Borrowing in Rust

January 27, 2022

  • rust

In Rust when you pass a variable to a function it is technically “moved”. That means the the caller looses ownership of the variable and can no longer read or modify it. When the other function finishes running it will take care of freeing up the memory used by the variable. For example:

fn main() {
    let singular = String::from("book");
    let plural = pluralize(singular);

    println!("The plural of {} is {}", singular, plural);
}

fn pluralize(singular: String) -> String {
    singular + "s"
}

The code above will produce the following error:

error[E0382]: borrow of moved value: `singular`
 --> src/main.rs:5:48
  |
2 |     let singular = String::from("book");
  |         -------- move occurs because `singular` has type `String`, which does not implement the `Copy` trait
3 |     let plural = pluralize(singular);
  |                            -------- value moved here
4 |
5 |     println!("The plural of {} is {}", singular, plural);
  |                                        ^^^^^^^^ value borrowed here after move

What the Rust compiler is trying to tell us here is that singular’s ownership was passed to pluralize, and therefore cannot be accessed anymore by main. When pluralize finishes running, the memory allocated to store singular will be freed automatically.

One solution is to clone the variable:

fn main() {
    let singular = String::from("book");
    let plural = pluralize(singular.clone());

    println!("The plural of {} is {}", singular, plural);
}

fn pluralize(singular: String) -> String {
    singular + "s"
}

However this solution has a downside of using more memory, and in cases where we are dealing with structs with lots of information this can hurt performance.

Rust solves this with the idea of Borrowing. With borrowing you can pass the variable to your function to use, but the caller is still responsible for the cleanup and ultimately “owns” that piece of memory allocation. This may seem similar to pointers in C / C++ but the difference is that with borrowing, the rust compiler is able to check your code at compile time and ensure the reference would never be invalid (pointing to nothing / invalid memory). T

Let’s update our previous example to use borrowing:

fn main() {
    let singular = String::from("book");
    let plural = pluralize(&singular);

    println!("The plural of {} is {}", singular, plural);
}

fn pluralize(singular: &str) -> String {
    singular.to_owned() + "s"
}

Notice what changed:

  1. Added an & when passing singular to pluralize, this signifies that we are passing a borrowed string slice to pluralize.
  2. Changed the type of singular argument to a string slice &str.
  3. Finally, since concatenation requires a resize/reallocation of the memory slot storing singular we call to_owned() on the string slice before appending "s".

Technically we didn’t save an allocation in this case since we needed to return a concatenated copy, but this won’t always be the case (sometimes all the function needs to do is read the value..).

In Sum

Borrowing reduces allocations which helps improve both memory usage and improve runtime performance. It is consider a more idiomatic way of passing values to functions if they are still need to be read/modified in the caller after the function has been called.

Finally, Rust’s compiler is able to use this feature to ensure that references are always valid, preventing an entire class of errors which are hard to debug and are often only discovered during runtime in many languages.


© 2022, Dorian Karter