Optimize unchecked indexing into chunks and chunks_mut

library/core/src/slice/iter.rs

@@ -1418,18 +1418,17 @@ impl<'a, T> Iterator for Chunks<'a, T> {
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
         let start = idx * self.chunk_size;
-        let end = match start.checked_add(self.chunk_size) {
-            None => self.v.len(),
-            Some(end) => cmp::min(end, self.v.len()),
-        };
         // SAFETY: the caller guarantees that `i` is in bounds,
         // which means that `start` must be in bounds of the
-        // underlying `self.v` slice, and we made sure that `end`
+        // underlying `self.v` slice, and we made sure that `len`
         // is also in bounds of `self.v`. Thus, `start` cannot overflow
         // an `isize`, and the slice constructed by `from_raw_parts`
         // is a subslice of `self.v` which is guaranteed to be valid
         // for the lifetime `'a` of `self.v`.
-        unsafe { from_raw_parts(self.v.as_ptr().add(start), end - start) }
+        unsafe {
+            let len = cmp::min(self.v.len().unchecked_sub(start), self.chunk_size);
+            from_raw_parts(self.v.as_ptr().add(start), len)
+        }
     }
 }
 
@@ -1457,7 +1456,7 @@ impl<'a, T> DoubleEndedIterator for Chunks<'a, T> {
         } else {
             let start = (len - 1 - n) * self.chunk_size;
             let end = match start.checked_add(self.chunk_size) {
-                Some(res) => cmp::min(res, self.v.len()),
+                Some(res) => cmp::min(self.v.len(), res),
                 None => self.v.len(),
             };
             let nth_back = &self.v[start..end];
@@ -1579,17 +1578,16 @@ impl<'a, T> Iterator for ChunksMut<'a, T> {
     #[doc(hidden)]
     unsafe fn __iterator_get_unchecked(&mut self, idx: usize) -> Self::Item {
         let start = idx * self.chunk_size;
-        let end = match start.checked_add(self.chunk_size) {
-            None => self.v.len(),
-            Some(end) => cmp::min(end, self.v.len()),
-        };
         // SAFETY: see comments for `Chunks::__iterator_get_unchecked`.
         //
         // Also note that the caller also guarantees that we're never called
         // with the same index again, and that no other methods that will
         // access this subslice are called, so it is valid for the returned
         // slice to be mutable.
-        unsafe { from_raw_parts_mut(self.v.as_mut_ptr().add(start), end - start) }
+        unsafe {
+            let len = cmp::min(self.v.len().unchecked_sub(start), self.chunk_size);
+            from_raw_parts_mut(self.v.as_mut_ptr().add(start), len)
+        }
     }
 }
 
@@ -1619,7 +1617,7 @@ impl<'a, T> DoubleEndedIterator for ChunksMut<'a, T> {
         } else {
             let start = (len - 1 - n) * self.chunk_size;
             let end = match start.checked_add(self.chunk_size) {
-                Some(res) => cmp::min(res, self.v.len()),
+                Some(res) => cmp::min(self.v.len(), res),
                 None => self.v.len(),
             };
             let (temp, _tail) = mem::replace(&mut self.v, &mut []).split_at_mut(end);