Interior Mutability

In some situations, it’s necessary to modify data behind a shared (read-only) reference. For example, a shared data structure might have an internal cache, and wish to update that cache from read-only methods.

The “interior mutability” pattern allows exclusive (mutable) access behind a shared reference. The standard library provides several ways to do this, all while still ensuring safety, typically by performing a runtime check.


use std::cell::RefCell;
use std::rc::Rc;

#[derive(Debug, Default)]
struct Node {
    value: i64,
    children: Vec<Rc<RefCell<Node>>>,

impl Node {
    fn new(value: i64) -> Rc<RefCell<Node>> {
        Rc::new(RefCell::new(Node { value, ..Node::default() }))

    fn sum(&self) -> i64 {
        self.value + self.children.iter().map(|c| c.borrow().sum()).sum::<i64>()

fn main() {
    let root = Node::new(1);
    let subtree = Node::new(10);

    println!("graph: {root:#?}");
    println!("graph sum: {}", root.borrow().sum());


Cell wraps a value and allows getting or setting the value, even with a shared reference to the Cell. However, it does not allow any references to the value. Since there are no references, borrowing rules cannot be broken.

This slide should take about 10 minutes.

The main thing to take away from this slide is that Rust provides safe ways to modify data behind a shared reference. There are a variety of ways to ensure that safety, and RefCell and Cell are two of them.

  • RefCell enforces Rust’s usual borrowing rules (either multiple shared references or a single exclusive reference) with a runtime check. In this case, all borrows are very short and never overlap, so the checks always succeed.

  • Rc only allows shared (read-only) access to its contents, since its purpose is to allow (and count) many references. But we want to modify the value, so we need interior mutability.

  • Cell is a simpler means to ensure safety: it has a set method that takes &self. This needs no runtime check, but requires moving values, which can have its own cost.

  • Demonstrate that reference loops can be created by adding root to subtree.children.

  • To demonstrate a runtime panic, add a fn inc(&mut self) that increments self.value and calls the same method on its children. This will panic in the presence of the reference loop, with thread 'main' panicked at 'already borrowed: BorrowMutError'.