The Rust Programming Language

RefCell<T> and the Interior Mutability Pattern

Interior mutability allows mutating data through immutable references by deferring borrow checking to runtime. RefCell<T> enforces borrowing rules at runtime instead of compile time.

Runtime vs Compile-time Borrow Checking

RefCell<T> is single-threaded only. For multithreaded scenarios, use Mutex<T>.

Interior Mutability Use Case

Testing scenario requiring mutation through immutable interface:

pub trait Messenger {
    fn send(&self, msg: &str);
}

pub struct LimitTracker<'a, T: Messenger> {
    messenger: &'a T,
    value: usize,
    max: usize,
}

impl<'a, T> LimitTracker<'a, T>
where
    T: Messenger,
{
    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
        LimitTracker {
            messenger,
            value: 0,
            max,
        }
    }

    pub fn set_value(&mut self, value: usize) {
        self.value = value;

        let percentage_of_max = self.value as f64 / self.max as f64;

        if percentage_of_max >= 1.0 {
            self.messenger.send("Error: You are over your quota!");
        } else if percentage_of_max >= 0.9 {
            self.messenger
                .send("Urgent warning: You've used up over 90% of your quota!");
        } else if percentage_of_max >= 0.75 {
            self.messenger
                .send("Warning: You've used up over 75% of your quota!");
        }
    }
}

Attempting to implement a mock that tracks sent messages:

pub trait Messenger {
    fn send(&self, msg: &str);
}

pub struct LimitTracker<'a, T: Messenger> {
    messenger: &'a T,
    value: usize,
    max: usize,
}

impl<'a, T> LimitTracker<'a, T>
where
    T: Messenger,
{
    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
        LimitTracker {
            messenger,
            value: 0,
            max,
        }
    }

    pub fn set_value(&mut self, value: usize) {
        self.value = value;

        let percentage_of_max = self.value as f64 / self.max as f64;

        if percentage_of_max >= 1.0 {
            self.messenger.send("Error: You are over your quota!");
        } else if percentage_of_max >= 0.9 {
            self.messenger
                .send("Urgent warning: You've used up over 90% of your quota!");
        } else if percentage_of_max >= 0.75 {
            self.messenger
                .send("Warning: You've used up over 75% of your quota!");
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    struct MockMessenger {
        sent_messages: Vec<String>,
    }

    impl MockMessenger {
        fn new() -> MockMessenger {
            MockMessenger {
                sent_messages: vec![],
            }
        }
    }

    impl Messenger for MockMessenger {
        fn send(&self, message: &str) {
            self.sent_messages.push(String::from(message));
        }
    }

    #[test]
    fn it_sends_an_over_75_percent_warning_message() {
        let mock_messenger = MockMessenger::new();
        let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);

        limit_tracker.set_value(80);

        assert_eq!(mock_messenger.sent_messages.len(), 1);
    }
}

This fails because send takes &self but we need to mutate sent_messages:

$ cargo test
   Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
error[E0596]: cannot borrow `self.sent_messages` as mutable, as it is behind a `&` reference
  --> src/lib.rs:58:13
   |
58 |             self.sent_messages.push(String::from(message));
   |             ^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
   |
help: consider changing this to be a mutable reference in the `impl` method and the `trait` definition
   |
2  ~     fn send(&mut self, msg: &str);
3  | }
...
56 |     impl Messenger for MockMessenger {
57 ~         fn send(&mut self, message: &str) {
   |

For more information about this error, try `rustc --explain E0596`.
error: could not compile `limit-tracker` (lib test) due to 1 previous error

Solution with RefCell<T>

pub trait Messenger {
    fn send(&self, msg: &str);
}

pub struct LimitTracker<'a, T: Messenger> {
    messenger: &'a T,
    value: usize,
    max: usize,
}

impl<'a, T> LimitTracker<'a, T>
where
    T: Messenger,
{
    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
        LimitTracker {
            messenger,
            value: 0,
            max,
        }
    }

    pub fn set_value(&mut self, value: usize) {
        self.value = value;

        let percentage_of_max = self.value as f64 / self.max as f64;

        if percentage_of_max >= 1.0 {
            self.messenger.send("Error: You are over your quota!");
        } else if percentage_of_max >= 0.9 {
            self.messenger
                .send("Urgent warning: You've used up over 90% of your quota!");
        } else if percentage_of_max >= 0.75 {
            self.messenger
                .send("Warning: You've used up over 75% of your quota!");
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::cell::RefCell;

    struct MockMessenger {
        sent_messages: RefCell<Vec<String>>,
    }

    impl MockMessenger {
        fn new() -> MockMessenger {
            MockMessenger {
                sent_messages: RefCell::new(vec![]),
            }
        }
    }

    impl Messenger for MockMessenger {
        fn send(&self, message: &str) {
            self.sent_messages.borrow_mut().push(String::from(message));
        }
    }

