Auto merge of #94802 - matthiaskrgr:rollup-4plu0fi, r=matthiaskrgr

Rollup of 5 pull requests

Successful merges:

 - #92150 (Improve suggestion when casting usize to (possibly) wide pointer)
 - #94635 (Merge `#[deprecated]` and `#[rustc_deprecated]`)
 - #94657 (Constify `Index{,Mut}` for `[T]`, `str`, and `[T; N]`)
 - #94746 (diagnostics: use rustc_on_unimplemented to recommend `[].iter()`)
 - #94788 (Account for suggestions for complete removal of lines)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup

12 files changed, 153 insertions, 53 deletions

diff --git a/library/core/src/array/mod.rs b/library/core/src/array/mod.rs
index ee79021ed53..20dfbc6347c 100644
--- a/library/core/src/array/mod.rs
+++ b/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 {
diff --git a/library/core/src/iter/traits/iterator.rs b/library/core/src/iter/traits/iterator.rs
index b38df1c2d02..c35d0784dd5 100644
--- a/library/core/src/iter/traits/iterator.rs
+++ b/library/core/src/iter/traits/iterator.rs
@@ -35,6 +35,11 @@ fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
               to have a bounded `RangeInclusive`: `0..=end`"
     ),
     on(
+        _Self = "[]",
+        label = "`{Self}` is not an iterator; try calling `.into_iter()` or `.iter()`"
+    ),
+    on(_Self = "&[]", label = "`{Self}` is not an iterator; try calling `.iter()`"),
+    on(
         _Self = "&str",
         label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
     ),
diff --git a/library/core/src/lib.rs b/library/core/src/lib.rs
index ddd8ae0c02c..97add8f2c6a 100644
--- a/library/core/src/lib.rs
+++ b/library/core/src/lib.rs
@@ -149,6 +149,8 @@
 #![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)]
@@ -167,6 +169,7 @@
 #![feature(const_refs_to_cell)]
 #![feature(decl_macro)]
 #![feature(derive_default_enum)]
+#![cfg_attr(not(bootstrap), feature(deprecated_suggestion))]
 #![feature(doc_cfg)]
 #![feature(doc_notable_trait)]
 #![feature(rustdoc_internals)]
diff --git a/library/core/src/num/mod.rs b/library/core/src/num/mod.rs
index 72105888f94..07fd317e074 100644
--- a/library/core/src/num/mod.rs
+++ b/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
     }
diff --git a/library/core/src/ops/range.rs b/library/core/src/ops/range.rs
index 11367220678..5029e0560b8 100644
--- a/library/core/src/ops/range.rs
+++ b/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.
diff --git a/library/core/src/ptr/const_ptr.rs b/library/core/src/ptr/const_ptr.rs
index 485a5965f4c..ee544b4842e 100644
--- a/library/core/src/ptr/const_ptr.rs
+++ b/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) }
diff --git a/library/core/src/ptr/mut_ptr.rs b/library/core/src/ptr/mut_ptr.rs
index 1412e836ebf..3374b48c88c 100644
--- a/library/core/src/ptr/mut_ptr.rs
+++ b/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) }
diff --git a/library/core/src/ptr/non_null.rs b/library/core/src/ptr/non_null.rs
index 77b93e0c24c..c744ad5dd2d 100644
--- a/library/core/src/ptr/non_null.rs
+++ b/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.
diff --git a/library/core/src/slice/index.rs b/library/core/src/slice/index.rs
index 0298bba8d32..7e6fbbe3538 100644
--- a/library/core/src/slice/index.rs
+++ b/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 {
@@ -31,31 +34,72 @@ 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");
 }
 
@@ -63,7 +107,7 @@ fn slice_start_index_overflow_fail() -> ! {
 #[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]
diff --git a/library/core/src/slice/mod.rs b/library/core/src/slice/mod.rs
index 9467c7f54ba..7311fe40e04 100644
--- a/library/core/src/slice/mod.rs
+++ b/library/core/src/slice/mod.rs
@@ -324,10 +324,11 @@ 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)
     }
@@ -348,10 +349,11 @@ 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)
     }
@@ -379,10 +381,11 @@ 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.
@@ -415,10 +418,11 @@ 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.
diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index 09709dc3cf6..f66bab999a9 100644
--- a/library/core/src/str/mod.rs
+++ b/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.
diff --git a/library/core/src/str/traits.rs b/library/core/src/str/traits.rs
index 95267624748..8b6b4fa02f8 100644
--- a/library/core/src/str/traits.rs
+++ b/library/core/src/str/traits.rs
@@ -53,9 +53,10 @@ impl PartialOrd for str {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> ops::Index<I> for str
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+impl<I> const ops::Index<I> for str
 where
-    I: SliceIndex<str>,
+    I: ~const SliceIndex<str>,
 {
     type Output = I::Output;
 
@@ -66,9 +67,10 @@ where
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<I> ops::IndexMut<I> for str
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+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 {
@@ -79,7 +81,7 @@ where
 #[inline(never)]
 #[cold]
 #[track_caller]
-fn str_index_overflow_fail() -> ! {
+const fn str_index_overflow_fail() -> ! {
     panic!("attempted to index str up to maximum usize");
 }
 
@@ -96,7 +98,8 @@ fn str_index_overflow_fail() -> ! {
 ///
 /// Equivalent to `&self[0 .. len]` or `&mut self[0 .. len]`.
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-unsafe impl SliceIndex<str> for ops::RangeFull {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl const SliceIndex<str> for ops::RangeFull {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
@@ -160,7 +163,8 @@ unsafe impl SliceIndex<str> for ops::RangeFull {
 /// // &s[3 .. 100];
 /// ```
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-unsafe impl SliceIndex<str> for ops::Range<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl const SliceIndex<str> for ops::Range<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
@@ -247,7 +251,8 @@ unsafe impl SliceIndex<str> for ops::Range<usize> {
 /// Panics if `end` does not point to the starting byte offset of a
 /// character (as defined by `is_char_boundary`), or if `end > len`.
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-unsafe impl SliceIndex<str> for ops::RangeTo<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl const SliceIndex<str> for ops::RangeTo<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
@@ -317,7 +322,8 @@ unsafe impl SliceIndex<str> for ops::RangeTo<usize> {
 /// Panics if `begin` does not point to the starting byte offset of
 /// a character (as defined by `is_char_boundary`), or if `begin > len`.
 #[stable(feature = "str_checked_slicing", since = "1.20.0")]
-unsafe impl SliceIndex<str> for ops::RangeFrom<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl const SliceIndex<str> for ops::RangeFrom<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
@@ -393,7 +399,8 @@ unsafe impl SliceIndex<str> for ops::RangeFrom<usize> {
 /// to the ending byte offset of a character (`end + 1` is either a starting
 /// byte offset or equal to `len`), if `begin > end`, or if `end >= len`.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-unsafe impl SliceIndex<str> for ops::RangeInclusive<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl const SliceIndex<str> for ops::RangeInclusive<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
@@ -444,7 +451,8 @@ unsafe impl SliceIndex<str> for ops::RangeInclusive<usize> {
 /// (`end + 1` is either a starting byte offset as defined by
 /// `is_char_boundary`, or equal to `len`), or if `end >= len`.
 #[stable(feature = "inclusive_range", since = "1.26.0")]
-unsafe impl SliceIndex<str> for ops::RangeToInclusive<usize> {
+#[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
+unsafe impl const SliceIndex<str> for ops::RangeToInclusive<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {