cannot be implemented by users as it is sealed. While it would be useful for the `get` method on slices, it seems weird to have a feature gate for that that isn't also gating index syntax at the same time, so I put them under the same feature gate.

r? ```````@fee1-dead```````

Author: samueltardieu

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/ptr/const_ptr.rs

@@ -1524,10 +1524,11 @@ impl<T> *const [T] {
     /// }
     /// ```
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
     #[inline]
-    pub unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
+    pub const unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
     where
-        I: SliceIndex<[T]>,
+        I: ~const SliceIndex<[T]>,
     {
         // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
         unsafe { index.get_unchecked(self) }

core/src/ptr/mut_ptr.rs

@@ -1881,10 +1881,11 @@ impl<T> *mut [T] {
     /// }
     /// ```
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
     #[inline(always)]
-    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> *mut I::Output
+    pub const unsafe fn get_unchecked_mut<I>(self, index: I) -> *mut I::Output
     where
-        I: SliceIndex<[T]>,
+        I: ~const SliceIndex<[T]>,
     {
         // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
         unsafe { index.get_unchecked_mut(self) }

core/src/ptr/non_null.rs

@@ -1597,10 +1597,11 @@ impl<T> NonNull<[T]> {
     /// }
     /// ```
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
     #[inline]
-    pub unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
+    pub const unsafe fn get_unchecked_mut<I>(self, index: I) -> NonNull<I::Output>
     where
-        I: SliceIndex<[T]>,
+        I: ~const SliceIndex<[T]>,
     {
         // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
         // As a consequence, the resulting pointer cannot be null.

core/src/range.rs

@@ -186,14 +186,17 @@ impl<T> IntoBounds<T> for Range<T> {
 }
 
 #[unstable(feature = "new_range_api", issue = "125687")]
-impl<T> From<Range<T>> for legacy::Range<T> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<Range<T>> for legacy::Range<T> {
     #[inline]
     fn from(value: Range<T>) -> Self {
         Self { start: value.start, end: value.end }
     }
 }
+
 #[unstable(feature = "new_range_api", issue = "125687")]
-impl<T> From<legacy::Range<T>> for Range<T> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<legacy::Range<T>> for Range<T> {
     #[inline]
     fn from(value: legacy::Range<T>) -> Self {
         Self { start: value.start, end: value.end }
@@ -362,7 +365,8 @@ impl<T> IntoBounds<T> for RangeInclusive<T> {
 }
 
 #[unstable(feature = "new_range_api", issue = "125687")]
-impl<T> From<RangeInclusive<T>> for legacy::RangeInclusive<T> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<RangeInclusive<T>> for legacy::RangeInclusive<T> {
     #[inline]
     fn from(value: RangeInclusive<T>) -> Self {
         Self::new(value.start, value.end)
@@ -506,14 +510,16 @@ impl<T> IntoBounds<T> for RangeFrom<T> {
 }
 
 #[unstable(feature = "new_range_api", issue = "125687")]
-impl<T> From<RangeFrom<T>> for legacy::RangeFrom<T> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<RangeFrom<T>> for legacy::RangeFrom<T> {
     #[inline]
     fn from(value: RangeFrom<T>) -> Self {
         Self { start: value.start }
     }
 }
 #[unstable(feature = "new_range_api", issue = "125687")]
-impl<T> From<legacy::RangeFrom<T>> for RangeFrom<T> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T> const From<legacy::RangeFrom<T>> for RangeFrom<T> {
     #[inline]
     fn from(value: legacy::RangeFrom<T>) -> Self {
         Self { start: value.start }

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 {
@@ -158,6 +160,8 @@ mod private_slice_index {
     message = "the type `{T}` cannot be indexed by `{Self}`",
     label = "slice indices are of type `usize` or ranges of `usize`"
 )]
+#[const_trait]
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
 pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
     /// The output type returned by methods.
     #[stable(feature = "slice_get_slice", since = "1.28.0")]
@@ -208,7 +212,8 @@ pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
 
 /// The methods `index` and `index_mut` panic if the index is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for usize {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for usize {
     type Output = T;
 
     #[inline]
@@ -278,7 +283,8 @@ unsafe impl<T> SliceIndex<[T]> for usize {
 
 /// Because `IndexRange` guarantees `start <= end`, fewer checks are needed here
 /// than there are for a general `Range<usize>` (which might be `100..3`).
-unsafe impl<T> SliceIndex<[T]> for ops::IndexRange {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::IndexRange {
     type Output = [T];
 
