There are times when Rust's insistence that each data value has only one owner just doesn't fit our program's data model. What if we're writing a word processing engine, and we wanted people to be able to include the same image in multiple places without wasting memory on duplicates; or what if we were modeling an organization where one person might be referenced by multiple roles?
We could have one definitive owner of the shared information, and use borrows everywhere else, and if that works, it's probably the way to go. There are two situations where it doesn't work, though:
- We don't know how long the lifetimes of each of the users of the shared data value will be
- We need write access to the shared data in at least one of the users ...
