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## `RefCell<T>` and the Interior Mutability Pattern

Interior mutability is a design pattern in Rust that allows you to mutate data even when there are immutable references to that data; normally, this action is disallowed by the borrowing rules. To mutate data, the pattern uses `unsafe` code inside a data structure to bend Rust’s usual rules that govern mutation and borrowing. Unsafe code indicates to the compiler that we’re checking the rules manually instead of relying on the compiler to check them for us; we will discuss unsafe code more in Chapter 20.

We can use types that use the interior mutability pattern only when we can ensure that the borrowing rules will be followed at runtime, even though the compiler can’t guarantee that. The `unsafe` code involved is then wrapped in a safe API, and the outer type is still immutable.

Let’s explore this concept by looking at the `RefCell<T>` type that follows the interior mutability pattern.

### Enforcing Borrowing Rules at Runtime

Unlike `Rc<T>`, the `RefCell<T>` type represents single ownership over the data it holds. So, what makes `RefCell<T>` different from a type like `Box<T>`? Recall the borrowing rules you learned in Chapter 4:

- At any given time, you can have either one mutable reference or any number of immutable references (but not both).
- References must always be valid.

With references and `Box<T>`, the borrowing rules’ invariants are enforced at compile time. With `RefCell<T>`, these invariants are enforced at runtime. With references, if you break these rules, you’ll get a compiler error. With `RefCell<T>`, if you break these rules, your program will panic and exit.

The advantages of checking the borrowing rules at compile time are that errors will be caught sooner in the development process, and there is no impact on runtime performance because all the analysis is completed beforehand. For those reasons, checking the borrowing rules at compile time is the best choice in the majority of cases, which is why this is Rust’s default.

The advantage of checking the borrowing rules at runtime instead is that certain memory-safe scenarios are then allowed, where they would’ve been disallowed by the compile-time checks. Static analysis, like the Rust compiler, is inherently conservative. Some properties of code are impossible to detect by analyzing the code: The most famous example is the Halting Problem, which is beyond the scope of this book but is an interesting topic to research.

Because some analysis is impossible, if the Rust compiler can’t be sure the code complies with the ownership rules, it might reject a correct program; in this way, it’s conservative. If Rust accepted an incorrect program, users wouldn’t be able to trust the guarantees Rust makes. However, if Rust rejects a correct program, the programmer will be inconvenienced, but nothing catastrophic can occur. The `RefCell<T>` type is useful when you’re sure your code follows the borrowing rules but the compiler is unable to understand and guarantee that.

Similar to `Rc<T>`, `RefCell<T>` is only for use in single-threaded scenarios and will give you a compile-time error if you try using it in a multithreaded context. We’ll talk about how to get the functionality of `RefCell<T>` in a multithreaded program in Chapter 16.

Here is a recap of the reasons to choose `Box<T>`, `Rc<T>`, or `RefCell<T>`:

- `Rc<T>` enables multiple owners of the same data; `Box<T>` and `RefCell<T>` have single owners.
- `Box<T>` allows immutable or mutable borrows checked at compile time; `Rc<T>` allows only immutable borrows checked at compile time; `RefCell<T>` allows immutable or mutable borrows checked at runtime.
- Because `RefCell<T>` allows mutable borrows checked at runtime, you can mutate the value inside the `RefCell<T>` even when the `RefCell<T>` is immutable.

Mutating the value inside an immutable value is the interior mutability pattern. Let’s look at a situation in which interior mutability is useful and examine how it’s possible.

### Using Interior Mutability

A consequence of the borrowing rules is that when you have an immutable value, you can’t borrow it mutably. For example, this code won’t compile:

    fn main() {
        let x = 5;
        let y = &mut x;
    }

If you tried to compile this code, you’d get the following error:

    $ cargo run
       Compiling borrowing v0.1.0 (file:///projects/borrowing)
    error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
     --> src/main.rs:3:13
      |
    3 |     let y = &mut x;
      |             ^^^^^^ cannot borrow as mutable
      |
    help: consider changing this to be mutable
      |
    2 |     let mut x = 5;
      |         +++

    For more information about this error, try `rustc --explain E0596`.
    error: could not compile `borrowing` (bin "borrowing") due to 1 previous error

However, there are situations in which it would be useful for a value to mutate itself in its methods but appear immutable to other code. Code outside the value’s methods would not be able to mutate the value. Using `RefCell<T>` is one way to get the ability to have interior mutability, but `RefCell<T>` doesn’t get around the borrowing rules completely: The borrow checker in the compiler allows this interior mutability, and the borrowing rules are checked at runtime instead. If you violate the rules, you’ll get a `panic!` instead of a compiler error.

Let’s work through a practical example where we can use `RefCell<T>` to mutate an immutable value and see why that is useful.

