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.

# // Copyright 2025 Google LLC
# // SPDX-License-Identifier: Apache-2.0
#
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:?}");
}

• 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 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}".

More to Explore

There are also OnceCell and OnceLock, which allow initialization on first use. Making these useful requires some more knowledge than students have at this time.