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.

Cell

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

use std::cell::Cell;
fn main() {
    // Note that `cell` is NOT declared as mutable.
    let cell = Cell::new(5);
    cell.set(123);
    println!("{}", cell.get());
}

RefCell

RefCell allows accessing and mutating a wrapped value by providing alternative types Ref and RefMut that emulate &T/&mut T without actually being Rust references.

These types perform dynamic checks using a counter in the RefCell to prevent existence of a RefMut alongside another Ref/RefMut.

By implementing Deref (and DerefMut for RefMut), these types allow calling methods on the inner value without allowing references to escape.

use std::cell::RefCell;
fn main() {
    // Note that `cell` is NOT declared as mutable.
    let cell = RefCell::new(5);
    {
        let mut cell_ref = cell.borrow_mut();
        *cell_ref = 123;
        // This triggers an error at runtime.
        // let other = cell.borrow();
        // println!("{}", *other);
    }
    println!("{cell:?}");
}

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.

  • The extra block in the RefCell example is to end the borrow created by the call to borrow_mut before we print the cell. Trying to print a borrowed RefCell just shows the message "{borrowed}".

• 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.

• Both RefCell and Cell are !Sync, which means &RefCell and &Cell can’t be passed between threads. This prevents two threads trying to access the cell at once.