Most methods on Pin use T: ?Sized to allow the underlying pointer to be dynamically sized. However both Pin::map_unchecked_mut and Pin::map_unchecked have another generic parameter U (the type to map to), which does not contain such (loosening of) trait bound. Is it intentional for U to be Sized, or is this an oversight? In other words would a change to allow U: ?Sized for the above mentioned methods be accepted?