Optimize indexing slices and strs with inclusive ranges

library/alloctests/tests/str.rs

@@ -631,13 +631,13 @@ mod slice_index {
                 // note: using 0 specifically ensures that the result of overflowing is 0..0,
                 //       so that `get` doesn't simply return None for the wrong reason.
                 bad: data[0..=usize::MAX];
-                message: "maximum usize";
+                message: "out of bounds";
             }
 
             in mod rangetoinclusive {
                 data: "hello";
                 bad: data[..=usize::MAX];
-                message: "maximum usize";
+                message: "out of bounds";
             }
         }
     }

library/core/src/range.rs

@@ -333,15 +333,6 @@ impl<Idx: Step> RangeInclusive<Idx> {
     }
 }
 
-impl RangeInclusive<usize> {
-    /// Converts to an exclusive `Range` for `SliceIndex` implementations.
-    /// The caller is responsible for dealing with `end == usize::MAX`.
-    #[inline]
-    pub(crate) const fn into_slice_range(self) -> Range<usize> {
-        Range { start: self.start, end: self.end + 1 }
-    }
-}
-
 #[unstable(feature = "new_range_api", issue = "125687")]
 impl<T> RangeBounds<T> for RangeInclusive<T> {
     fn start_bound(&self) -> Bound<&T> {

library/core/src/slice/index.rs

@@ -650,7 +650,6 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeFull {
 }
 
 /// The methods `index` and `index_mut` panic if:
-/// - the end of the range is `usize::MAX` or
 /// - 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")]
@@ -660,12 +659,14 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
 
     #[inline]
     fn get(self, slice: &[T]) -> Option<&[T]> {
-        if *self.end() == usize::MAX { None } else { self.into_slice_range().get(slice) }
+        // `self.into_slice_range()` cannot overflow, because `*self.end() <
+        // slice.len()` implies `*self.end() < usize::MAX`.
+        if *self.end() >= slice.len() { None } else { self.into_slice_range().get(slice) }
     }
 
     #[inline]
     fn get_mut(self, slice: &mut [T]) -> Option<&mut [T]> {
-        if *self.end() == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
+        if *self.end() >= slice.len() { None } else { self.into_slice_range().get_mut(slice) }
     }
 
     #[inline]
@@ -865,6 +866,7 @@ where
 
         ops::Bound::Excluded(&end) if end > len => slice_index_fail(0, end, len),
         ops::Bound::Excluded(&end) => end,
+
         ops::Bound::Unbounded => len,
     };
 
@@ -920,19 +922,29 @@ where
 {
     let len = bounds.end;
 
-    let start = match range.start_bound() {
-        ops::Bound::Included(&start) => start,
-        ops::Bound::Excluded(start) => start.checked_add(1)?,
-        ops::Bound::Unbounded => 0,
-    };
-
     let end = match range.end_bound() {
-        ops::Bound::Included(end) => end.checked_add(1)?,
+        ops::Bound::Included(&end) if end >= len => return None,
+        // Cannot overflow because `end < len` implies `end < usize::MAX`.
+        ops::Bound::Included(end) => end + 1,
+
+        ops::Bound::Excluded(&end) if end > len => return None,
         ops::Bound::Excluded(&end) => end,
+
         ops::Bound::Unbounded => len,
     };
 
-    if start > end || end > len { None } else { Some(ops::Range { start, end }) }
+    let start = match range.start_bound() {
+        ops::Bound::Excluded(&start) if start >= end => return None,
+        // Cannot overflow because `start < end` implies `start < usize::MAX`.
+        ops::Bound::Excluded(&start) => start + 1,
+
+        ops::Bound::Included(&start) if start > end => return None,
+        ops::Bound::Included(&start) => start,
+
+        ops::Bound::Unbounded => 0,
+    };
+
+    Some(ops::Range { start, end })
 }
 
 /// Converts a pair of `ops::Bound`s into `ops::Range` without performing any
@@ -961,21 +973,27 @@ pub(crate) fn into_range(
     len: usize,
     (start, end): (ops::Bound<usize>, ops::Bound<usize>),
 ) -> Option<ops::Range<usize>> {
-    use ops::Bound;
-    let start = match start {
-        Bound::Included(start) => start,
-        Bound::Excluded(start) => start.checked_add(1)?,
-        Bound::Unbounded => 0,
-    };
-
     let end = match end {
-        Bound::Included(end) => end.checked_add(1)?,
-        Bound::Excluded(end) => end,
-        Bound::Unbounded => len,
+        ops::Bound::Included(end) if end >= len => return None,
+        // Cannot overflow because `end < len` implies `end < usize::MAX`.
+        ops::Bound::Included(end) => end + 1,
+
+        ops::Bound::Excluded(end) if end > len => return None,
+        ops::Bound::Excluded(end) => end,
+
+        ops::Bound::Unbounded => len,
     };
 
