rust•armanriazi•error•panicked•at 'already borrowed: BorrowMutError'
If we try to violate these rules, rather than getting a compiler error as we would with references, the implementation of RefCell<T> will panic at runtime.
We’re deliberately trying to create two mutable borrows active for the same scope to illustrate that RefCell<T> prevents us from doing this at runtime.
//+//let mut one_borrow = self.sent_messages.borrow_mut();
//-//let mut two_borrow = self.sent_messages.borrow_mut();