This would lead to undefined behaviour.

The vec field in the IntoIter struct is not in a valid state after iteration has begun. All elements before next have been "removed" from the Vec through unsafe code even though it's still there. All methods on IntoIter must take care to not read from them.

If you want a Debug implementation, you will need not to go through derive, but instead write a manual implementation. You can take a look at the Clone implementation to see how Debug could be implemented manually.