     #[inline]
@@ -354,7 +360,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::IndexRange {
 /// - the start of the range is greater than the end of the range or
 /// - the end of the range is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::Range<usize> {
     type Output = [T];
 
     #[inline]
@@ -453,7 +460,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
 }
 
 #[unstable(feature = "new_range_api", issue = "125687")]
-unsafe impl<T> SliceIndex<[T]> for range::Range<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for range::Range<usize> {
     type Output = [T];
 
     #[inline]
@@ -491,7 +499,8 @@ unsafe impl<T> SliceIndex<[T]> for range::Range<usize> {
 
 /// The methods `index` and `index_mut` panic if the end of the range is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeTo<usize> {
     type Output = [T];
 
     #[inline]
@@ -529,7 +538,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
 
 /// The methods `index` and `index_mut` panic if the start of the range is out of bounds.
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeFrom<usize> {
     type Output = [T];
 
     #[inline]
@@ -574,7 +584,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
 }
 
 #[unstable(feature = "new_range_api", issue = "125687")]
-unsafe impl<T> SliceIndex<[T]> for range::RangeFrom<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for range::RangeFrom<usize> {
     type Output = [T];
 
     #[inline]
@@ -611,7 +622,8 @@ unsafe impl<T> SliceIndex<[T]> for range::RangeFrom<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeFull {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeFull {
     type Output = [T];
 
     #[inline]
@@ -650,7 +662,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeFull {
 /// - the start of the range is greater than the end of the range or
 /// - the end of the range is out of bounds.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
     type Output = [T];
 
     #[inline]
@@ -693,7 +706,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
 }
 
 #[unstable(feature = "new_range_api", issue = "125687")]
-unsafe impl<T> SliceIndex<[T]> for range::RangeInclusive<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for range::RangeInclusive<usize> {
     type Output = [T];
 
     #[inline]
@@ -731,7 +745,8 @@ unsafe impl<T> SliceIndex<[T]> for range::RangeInclusive<usize> {
 
 /// The methods `index` and `index_mut` panic if the end of the range is out of bounds.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeToInclusive<usize> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeToInclusive<usize> {
     type Output = [T];
 
     #[inline]

core/src/slice/mod.rs

@@ -568,9 +568,10 @@ impl<T> [T] {
     #[rustc_no_implicit_autorefs]
     #[inline]
     #[must_use]
-    pub fn get<I>(&self, index: I) -> Option<&I::Output>
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
+    pub const fn get<I>(&self, index: I) -> Option<&I::Output>
     where
-        I: SliceIndex<Self>,
+        I: ~const SliceIndex<Self>,
     {
         index.get(self)
     }
@@ -594,9 +595,10 @@ impl<T> [T] {
     #[rustc_no_implicit_autorefs]
     #[inline]
     #[must_use]
-    pub fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
+    pub const fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
     where
-        I: SliceIndex<Self>,
+        I: ~const SliceIndex<Self>,
     {
         index.get_mut(self)
     }
@@ -633,9 +635,10 @@ impl<T> [T] {
     #[inline]
     #[must_use]
     #[track_caller]
-    pub unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
+    pub const unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
     where
-        I: SliceIndex<Self>,
+        I: ~const SliceIndex<Self>,
     {
         // SAFETY: the caller must uphold most of the safety requirements for `get_unchecked`;
         // the slice is dereferenceable because `self` is a safe reference.
@@ -677,9 +680,10 @@ impl<T> [T] {
     #[inline]
     #[must_use]
     #[track_caller]
-    pub unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
+    pub const unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
     where
-        I: SliceIndex<Self>,
+        I: ~const SliceIndex<Self>,
     {
         // SAFETY: the caller must uphold the safety requirements for `get_unchecked_mut`;
         // the slice is dereferenceable because `self` is a safe reference.

core/src/str/mod.rs

@@ -589,8 +589,9 @@ impl str {
     /// assert!(v.get(..42).is_none());
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
     #[inline]
-    pub fn get<I: SliceIndex<str>>(&self, i: I) -> Option<&I::Output> {
+    pub const fn get<I: ~const SliceIndex<str>>(&self, i: I) -> Option<&I::Output> {
         i.get(self)
     }
 
@@ -621,8 +622,9 @@ impl str {
     /// assert_eq!("HEllo", v);
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_index", issue = "143775")]
     #[inline]
-    pub fn get_mut<I: SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output> {
+    pub const fn get_mut<I: ~const SliceIndex<str>>(&mut self, i: I) -> Option<&mut I::Output> {
         i.get_mut(self)
     }