It doesn't sound like they're making multiple implementations that can write? If they were, they could make a trait for a Writable fs. But from their descripti

It doesn't sound like they're making multiple implementations that can write? If they were, they could make a trait for a Writable fs. But from their description it sounded like exactly two concrete implementations, one read-only, one read-write. If you want to share code or implementations, there's lots of ways still. For example, if the read-write filesystem implementation is a superset of the read-only, put a read-only struct inside the read-write.