Use add instead of offset to avoid casting to isize.

src/array_string.rs

@@ -230,7 +230,7 @@ impl<A> ArrayString<A>
             return Err(CapacityError::new(s));
         }
         unsafe {
-            let dst = self.xs.ptr_mut().offset(self.len() as isize);
+            let dst = self.xs.ptr_mut().add(self.len());
             let src = s.as_ptr();
             ptr::copy_nonoverlapping(src, dst, s.len());
             let newl = self.len() + s.len();
@@ -321,8 +321,8 @@ impl<A> ArrayString<A>
         let next = idx + ch.len_utf8();
         let len = self.len();
         unsafe {
-            ptr::copy(self.xs.ptr().offset(next as isize),
-                      self.xs.ptr_mut().offset(idx as isize),
+            ptr::copy(self.xs.ptr().add(next),
+                      self.xs.ptr_mut().add(idx),
                       len - next);
             self.set_len(len - (next - idx));
         }