Interior Mutability: RefCell, Cell

In Rust, the borrowing rules usually prevent you from having a mutable reference to data if you already have an immutable reference to it. This ensures data safety.

Interior mutability is a design pattern that allows you to mutate data even when you only have an immutable reference to it. It's like a controlled exception to Rust's usual rules, used in specific situations.

The Cell<T> type provides interior mutability for types that implement the Copy trait (like integers, booleans, characters). It works by replacing the value inside.

• get(): Returns a copy of the value inside the Cell.
• set(value): Replaces the value inside the Cell with a new one.

It's simple and efficient for small, copyable data.