Implement BytesMut::extend_from_within (#818)

src/bytes.rs

@@ -367,34 +367,7 @@ impl Bytes {
     /// Requires that `begin <= end` and `end <= self.len()`, otherwise slicing
     /// will panic.
     pub fn slice(&self, range: impl RangeBounds<usize>) -> Self {
-        use core::ops::Bound;
-
-        let len = self.len();
-
-        let begin = match range.start_bound() {
-            Bound::Included(&n) => n,
-            Bound::Excluded(&n) => n.checked_add(1).expect("out of range"),
-            Bound::Unbounded => 0,
-        };
-
-        let end = match range.end_bound() {
-            Bound::Included(&n) => n.checked_add(1).expect("out of range"),
-            Bound::Excluded(&n) => n,
-            Bound::Unbounded => len,
-        };
-
-        assert!(
-            begin <= end,
-            "range start must not be greater than end: {:?} <= {:?}",
-            begin,
-            end,
-        );
-        assert!(
-            end <= len,
-            "range end out of bounds: {:?} <= {:?}",
-            end,
-            len,
-        );
+        let (begin, end) = crate::range(range, self.len());
 
         if end == begin {
             return Bytes::new_empty_with_ptr(self.ptr.wrapping_add(begin));