    #[test]
    fn it_sends_an_over_75_percent_warning_message() {
        // --snip--
        let mock_messenger = MockMessenger::new();
        let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);

        limit_tracker.set_value(80);

        assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
    }
}

RefCell<T> provides:

• borrow() returns Ref<T> (immutable smart pointer)
• borrow_mut() returns RefMut<T> (mutable smart pointer)

Both implement Deref for transparent access.

Runtime Borrow Checking

RefCell<T> tracks active borrows at runtime. Violating borrowing rules causes panics:

pub trait Messenger {
    fn send(&self, msg: &str);
}

pub struct LimitTracker<'a, T: Messenger> {
    messenger: &'a T,
    value: usize,
    max: usize,
}

impl<'a, T> LimitTracker<'a, T>
where
    T: Messenger,
{
    pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
        LimitTracker {
            messenger,
            value: 0,
            max,
        }
    }

    pub fn set_value(&mut self, value: usize) {
        self.value = value;

        let percentage_of_max = self.value as f64 / self.max as f64;

        if percentage_of_max >= 1.0 {
            self.messenger.send("Error: You are over your quota!");
        } else if percentage_of_max >= 0.9 {
            self.messenger
                .send("Urgent warning: You've used up over 90% of your quota!");
        } else if percentage_of_max >= 0.75 {
            self.messenger
                .send("Warning: You've used up over 75% of your quota!");
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::cell::RefCell;

    struct MockMessenger {
        sent_messages: RefCell<Vec<String>>,
    }

    impl MockMessenger {
        fn new() -> MockMessenger {
            MockMessenger {
                sent_messages: RefCell::new(vec![]),
            }
        }
    }

    impl Messenger for MockMessenger {
        fn send(&self, message: &str) {
            let mut one_borrow = self.sent_messages.borrow_mut();
            let mut two_borrow = self.sent_messages.borrow_mut();

            one_borrow.push(String::from(message));
            two_borrow.push(String::from(message));
        }
    }

    #[test]
    fn it_sends_an_over_75_percent_warning_message() {
        let mock_messenger = MockMessenger::new();
        let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);

        limit_tracker.set_value(80);

        assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
    }
}

$ cargo test
   Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.91s
     Running unittests src/lib.rs (target/debug/deps/limit_tracker-e599811fa246dbde)

running 1 test
test tests::it_sends_an_over_75_percent_warning_message ... FAILED

failures:

---- tests::it_sends_an_over_75_percent_warning_message stdout ----

thread 'tests::it_sends_an_over_75_percent_warning_message' panicked at src/lib.rs:60:53:
already borrowed: BorrowMutError
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace


failures:
    tests::it_sends_an_over_75_percent_warning_message

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

error: test failed, to rerun pass `--lib`

Combining Rc<T> and RefCell<T>

Multiple ownership with mutability:

#[derive(Debug)]
enum List {
    Cons(Rc<RefCell<i32>>, Rc<List>),
    Nil,
}

use crate::List::{Cons, Nil};
use std::cell::RefCell;
use std::rc::Rc;

fn main() {
    let value = Rc::new(RefCell::new(5));

    let a = Rc::new(Cons(Rc::clone(&value), Rc::new(Nil)));

    let b = Cons(Rc::new(RefCell::new(3)), Rc::clone(&a));
    let c = Cons(Rc::new(RefCell::new(4)), Rc::clone(&a));

    *value.borrow_mut() += 10;

    println!("a after = {a:?}");
    println!("b after = {b:?}");
    println!("c after = {c:?}");
}

Output:

$ cargo run
   Compiling cons-list v0.1.0 (file:///projects/cons-list)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.63s
     Running `target/debug/cons-list`
a after = Cons(RefCell { value: 15 }, Nil)
b after = Cons(RefCell { value: 3 }, Cons(RefCell { value: 15 }, Nil))
c after = Cons(RefCell { value: 4 }, Cons(RefCell { value: 15 }, Nil))

Rc<RefCell<T>> enables multiple owners who can all mutate the shared data, checked at runtime rather than compile time.