How to Use Cell<T> in Rust

Use RefCell<T> to enable interior mutability, allowing you to mutate data even when you only have an immutable reference to the container. Wrap your data in RefCell, then call .borrow_mut() to get a mutable reference to the inner value.

use std::cell::RefCell;

struct MockMessenger {
    sent_messages: RefCell<Vec<String>>,
}

impl MockMessenger {
    fn new() -> MockMessenger {
        MockMessenger {
            sent_messages: RefCell::new(vec![]),
        }
    }

    fn send(&self, message: &str) {
        self.sent_messages.borrow_mut().push(String::from(message));
    }
}

RefCell lets you change data inside a container even when you only have read-only access to the container itself. It acts like a safety guard that checks if anyone else is using the data before letting you make changes. You use this when you need to share data across different parts of your program but still need to update it occasionally.