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The Ultimate Guide to Rust Programming

Borrow

Explore Rust’s borrowing system to understand how to pass references without ownership transferring. Learn to use immutable and mutable references, fix common errors, and manage lifetimes and dereferencing for cleaner, safer code.

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Let’s keep the momentum going from our lesson on ownership! We spent a lot of time playing around with ownership and moving, and we found it really annoying (or, at least, I did) that we had to keep moving values into functions and then back out. It would be nice to have a better solution for it. With that kind of introduction, you better believe that’s what we’re about to learn.

Here’s another look at some of that move-in-move-out code we had before:

Rust 1.40.0
struct Fruit {
    apples: i32,
    bananas: i32,
}
fn increase_fruit(mut fruit: Fruit) -> Fruit {
    fruit.apples *= 2;
    fruit.bananas *= 3;
    fruit
}
fn print_fruit(fruit: Fruit) -> Fruit {
    println!("You have {} apples and {} bananas", fruit.apples, fruit.bananas);
    fruit
}
fn main() {
    let fruit = Fruit {
        apples: 10,
        bananas: 5,
    };
    let fruit = print_fruit(fruit);
    let fruit = increase_fruit(fruit);
    print_fruit(fruit);
}

In particular, this line really bothers me:

Rust 1.40.0
let fruit = print_fruit(fruit);

I don’t want to have to move the value in and back out. Instead, I’d like to be able to let print_fruit borrow the value I own in main, without moving it completely. Rust supports exactly that! Let’s work through changing the code, and then we’ll explain some details.

First, instead of passing print_fruit the fruit value itself, we need to pass it a borrowed reference. There’s a new unary operator to learn for this, &.

NOTE: Does that ...