Constify Index{,Mut} for [T], str, and [T; N]

library/core/src/array/mod.rs

@@ -276,9 +276,10 @@ 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_slice_index", issue = "none")]
+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;
 
@@ -289,9 +290,10 @@ 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_slice_index", issue = "none")]
+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 {

library/core/src/lib.rs

@@ -148,6 +148,8 @@
 #![feature(variant_count)]
 #![feature(const_array_from_ref)]
 #![feature(const_slice_from_ref)]
+#![feature(const_slice_index)]
+#![feature(const_is_char_boundary)]
 //
 // Language features:
 #![feature(abi_unadjusted)]

library/core/src/num/mod.rs

@@ -809,7 +809,7 @@ impl u8 {
         ascii::escape_default(self)
     }
 
-    pub(crate) fn is_utf8_char_boundary(self) -> bool {
+    pub(crate) const fn is_utf8_char_boundary(self) -> bool {
         // This is bit magic equivalent to: b < 128 || b >= 192
         (self as i8) >= -0x40
     }

library/core/src/ops/range.rs

@@ -446,7 +446,7 @@ impl RangeInclusive<usize> {
     /// Converts to an exclusive `Range` for `SliceIndex` implementations.
     /// The caller is responsible for dealing with `end == usize::MAX`.
     #[inline]
-    pub(crate) fn into_slice_range(self) -> Range<usize> {
+    pub(crate) const fn into_slice_range(self) -> Range<usize> {
         // If we're not exhausted, we want to simply slice `start..end + 1`.
         // If we are exhausted, then slicing with `end + 1..end + 1` gives us an
         // empty range that is still subject to bounds-checks for that endpoint.

library/core/src/ptr/const_ptr.rs

@@ -1032,10 +1032,11 @@ impl<T> *const [T] {
     /// }
     /// ```
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[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) }

library/core/src/ptr/mut_ptr.rs

@@ -1302,10 +1302,11 @@ impl<T> *mut [T] {
     /// }
     /// ```
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[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) }

library/core/src/ptr/non_null.rs

@@ -630,10 +630,11 @@ impl<T> NonNull<[T]> {
     /// }
     /// ```
     #[unstable(feature = "slice_ptr_get", issue = "74265")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[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.

library/core/src/slice/index.rs

@@ -1,12 +1,14 @@
 //! Indexing implementations for `[T]`.
 
+use crate::intrinsics::const_eval_select;
 use crate::ops;
 use crate::ptr;
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, I> ops::Index<I> for [T]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+impl<T, I> const ops::Index<I> for [T]
 where
-    I: SliceIndex<[T]>,
+    I: ~const SliceIndex<[T]>,
 {
     type Output = I::Output;
 
@@ -17,9 +19,10 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, I> ops::IndexMut<I> for [T]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+impl<T, I> const ops::IndexMut<I> for [T]
 where
-    I: SliceIndex<[T]>,
+    I: ~const SliceIndex<[T]>,
 {
     #[inline]
     fn index_mut(&mut self, index: I) -> &mut I::Output {
@@ -31,39 +34,80 @@ where
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+const fn slice_start_index_len_fail(index: usize, len: usize) -> ! {
+    // SAFETY: we are just panicking here
+    unsafe {
+        const_eval_select(
+            (index, len),
+            slice_start_index_len_fail_ct,
+            slice_start_index_len_fail_rt,
+        )
+    }
+}
+
+// FIXME const-hack
+fn slice_start_index_len_fail_rt(index: usize, len: usize) -> ! {
     panic!("range start index {} out of range for slice of length {}", index, len);
 }
 
+const fn slice_start_index_len_fail_ct(_: usize, _: usize) -> ! {
+    panic!("slice start index is out of range for slice");
+}
+
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+const fn slice_end_index_len_fail(index: usize, len: usize) -> ! {
+    // SAFETY: we are just panicking here
+    unsafe {
+        const_eval_select((index, len), slice_end_index_len_fail_ct, slice_end_index_len_fail_rt)
+    }
+}
+
+// FIXME const-hack
+fn slice_end_index_len_fail_rt(index: usize, len: usize) -> ! {
     panic!("range end index {} out of range for slice of length {}", index, len);
 }
 
+const fn slice_end_index_len_fail_ct(_: usize, _: usize) -> ! {
+    panic!("slice end index is out of range for slice");
+}
+
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-fn slice_index_order_fail(index: usize, end: usize) -> ! {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+const fn slice_index_order_fail(index: usize, end: usize) -> ! {
+    // SAFETY: we are just panicking here
+    unsafe { const_eval_select((index, end), slice_index_order_fail_ct, slice_index_order_fail_rt) }
+}
+
+// FIXME const-hack
+fn slice_index_order_fail_rt(index: usize, end: usize) -> ! {
     panic!("slice index starts at {} but ends at {}", index, end);
 }
 
+const fn slice_index_order_fail_ct(_: usize, _: usize) -> ! {
+    panic!("slice index start is larger than end");
+}
+
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-fn slice_start_index_overflow_fail() -> ! {
+const fn slice_start_index_overflow_fail() -> ! {
     panic!("attempted to index slice from after maximum usize");
 }
 
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-fn slice_end_index_overflow_fail() -> ! {
+const fn slice_end_index_overflow_fail() -> ! {
     panic!("attempted to index slice up to maximum usize");
 }
 
@@ -153,7 +197,8 @@ pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for usize {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for usize {
     type Output = T;
 
     #[inline]
@@ -197,7 +242,8 @@ unsafe impl<T> SliceIndex<[T]> for usize {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for ops::Range<usize> {
     type Output = [T];
 
     #[inline]
@@ -261,7 +307,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::Range<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeTo<usize> {
     type Output = [T];
 
     #[inline]
@@ -298,7 +345,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeTo<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeFrom<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeFrom<usize> {
     type Output = [T];
 
     #[inline]
@@ -343,7 +391,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::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_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeFull {
     type Output = [T];
 
     #[inline]
@@ -378,7 +427,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeFull {
 }
 
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
     type Output = [T];
 
     #[inline]
@@ -421,7 +471,8 @@ unsafe impl<T> SliceIndex<[T]> for ops::RangeInclusive<usize> {
 }
 
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-unsafe impl<T> SliceIndex<[T]> for ops::RangeToInclusive<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl<T> const SliceIndex<[T]> for ops::RangeToInclusive<usize> {
     type Output = [T];
 
     #[inline]

library/core/src/slice/mod.rs

@@ -321,10 +321,11 @@ impl<T> [T] {
     /// assert_eq!(None, v.get(0..4));
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[inline]
-    pub fn get<I>(&self, index: I) -> Option<&I::Output>
+    pub const fn get<I>(&self, index: I) -> Option<&I::Output>
     where
-        I: SliceIndex<Self>,
+        I: ~const SliceIndex<Self>,
     {
         index.get(self)
     }
@@ -345,10 +346,11 @@ impl<T> [T] {
     /// assert_eq!(x, &[0, 42, 2]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[inline]
-    pub fn get_mut<I>(&mut self, index: I) -> Option<&mut I::Output>
+    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)
     }
@@ -376,10 +378,11 @@ impl<T> [T] {
     /// }
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[inline]
-    pub unsafe fn get_unchecked<I>(&self, index: I) -> &I::Output
+    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.
@@ -412,10 +415,11 @@ impl<T> [T] {
     /// assert_eq!(x, &[1, 13, 4]);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[inline]
-    pub unsafe fn get_unchecked_mut<I>(&mut self, index: I) -> &mut I::Output
+    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.