We've seen how Rc enables multiple owners and how RefCell allows interior mutability. Individually, they solve specific problems. But when combined, they unlock a powerful pattern for managing shared, mutable state in a single-threaded context. The combination Rc> allows a single piece of data to have multiple owners, any of whom can mutate it.
We've seen how Rust's borrowing rules are enforced at compile time, and how Rc allows shared ownership of immutable data. But what if we need to mutate data that is shared and theoretically immutable? For this, Rust provides the interior mutability pattern, and its primary tool is the RefCell smart pointer.
The Rc and RefCell patterns are incredibly powerful, but they come with a potential danger: reference cycles. A reference cycle is a situation where items reference each other in a loop, causing their reference counts to never drop to zero. This is a form of memory leak, and while Rust's ownership system prevents most memory safety issues, this is one you must handle carefully yourself. In this article, we'll learn how to create a reference cycle and how to prevent it using weak references.