Optimize SliceIndex impl for RangeInclusive

The check for `self.end() == usize::MAX` can be combined with the `self.end() +
1 > slice.len()` check into `self.en() >= slice.len()`, since `self.end() <
slice.len()` implies both `self.end() <= slice.len()` and `self.end() <
usize::MAX`.

The tradeoff is slightly worse error reporting: previously there would be a
special panic message in the `range.end() == usize::MAX` case.

Author: Kmeakin

library/core/src/slice/index.rs

@@ -658,7 +658,6 @@ 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")]
@@ -668,12 +667,14 @@ 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]
@@ -690,16 +691,16 @@ unsafe impl<T> const SliceIndex<[T]> for ops::RangeInclusive<usize> {
 
     #[inline]
     fn index(self, slice: &[T]) -> &[T] {
-        if *self.end() == usize::MAX {
-            slice_end_index_overflow_fail();
+        if *self.end() >= slice.len() {
+            slice_end_index_len_fail(*self.end(), slice.len());
         }
         self.into_slice_range().index(slice)
     }
 
     #[inline]
     fn index_mut(self, slice: &mut [T]) -> &mut [T] {
-        if *self.end() == usize::MAX {
-            slice_end_index_overflow_fail();
+        if *self.end() >= slice.len() {
+            slice_end_index_len_fail(*self.end(), slice.len());
         }
         self.into_slice_range().index_mut(slice)
     }

library/coretests/tests/slice.rs

@@ -1492,14 +1492,14 @@ 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 range";
         }
 
         in mod rangetoinclusive_overflow {
             data: [0, 1];
 
             bad: data[..= usize::MAX];
-            message: "maximum usize";
+            message: "out of range";
         }
 
         in mod boundpair_overflow_end {

src/tools/miri/tests/panic/oob_subslice.stderr

@@ -1,5 +1,5 @@
 
-thread 'main' ($TID) panicked at tests/panic/oob_subslice.rs:LL:CC:
-range end index 5 out of range for slice of length 4
+thread 'main' panicked at tests/panic/oob_subslice.rs:LL:CC:
+range end index 4 out of range for slice of length 4
 note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
 note: in Miri, you may have to set `MIRIFLAGS=-Zmiri-env-forward=RUST_BACKTRACE` for the environment variable to have an effect