Rollup merge of #120291 - pitaj:string-sliceindex, r=Amanieu

Have `String` use `SliceIndex` impls from `str`

This PR simplifies the implementation of `Index` and `IndexMut` on `String`, and in the process enables indexing `String` by any user types that implement `SliceIndex<str>`.

Similar to #47832

r? libs

Not sure if this warrants a crater run.

library/alloc/src/lib.rs

@@ -152,6 +152,7 @@
 #![feature(set_ptr_value)]
 #![feature(sized_type_properties)]
 #![feature(slice_from_ptr_range)]
+#![feature(slice_index_methods)]
 #![feature(slice_ptr_get)]
 #![feature(slice_ptr_len)]
 #![feature(slice_range)]

library/alloc/src/string.rs

@@ -54,7 +54,7 @@ use core::ops::Add;
 use core::ops::AddAssign;
 #[cfg(not(no_global_oom_handling))]
 use core::ops::Bound::{Excluded, Included, Unbounded};
-use core::ops::{self, Index, IndexMut, Range, RangeBounds};
+use core::ops::{self, Range, RangeBounds};
 use core::ptr;
 use core::slice;
 use core::str::pattern::Pattern;
@@ -2433,100 +2433,26 @@ impl AddAssign<&str> for String {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl ops::Index<ops::Range<usize>> for String {
-    type Output = str;
+impl<I> ops::Index<I> for String
+where
+    I: slice::SliceIndex<str>,
+{
+    type Output = I::Output;
 
     #[inline]
-    fn index(&self, index: ops::Range<usize>) -> &str {
-        &self[..][index]
+    fn index(&self, index: I) -> &I::Output {
+        index.index(self.as_str())
     }
 }
-#[stable(feature = "rust1", since = "1.0.0")]
-impl ops::Index<ops::RangeTo<usize>> for String {
-    type Output = str;
 
-    #[inline]
-    fn index(&self, index: ops::RangeTo<usize>) -> &str {
-        &self[..][index]
-    }
-}
 #[stable(feature = "rust1", since = "1.0.0")]
-impl ops::Index<ops::RangeFrom<usize>> for String {
-    type Output = str;
-
-    #[inline]
-    fn index(&self, index: ops::RangeFrom<usize>) -> &str {
-        &self[..][index]
-    }
-}
-#[stable(feature = "rust1", since = "1.0.0")]
-impl ops::Index<ops::RangeFull> for String {
-    type Output = str;
-
-    #[inline]
-    fn index(&self, _index: ops::RangeFull) -> &str {
-        unsafe { str::from_utf8_unchecked(&self.vec) }
-    }
-}
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl ops::Index<ops::RangeInclusive<usize>> for String {
-    type Output = str;
-
-    #[inline]
-    fn index(&self, index: ops::RangeInclusive<usize>) -> &str {
-        Index::index(&**self, index)
-    }
-}
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl ops::Index<ops::RangeToInclusive<usize>> for String {
-    type Output = str;
-
-    #[inline]
-    fn index(&self, index: ops::RangeToInclusive<usize>) -> &str {
-        Index::index(&**self, index)
-    }
-}
-
-#[stable(feature = "derefmut_for_string", since = "1.3.0")]
-impl ops::IndexMut<ops::Range<usize>> for String {
-    #[inline]
-    fn index_mut(&mut self, index: ops::Range<usize>) -> &mut str {
-        &mut self[..][index]
-    }
-}
-#[stable(feature = "derefmut_for_string", since = "1.3.0")]
-impl ops::IndexMut<ops::RangeTo<usize>> for String {
-    #[inline]
-    fn index_mut(&mut self, index: ops::RangeTo<usize>) -> &mut str {
-        &mut self[..][index]
-    }
-}
-#[stable(feature = "derefmut_for_string", since = "1.3.0")]
-impl ops::IndexMut<ops::RangeFrom<usize>> for String {
-    #[inline]
-    fn index_mut(&mut self, index: ops::RangeFrom<usize>) -> &mut str {
-        &mut self[..][index]
-    }
-}
-#[stable(feature = "derefmut_for_string", since = "1.3.0")]
-impl ops::IndexMut<ops::RangeFull> for String {
-    #[inline]
-    fn index_mut(&mut self, _index: ops::RangeFull) -> &mut str {
-        unsafe { str::from_utf8_unchecked_mut(&mut *self.vec) }
-    }
-}
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl ops::IndexMut<ops::RangeInclusive<usize>> for String {
-    #[inline]
-    fn index_mut(&mut self, index: ops::RangeInclusive<usize>) -> &mut str {
-        IndexMut::index_mut(&mut **self, index)
-    }
-}
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl ops::IndexMut<ops::RangeToInclusive<usize>> for String {
+impl<I> ops::IndexMut<I> for String
+where
+    I: slice::SliceIndex<str>,
+{
     #[inline]
-    fn index_mut(&mut self, index: ops::RangeToInclusive<usize>) -> &mut str {
-        IndexMut::index_mut(&mut **self, index)
+    fn index_mut(&mut self, index: I) -> &mut I::Output {
+        index.index_mut(self.as_mut_str())
     }
 }