#### Testing with Mock Objects

Sometimes during testing a programmer will use a type in place of another type, in order to observe particular behavior and assert that it’s implemented correctly. This placeholder type is called a test double. Think of it in the sense of a stunt double in filmmaking, where a person steps in and substitutes for an actor to do a particularly tricky scene. Test doubles stand in for other types when we’re running tests. Mock objects are specific types of test doubles that record what happens during a test so that you can assert that the correct actions took place.

Rust doesn’t have objects in the same sense as other languages have objects, and Rust doesn’t have mock object functionality built into the standard library as some other languages do. However, you can definitely create a struct that will serve the same purposes as a mock object.

Here’s the scenario we’ll test: We’ll create a library that tracks a value against a maximum value and sends messages based on how close to the maximum value the current value is. This library could be used to keep track of a user’s quota for the number of API calls they’re allowed to make, for example.

Our library will only provide the functionality of tracking how close to the maximum a value is and what the messages should be at what times. Applications that use our library will be expected to provide the mechanism for sending the messages: The application could show the message to the user directly, send an email, send a text message, or do something else. The library doesn’t need to know that detail. All it needs is something that implements a trait we’ll provide, called `Messenger`. Listing 15-20 shows the library code.

Filename: src/lib.rs

    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!");
            }
        }
    }

Listing 15-20: A library to keep track of how close a value is to a maximum value and warn when the value is at certain levels

One important part of this code is that the `Messenger` trait has one method called `send` that takes an immutable reference to `self` and the text of the message. This trait is the interface our mock object needs to implement so that the mock can be used in the same way a real object is. The other important part is that we want to test the behavior of the `set_value` method on the `LimitTracker`. We can change what we pass in for the `value` parameter, but `set_value` doesn’t return anything for us to make assertions on. We want to be able to say that if we create a `LimitTracker` with something that implements the `Messenger` trait and a particular value for `max`, the messenger is told to send the appropriate messages when we pass different numbers for `value`.

We need a mock object that, instead of sending an email or text message when we call `send`, will only keep track of the messages it’s told to send. We can create a new instance of the mock object, create a `LimitTracker` that uses the mock object, call the `set_value` method on `LimitTracker`, and then check that the mock object has the messages we expect. Listing 15-21 shows an attempt to implement a mock object to do just that, but the borrow checker won’t allow it.

Filename: src/lib.rs

    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);
        }
    }

Listing 15-21: An attempt to implement a `MockMessenger` that isn’t allowed by the borrow checker

This test code defines a `MockMessenger` struct that has a `sent_messages` field with a `Vec` of `String` values to keep track of the messages it’s told to send. We also define an associated function `new` to make it convenient to create new `MockMessenger` values that start with an empty list of messages. We then implement the `Messenger` trait for `MockMessenger` so that we can give a `MockMessenger` to a `LimitTracker`. In the definition of the `send` method, we take the message passed in as a parameter and store it in the `MockMessenger` list of `sent_messages`.

In the test, we’re testing what happens when the `LimitTracker` is told to set `value` to something that is more than 75 percent of the `max` value. First, we create a new `MockMessenger`, which will start with an empty list of messages. Then, we create a new `LimitTracker` and give it a reference to the new `MockMessenger` and a `max` value of `100`. We call the `set_value` method on the `LimitTracker` with a value of `80`, which is more than 75 percent of 100. Then, we assert that the list of messages that the `MockMessenger` is keeping track of should now have one message in it.

However, there’s one problem with this test, as shown here:

    $ 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

We can’t modify the `MockMessenger` to keep track of the messages, because the `send` method takes an immutable reference to `self`. We also can’t take the suggestion from the error text to use `&mut self` in both the `impl` method and the trait definition. We do not want to change the `Messenger` trait solely for the sake of testing. Instead, we need to find a way to make our test code work correctly with our existing design.

This is a situation in which interior mutability can help! We’ll store the `sent_messages` within a `RefCell<T>`, and then the `send` method will be able to modify `sent_messages` to store the messages we’ve seen. Listing 15-22 shows what that looks like.

Filename: src/lib.rs

    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);
        }
    }

Listing 15-22: Using `RefCell<T>` to mutate an inner value while the outer value is considered immutable

The `sent_messages` field is now of type `RefCell<Vec<String>>` instead of `Vec<String>`. In the `new` function, we create a new `RefCell<Vec<String>>` instance around the empty vector.

For the implementation of the `send` method, the first parameter is still an immutable borrow of `self`, which matches the trait definition. We call `borrow_mut` on the `RefCell<Vec<String>>` in `self.sent_messages` to get a mutable reference to the value inside the `RefCell<Vec<String>>`, which is the vector. Then, we can call `push` on the mutable reference to the vector to keep track of the messages sent during the test.

The last change we have to make is in the assertion: To see how many items are in the inner vector, we call `borrow` on the `RefCell<Vec<String>>` to get an immutable reference to the vector.

Now that you’ve seen how to use `RefCell<T>`, let’s dig into how it works!

