How to Use std::cell for Interior Mutability

Use std::cell::RefCell<T> to wrap data that needs to be mutated through an immutable reference by calling borrow_mut() to get a mutable reference at runtime.

use std::cell::RefCell;

fn main() {
    let data = RefCell::new(5);
    *data.borrow_mut() = 10;
    println!("{}", *data.borrow());
}

The RefCell enforces Rust's borrowing rules at runtime instead of compile time, panicking if you violate them (e.g., holding multiple mutable borrows).

RefCell lets you change data inside a variable that is normally locked against changes. It acts like a security guard that checks if you are allowed to edit the data right now, preventing crashes from conflicting edits. Use it when you need to modify data shared between parts of your program that don't have direct ownership.