Modelling a simple User/Group system

As a little exercise for myself, I wanted to model a User-Group system in Rust, similar to other systems that manage permissions (e.g. LDAP). The requirements I set for myself are: Two types of entities: Users and Groups Users can be added to Groups. Groups can also be added to other groups. I want to be able to see all members of a group, as well as all groups the user is a member of. This action is explicitly non-recursive (no need to traverse nested groups - I only need to see direct group ...