#### Tracking Borrows at Runtime

When creating immutable and mutable references, we use the `&` and `&mut` syntax, respectively. With `RefCell<T>`, we use the `borrow` and `borrow_mut` methods, which are part of the safe API that belongs to `RefCell<T>`. The `borrow` method returns the smart pointer type `Ref<T>`, and `borrow_mut` returns the smart pointer type `RefMut<T>`. Both types implement `Deref`, so we can treat them like regular references.

The `RefCell<T>` keeps track of how many `Ref<T>` and `RefMut<T>` smart pointers are currently active. Every time we call `borrow`, the `RefCell<T>` increases its count of how many immutable borrows are active. When a `Ref<T>` value goes out of scope, the count of immutable borrows goes down by 1. Just like the compile-time borrowing rules, `RefCell<T>` lets us have many immutable borrows or one mutable borrow at any point in time.

If we try to violate these rules, rather than getting a compiler error as we would with references, the implementation of `RefCell<T>` will panic at runtime. Listing 15-23 shows a modification of the implementation of `send` in Listing 15-22. We’re deliberately trying to create two mutable borrows active for the same scope to illustrate that `RefCell<T>` prevents us from doing this at runtime.

Filename: src/lib.rs

    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);
        }
    }

Listing 15-23: Creating two mutable references in the same scope to see that `RefCell<T>` will panic

We create a variable `one_borrow` for the `RefMut<T>` smart pointer returned from `borrow_mut`. Then, we create another mutable borrow in the same way in the variable `two_borrow`. This makes two mutable references in the same scope, which isn’t allowed. When we run the tests for our library, the code in Listing 15-23 will compile without any errors, but the test will fail:

    $ 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:
    RefCell already borrowed
    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`

Notice that the code panicked with the message `already borrowed: BorrowMutError`. This is how `RefCell<T>` handles violations of the borrowing rules at runtime.

Choosing to catch borrowing errors at runtime rather than compile time, as we’ve done here, means you’d potentially be finding mistakes in your code later in the development process: possibly not until your code was deployed to production. Also, your code would incur a small runtime performance penalty as a result of keeping track of the borrows at runtime rather than compile time. However, using `RefCell<T>` makes it possible to write a mock object that can modify itself to keep track of the messages it has seen while you’re using it in a context where only immutable values are allowed. You can use `RefCell<T>` despite its trade-offs to get more functionality than regular references provide.

### Allowing Multiple Owners of Mutable Data

A common way to use `RefCell<T>` is in combination with `Rc<T>`. Recall that `Rc<T>` lets you have multiple owners of some data, but it only gives immutable access to that data. If you have an `Rc<T>` that holds a `RefCell<T>`, you can get a value that can have multiple owners and that you can mutate!

For example, recall the cons list example in Listing 15-18 where we used `Rc<T>` to allow multiple lists to share ownership of another list. Because `Rc<T>` holds only immutable values, we can’t change any of the values in the list once we’ve created them. Let’s add in `RefCell<T>` for its ability to change the values in the lists. Listing 15-24 shows that by using a `RefCell<T>` in the `Cons` definition, we can modify the value stored in all the lists.

Filename: src/main.rs

    #[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:?}");
    }

Listing 15-24: Using `Rc<RefCell<i32>>` to create a `List` that we can mutate

We create a value that is an instance of `Rc<RefCell<i32>>` and store it in a variable named `value` so that we can access it directly later. Then, we create a `List` in `a` with a `Cons` variant that holds `value`. We need to clone `value` so that both `a` and `value` have ownership of the inner `5` value rather than transferring ownership from `value` to `a` or having `a` borrow from `value`.

We wrap the list `a` in an `Rc<T>` so that when we create lists `b` and `c`, they can both refer to `a`, which is what we did in Listing 15-18.

After we’ve created the lists in `a`, `b`, and `c`, we want to add 10 to the value in `value`. We do this by calling `borrow_mut` on `value`, which uses the automatic dereferencing feature we discussed in “Where’s the `->` Operator?” in Chapter 5 to dereference the `Rc<T>` to the inner `RefCell<T>` value. The `borrow_mut` method returns a `RefMut<T>` smart pointer, and we use the dereference operator on it and change the inner value.

When we print `a`, `b`, and `c`, we can see that they all have the modified value of `15` rather than `5`:

    $ 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))

This technique is pretty neat! By using `RefCell<T>`, we have an outwardly immutable `List` value. But we can use the methods on `RefCell<T>` that provide access to its interior mutability so that we can modify our data when we need to. The runtime checks of the borrowing rules protect us from data races, and it’s sometimes worth trading a bit of speed for this flexibility in our data structures. Note that `RefCell<T>` does not work for multithreaded code! `Mutex<T>` is the thread-safe version of `RefCell<T>`, and we’ll discuss `Mutex<T>` in Chapter 16.