How to Use RefCell<T> for Interior Mutability in Rust

Use RefCell<T> to wrap a value when you need to mutate it through an immutable reference, deferring borrow checking to runtime.

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));
    }
}

The borrow_mut() method returns a mutable reference to the inner data, while borrow() returns an immutable one. This allows mutation inside methods that only take &self.

RefCell is a tool that lets you change data inside a structure even when you only have a read-only handle to it. It acts like a security guard that checks if anyone is currently using the data before letting you modify it, preventing crashes. You use this when you need to share data between parts of your program that shouldn't normally be allowed to change it.