FIX: Use raw pointers for .swap(i, j)

This fixes a minor issue where we want to avoid overlapping &mut from
the same array.

src/impl_methods.rs

@@ -630,11 +630,18 @@ where
         S: DataMut,
         I: NdIndex<D>,
     {
-        let ptr1: *mut _ = &mut self[index1];
-        let ptr2: *mut _ = &mut self[index2];
-        unsafe {
-            std_ptr::swap(ptr1, ptr2);
+        let ptr = self.as_mut_ptr();
+        let offset1 = index1.index_checked(&self.dim, &self.strides);
+        let offset2 = index2.index_checked(&self.dim, &self.strides);
+        if let Some(offset1) = offset1 {
+            if let Some(offset2) = offset2 {
+                unsafe {
+                    std_ptr::swap(ptr.offset(offset1), ptr.offset(offset2));
+                }
+                return;
+            }
         }
+        panic!("swap: index out of bounds for indices {:?} {:?}", index1, index2);
     }
 
     /// Swap elements *unchecked* at indices `index1` and `index2`.