-    // Don't bother with checking `start < end` and `end <= len`
-    // since these checks are handled by `Range` impls
+    let start = match start {
+        ops::Bound::Excluded(start) if start >= end => return None,
+        // Cannot overflow because `start < end` implies `start < usize::MAX`.
+        ops::Bound::Excluded(start) => start + 1,
+
+        ops::Bound::Included(start) if start > end => return None,
+        ops::Bound::Included(start) => start,
+
+        ops::Bound::Unbounded => 0,
+    };
 
     Some(start..end)
 }

library/core/src/str/mod.rs

@@ -87,7 +87,7 @@ fn slice_error_fail_rt(s: &str, begin: usize, end: usize) -> ! {
     let ellipsis = if trunc_len < s.len() { "[...]" } else { "" };
 
     // 1. out of bounds
-    if begin > s.len() || end > s.len() {
+    if begin > s.len() || end > s.len() || (end == s.len() && s.is_char_boundary(begin)) {
         let oob_index = if begin > s.len() { begin } else { end };
         panic!("byte index {oob_index} is out of bounds of `{s_trunc}`{ellipsis}");
     }

library/core/src/str/traits.rs

@@ -75,13 +75,6 @@ where
     }
 }
 
-#[inline(never)]
-#[cold]
-#[track_caller]
-const fn str_index_overflow_fail() -> ! {
-    panic!("attempted to index str up to maximum usize");
-}
-
 /// Implements substring slicing with syntax `&self[..]` or `&mut self[..]`.
 ///
 /// Returns a slice of the whole string, i.e., returns `&self` or `&mut
@@ -639,11 +632,11 @@ unsafe impl const SliceIndex<str> for ops::RangeInclusive<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
-        if *self.end() == usize::MAX { None } else { self.into_slice_range().get(slice) }
+        if *self.end() >= slice.len() { None } else { self.into_slice_range().get(slice) }
     }
     #[inline]
     fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
-        if *self.end() == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
+        if *self.end() >= slice.len() { None } else { self.into_slice_range().get_mut(slice) }
     }
     #[inline]
     unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
@@ -657,15 +650,15 @@ unsafe impl const SliceIndex<str> for ops::RangeInclusive<usize> {
     }
     #[inline]
     fn index(self, slice: &str) -> &Self::Output {
-        if *self.end() == usize::MAX {
-            str_index_overflow_fail();
+        if *self.end() >= slice.len() {
+            super::slice_error_fail(slice, *self.start(), *self.end());
         }
         self.into_slice_range().index(slice)
     }
     #[inline]
     fn index_mut(self, slice: &mut str) -> &mut Self::Output {
-        if *self.end() == usize::MAX {
-            str_index_overflow_fail();
+        if *self.end() >= slice.len() {
+            super::slice_error_fail(slice, *self.start(), *self.end());
         }
         self.into_slice_range().index_mut(slice)
     }
@@ -677,35 +670,29 @@ unsafe impl const SliceIndex<str> for range::RangeInclusive<usize> {
     type Output = str;
     #[inline]
     fn get(self, slice: &str) -> Option<&Self::Output> {
-        if self.end == usize::MAX { None } else { self.into_slice_range().get(slice) }
+        ops::RangeInclusive::from(self).get(slice)
     }
     #[inline]
     fn get_mut(self, slice: &mut str) -> Option<&mut Self::Output> {
-        if self.end == usize::MAX { None } else { self.into_slice_range().get_mut(slice) }
+        ops::RangeInclusive::from(self).get_mut(slice)
     }
     #[inline]
     unsafe fn get_unchecked(self, slice: *const str) -> *const Self::Output {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked`.
-        unsafe { self.into_slice_range().get_unchecked(slice) }
+        unsafe { ops::RangeInclusive::from(self).get_unchecked(slice) }
     }
     #[inline]
     unsafe fn get_unchecked_mut(self, slice: *mut str) -> *mut Self::Output {
         // SAFETY: the caller must uphold the safety contract for `get_unchecked_mut`.
-        unsafe { self.into_slice_range().get_unchecked_mut(slice) }
+        unsafe { ops::RangeInclusive::from(self).get_unchecked_mut(slice) }
     }
     #[inline]
     fn index(self, slice: &str) -> &Self::Output {
-        if self.end == usize::MAX {
-            str_index_overflow_fail();
-        }
-        self.into_slice_range().index(slice)
+        ops::RangeInclusive::from(self).index(slice)
     }
     #[inline]
     fn index_mut(self, slice: &mut str) -> &mut Self::Output {
-        if self.end == usize::MAX {
-            str_index_overflow_fail();
-        }
-        self.into_slice_range().index_mut(slice)
+        ops::RangeInclusive::from(self).index_mut(slice)
     }
 }