RefCell is similar to Cell, except that it accepts non-Copy data. This also means that when modifying the underlying object, it cannot simply copy it when returning it, it will need to return references. The same way, when you want to mutate the object inside, it will return a mutable reference. This only works because it will dynamically check at runtime whether a borrow exists before returning a mutable borrow, or the other way around, and if it does, the thread will panic.
Instead of using the get() method as in Cell, RefCell has two methods to get the underlying data: borrow() and borrow_mut(). The first will get a read-only borrow, and you can have as many immutable borrows in a scope. The second one will return a read-write ...
