Constify slice index for strings

Author: fee1-dead

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

@@ -148,7 +148,8 @@
 #![feature(variant_count)]
 #![feature(const_array_from_ref)]
 #![feature(const_slice_from_ref)]
-#![feature(const_slice_index_impls)]
+#![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/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

@@ -5,7 +5,7 @@ use crate::ops;
 use crate::ptr;
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 impl<T, I> const ops::Index<I> for [T]
 where
     I: ~const SliceIndex<[T]>,
@@ -19,7 +19,7 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 impl<T, I> const ops::IndexMut<I> for [T]
 where
     I: ~const SliceIndex<[T]>,
@@ -30,16 +30,19 @@ where
     }
 }
 
-
 #[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[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)
+        const_eval_select(
+            (index, len),
+            slice_start_index_len_fail_ct,
+            slice_start_index_len_fail_rt,
+        )
     }
 }
 
@@ -56,7 +59,7 @@ const fn slice_start_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[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 {
@@ -77,7 +80,7 @@ const fn slice_end_index_len_fail_ct(_: usize, _: usize) -> ! {
 #[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[cold]
 #[track_caller]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[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) }
@@ -194,7 +197,7 @@ pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for usize {
     type Output = T;
 
@@ -239,7 +242,7 @@ unsafe impl<T> const SliceIndex<[T]> for usize {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for ops::Range<usize> {
     type Output = [T];
 
@@ -304,7 +307,7 @@ unsafe impl<T> const SliceIndex<[T]> for ops::Range<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for ops::RangeTo<usize> {
     type Output = [T];
 
@@ -342,7 +345,7 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeTo<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for ops::RangeFrom<usize> {
     type Output = [T];
 
@@ -388,7 +391,7 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeFrom<usize> {
 }
 
 #[stable(feature = "slice_get_slice_impls", since = "1.15.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for ops::RangeFull {
     type Output = [T];
 
@@ -424,7 +427,7 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeFull {
 }
 
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
     type Output = [T];
 
@@ -468,7 +471,7 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
 }
 
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-#[rustc_const_unstable(feature = "const_slice_index_impls", issue = "none")]
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
 unsafe impl<T> const SliceIndex<[T]> for ops::RangeToInclusive<usize> {
     type Output = [T];
 

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.

library/core/src/str/mod.rs

@@ -79,7 +79,23 @@ use iter::{MatchesInternal, SplitNInternal};
 #[inline(never)]
 #[cold]
 #[track_caller]
-fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
+#[rustc_allow_const_fn_unstable(const_eval_select)]
+const fn slice_error_fail(s: &str, begin: usize, end: usize) -> ! {
+    // SAFETY: panics for both branches
+    unsafe {
+        crate::intrinsics::const_eval_select(
+            (s, begin, end),
+            slice_error_fail_ct,
+            slice_error_fail_rt,
+        )
+    }
+}
+
+const fn slice_error_fail_ct(_: &str, _: usize, _: usize) -> ! {
+    panic!("failed to slice string");
+}
+
+fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
     const MAX_DISPLAY_LENGTH: usize = 256;
     let trunc_len = s.floor_char_boundary(MAX_DISPLAY_LENGTH);
     let s_trunc = &s[..trunc_len];
@@ -189,8 +205,9 @@ impl str {
     /// ```
     #[must_use]
     #[stable(feature = "is_char_boundary", since = "1.9.0")]
+    #[rustc_const_unstable(feature = "const_is_char_boundary", issue = "none")]
     #[inline]
-    pub fn is_char_boundary(&self, index: usize) -> bool {
+    pub const fn is_char_boundary(&self, index: usize) -> bool {
         // 0 is always ok.
         // Test for 0 explicitly so that it can optimize out the check
         // easily and skip reading string data for that case.
@@ -418,8 +435,9 @@ impl str {
     /// assert!(v.get(..42).is_none());
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[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)
     }
 
@@ -450,8 +468,9 @@ impl str {
     /// assert_eq!("HEllo", v);
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[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)
     }
 
@@ -482,8 +501,9 @@ impl str {
     /// }
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[inline]
-    pub unsafe fn get_unchecked<I: SliceIndex<str>>(&self, i: I) -> &I::Output {
+    pub const unsafe fn get_unchecked<I: ~const SliceIndex<str>>(&self, i: I) -> &I::Output {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked`;
         // the slice is dereferenceable because `self` is a safe reference.
         // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.
@@ -517,8 +537,12 @@ impl str {
     /// }
     /// ```
     #[stable(feature = "str_checked_slicing", since = "1.20.0")]
+    #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
     #[inline]
-    pub unsafe fn get_unchecked_mut<I: SliceIndex<str>>(&mut self, i: I) -> &mut I::Output {
+    pub const unsafe fn get_unchecked_mut<I: ~const SliceIndex<str>>(
+        &mut self,
+        i: I,
+    ) -> &mut I::Output {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`;
         // the slice is dereferenceable because `self` is a safe reference.
         // The returned pointer is safe because impls of `SliceIndex` have to guarantee that it is.