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 type that follows the interior mutability pattern.

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

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

    With references and Box, the borrowing rules’ invariants are enforced at compile time. With RefCell, these invariants are enforced at runtime. With references, if you break these rules, you’ll get a compiler error. With RefCell, 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 in the guarantees Rust makes. However, if Rust rejects a correct program, the programmer will be inconvenienced, but nothing catastrophic can occur. The RefCell 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, RefCell 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 in a multithreaded program in Chapter 16.

    Here is a recap of the reasons to choose Box, Rc, or RefCell:

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

    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:

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

    1. $ cargo run
    2.    Compiling borrowing v0.1.0 (file:///projects/borrowing)
    3. error[E0596]: cannot borrow `x` as mutable, as it is not declared as mutable
    4.  --> src/main.rs:3:13
    5.   |
    6. 2 |     let x = 5;
    7.   |         - help: consider changing this to be mutable: `mut x`
    8. 3 |     let y = &mut x;
    9.   |             ^^^^^^ cannot borrow as mutable
    11. For more information about this error, try `rustc --explain E0596`.
    12. error: could not compile `borrowing` due to 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 is one way to get the ability to have interior mutability, but RefCell 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 to mutate an immutable value and see why that is useful.

    Sometimes during testing a programmer will use a type in place of another type, in order to observe particular behavior and assert 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 particular 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 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.

    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 put a message in the application, send an email, send a text message, or 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

    1. pub trait Messenger {
    2.     fn send(&self, msg: &str);
    3. }
    5. pub struct LimitTracker<'a, T: Messenger> {
    6.     messenger: &'a T,
    7.     value: usize,
    8.     max: usize,
    9. }
    11. impl<'a, T> LimitTracker<'a, T>
    12. where
    13.     T: Messenger,
    14. {
    15.     pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
    16.         LimitTracker {
    17.             messenger,
    18.             value: 0,
    19.             max,
    20.         }
    21.     }
    23.     pub fn set_value(&mut self, value: usize) {
    24.         self.value = value;
    26.         let percentage_of_max = self.value as f64 / self.max as f64;
    28.         if percentage_of_max >= 1.0 {
    29.             self.messenger.send("Error: You are over your quota!");
    30.         } else if percentage_of_max >= 0.9 {
    31.             self.messenger
    32.                 .send("Urgent warning: You've used up over 90% of your quota!");
    33.         } else if percentage_of_max >= 0.75 {
    34.             self.messenger
    35.                 .send("Warning: You've used up over 75% of your quota!");
    36.         }
    37.     }
    38. }

    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, when we pass different numbers for value, the messenger is told to send the appropriate messages.

    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

    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 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 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:

    1. $ cargo test
    2.    Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
    3. error[E0596]: cannot borrow `self.sent_messages` as mutable, as it is behind a `&` reference
    4.    |
    5. 2  |     fn send(&self, msg: &str);
    6.    |             ----- help: consider changing that to be a mutable reference: `&mut self`
    7. ...
    8. 58 |             self.sent_messages.push(String::from(message));
    9.    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `self` is a `&` reference, so the data it refers to cannot be borrowed as mutable
    11. For more information about this error, try `rustc --explain E0596`.
    12. error: could not compile `limit-tracker` due to previous error
    13. warning: build failed, waiting for other jobs to finish...

    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 instead, because then the signature of send wouldn’t match the signature in the Messenger trait definition (feel free to try and see what error message you get).

    This is a situation in which interior mutability can help! We’ll store the sent_messages within a RefCell, 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

