Cell, RefCell and Runtime Borrows

Start with the idea

Cell<T> is useful when a value can be read or replaced through a shared reference without handing out a reference to its contents. RefCell<T> instead hands out guarded borrows and checks their exclusivity at runtime. borrow_mut can panic on a conflict; try_borrow_mut lets you handle that conflict as an error. The example retains a reader, observes a rejected writer, then drops the reader and mutates successfully.

Neither type automatically supplies synchronized shared access between threads. RefCell<T> can be Send when T is Send, but it is not Sync: transferring a uniquely owned cell is different from sharing references to it concurrently.