    1. pub trait Messenger {
    2.     fn send(&self, msg: &str);
    3. }
    5. pub struct LimitTracker<'a, T: Messenger> {
    6.     messenger: &'a T,
    7.     value: usize,
    8.     max: usize,
    9. }
    11. impl<'a, T> LimitTracker<'a, T>
    12. where
    13.     T: Messenger,
    14. {
    15.     pub fn new(messenger: &'a T, max: usize) -> LimitTracker<'a, T> {
    16.         LimitTracker {
    17.             messenger,
    18.             value: 0,
    19.             max,
    20.         }
    21.     }
    23.     pub fn set_value(&mut self, value: usize) {
    24.         self.value = value;
    26.         let percentage_of_max = self.value as f64 / self.max as f64;
    28.         if percentage_of_max >= 1.0 {
    29.             self.messenger.send("Error: You are over your quota!");
    30.         } else if percentage_of_max >= 0.9 {
    31.             self.messenger
    32.                 .send("Urgent warning: You've used up over 90% of your quota!");
    33.         } else if percentage_of_max >= 0.75 {
    34.             self.messenger
    35.                 .send("Warning: You've used up over 75% of your quota!");
    36.         }
    37.     }
    38. }
    40. #[cfg(test)]
    41. mod tests {
    42.     use super::*;
    43.     use std::cell::RefCell;
    45.     struct MockMessenger {
    46.         sent_messages: RefCell<Vec<String>>,
    47.     }
    49.     impl MockMessenger {
    50.         fn new() -> MockMessenger {
    51.             MockMessenger {
    52.                 sent_messages: RefCell::new(vec![]),
    53.             }
    54.     }
    56.     impl Messenger for MockMessenger {
    57.         fn send(&self, message: &str) {
    58.             self.sent_messages.borrow_mut().push(String::from(message));
    59.         }
    60.     }
    62.     #[test]
    63.     fn it_sends_an_over_75_percent_warning_message() {
    64.         // --snip--
    65.         let mock_messenger = MockMessenger::new();
    66.         let mut limit_tracker = LimitTracker::new(&mock_messenger, 100);
    68.         limit_tracker.set_value(80);
    70.         assert_eq!(mock_messenger.sent_messages.borrow().len(), 1);
    71.     }
    72. }

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

    The sent_messages field is now of type RefCell> instead of Vec. In the new function, we create a new RefCell> 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> in self.sent_messages to get a mutable reference to the value inside the RefCell>, 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.

    Now that you’ve seen how to use RefCell, let’s dig into how it works!

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

    The RefCell keeps track of how many Ref and RefMut smart pointers are currently active. Every time we call borrow, the RefCell increases its count of how many immutable borrows are active. When a Ref value goes out of scope, the count of immutable borrows goes down by one. Just like the compile-time borrowing rules, RefCell 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 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 prevents us from doing this at runtime.

    Filename: src/lib.rs

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

    We create a variable one_borrow for the RefMut 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:

    1. $ cargo test
    2.    Compiling limit-tracker v0.1.0 (file:///projects/limit-tracker)
    3.     Finished test [unoptimized + debuginfo] target(s) in 0.91s
    4.      Running unittests src/lib.rs (target/debug/deps/limit_tracker-e599811fa246dbde)
    6. running 1 test
    7. test tests::it_sends_an_over_75_percent_warning_message ... FAILED
    9. failures:
    11. ---- tests::it_sends_an_over_75_percent_warning_message stdout ----
    12. thread 'main' panicked at 'already borrowed: BorrowMutError', src/lib.rs:60:53
    13. note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
    15. failures:
    16.     tests::it_sends_an_over_75_percent_warning_message
    18. test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
    20. error: test failed, to rerun pass '--lib'

    Notice that the code panicked with the message already borrowed: BorrowMutError. This is how RefCell 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 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 despite its trade-offs to get more functionality than regular references provide.

    A common way to use RefCell is in combination with Rc. Recall that Rc lets you have multiple owners of some data, but it only gives immutable access to that data. If you have an Rc that holds a RefCell, 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 to allow multiple lists to share ownership of another list. Because Rc 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 to gain the ability to change the values in the lists. Listing 15-24 shows that by using a RefCell in the Cons definition, we can modify the value stored in all the lists:

    Filename: src/main.rs

    1. #[derive(Debug)]
    2. enum List {
    3.     Cons(Rc<RefCell<i32>>, Rc<List>),
    4.     Nil,
    5. }
    7. use crate::List::{Cons, Nil};
    8. use std::cell::RefCell;
    9. use std::rc::Rc;
    11. fn main() {
    12.     let value = Rc::new(RefCell::new(5));
    14.     let a = Rc::new(Cons(Rc::clone(&value), Rc::new(Nil)));
    16.     let b = Cons(Rc::new(RefCell::new(3)), Rc::clone(&a));
    17.     let c = Cons(Rc::new(RefCell::new(4)), Rc::clone(&a));
    19.     *value.borrow_mut() += 10;
    21.     println!("a after = {:?}", a);
    22.     println!("b after = {:?}", b);
    23.     println!("c after = {:?}", c);
    24. }

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

    We create a value that is an instance of Rc> and store it in a variable named value so 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 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 so 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 Chapter 5 (see the section ) to dereference the Rc to the inner RefCell value. The borrow_mut method returns a RefMut smart pointer, and we use the dereference operator on it and change the inner value.

    This technique is pretty neat! By using RefCell, we have an outwardly immutable List value. But we can use the methods on RefCell that provide access to its interior mutability so 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 does not work for multithreaded code! Mutex is the thread-safe version of RefCell and we’ll discuss Mutex in Chapter 16.