References and Borrowing

Slices

We saw how to use indexing to get a single element out of an array. But we can also use indexing to get a reference to multiple elements:

fn main() {
    let a = [1, 2, 3, 4, 5];

    let s = &a[0..2];

    println!("The value of s is: {:?}", s);
}

Let’s try running it:

$ cargo run
   Compiling functions v0.1.0 (file:///home/steve/tmp/functions)
     Running `target/debug/functions`
The value of s is: [1, 2]

This guide is two of three presenting Rust’s ownership system. This is one of Rust’s most unique and compelling features, with which Rust developers should become quite acquainted. Ownership is how Rust achieves its largest goal, memory safety. There are a few distinct concepts, each with its own chapter:

ownership, the key concept
borrowing, which you’re reading now
lifetimes, an advanced concept of borrowing

These three chapters are related, and in order. You’ll need all three to fully understand the ownership system.

Borrowing

At the end of the ownership section, we had a nasty function that looked like this:

fn foo(v1: Vec<i32>, v2: Vec<i32>) -> (Vec<i32>, Vec<i32>, i32) {
    // do stuff with v1 and v2

    // hand back ownership, and the result of our function
    (v1, v2, 42)
}

let v1 = vec![1, 2, 3];
let v2 = vec![1, 2, 3];

let (v1, v2, answer) = foo(v1, v2);

This is not idiomatic Rust, however, as it doesn’t take advantage of borrowing. Here’s the first step:

fn foo(v1: &Vec<i32>, v2: &Vec<i32>) -> i32 {
    // do stuff with v1 and v2

    // return the answer
    42
}

let v1 = vec![1, 2, 3];
let v2 = vec![1, 2, 3];

let answer = foo(&v1, &v2);

// we can use v1 and v2 here!

Instead of taking Vec<i32>s as our arguments, we take a reference: &Vec<i32>. And instead of passing v1 and v2 directly, we pass &v1 and &v2. We call the &T type a ‘reference’, and rather than owning the resource, it borrows ownership. A binding that borrows something does not deallocate the resource when it goes out of scope. This means that after the call to foo(), we can use our original bindings again.

References are immutable, just like bindings. This means that inside of foo(), the vectors can’t be changed at all:

fn foo(v: &Vec<i32>) {
     v.push(5);
}

let v = vec![];

foo(&v);

errors with:

error: cannot borrow immutable borrowed content `*v` as mutable
v.push(5);
^

Pushing a value mutates the vector, and so we aren’t allowed to do it.

&mut references

There’s a second kind of reference: &mut T. A ‘mutable reference’ allows you to mutate the resource you’re borrowing. For example:

let mut x = 5;
{
    let y = &mut x;
    *y += 1;
}
println!("{}", x);

This will print 6. We make y a mutable reference to x, then add one to the thing y points at. You’ll notice that x had to be marked mut as well. If it wasn’t, we couldn’t take a mutable borrow to an immutable value.

You'll also notice we added an asterisk (*) in front of y, making it *y, this is because y is an &mut reference. You'll also need to use them for accessing the contents of a reference as well.

Otherwise, &mut references are just like references. There is a large difference between the two, and how they interact, though. You can tell something is fishy in the above example, because we need that extra scope, with the { and }. If we remove them, we get an error:

error: cannot borrow `x` as immutable because it is also borrowed as mutable
    println!("{}", x);
                   ^
note: previous borrow of `x` occurs here; the mutable borrow prevents
subsequent moves, borrows, or modification of `x` until the borrow ends
        let y = &mut x;
                     ^
note: previous borrow ends here
fn main() {

}
^

As it turns out, there are rules.

The Rules

Here’s the rules about borrowing in Rust:

First, any borrow must last for a scope no greater than that of the owner. Second, you may have one or the other of these two kinds of borrows, but not both at the same time:

one or more references (&T) to a resource,
exactly one mutable reference (&mut T).

You may notice that this is very similar, though not exactly the same as, to the definition of a data race:

There is a ‘data race’ when two or more pointers access the same memory location at the same time, where at least one of them is writing, and the operations are not synchronized.

With references, you may have as many as you’d like, since none of them are writing. However, as we can only have one &mut at a time, it is impossible to have a data race. This is how Rust prevents data races at compile time: we’ll get errors if we break the rules.

With this in mind, let’s consider our example again.

Thinking in scopes

Here’s the code:

let mut x = 5;
let y = &mut x;

*y += 1;

println!("{}", x);

This code gives us this error:

error: cannot borrow `x` as immutable because it is also borrowed as mutable
    println!("{}", x);
                   ^

This is because we’ve violated the rules: we have a &mut T pointing to x, and so we aren’t allowed to create any &Ts. One or the other. The note hints at how to think about this problem:

note: previous borrow ends here
fn main() {

}
^

In other words, the mutable borrow is held through the rest of our example. What we want is for the mutable borrow to end before we try to call println! and make an immutable borrow. In Rust, borrowing is tied to the scope that the borrow is valid for. And our scopes look like this:

let mut x = 5;

let y = &mut x;    // -+ &mut borrow of x starts here
                   //  |
*y += 1;           //  |
                   //  |
println!("{}", x); // -+ - try to borrow x here
                   // -+ &mut borrow of x ends here

The scopes conflict: we can’t make an &x while y is in scope.

So when we add the curly braces:

let mut x = 5;

{
    let y = &mut x; // -+ &mut borrow starts here
    *y += 1;        //  |
}                   // -+ ... and ends here

println!("{}", x);  // <- try to borrow x here

There’s no problem. Our mutable borrow goes out of scope before we create an immutable one. But scope is the key to seeing how long a borrow lasts for.

Issues borrowing prevents

Why have these restrictive rules? Well, as we noted, these rules prevent data races. What kinds of issues do data races cause? Here’s a few.

Iterator invalidation

One example is ‘iterator invalidation’, which happens when you try to mutate a collection that you’re iterating over. Rust’s borrow checker prevents this from happening:

let mut v = vec![1, 2, 3];

for i in &v {
    println!("{}", i);
}

This prints out one through three. As we iterate through the vectors, we’re only given references to the elements. And v is itself borrowed as immutable, which means we can’t change it while we’re iterating:

let mut v = vec![1, 2, 3];

for i in &v {
    println!("{}", i);
    v.push(34);
}

Here’s the error:

error: cannot borrow `v` as mutable because it is also borrowed as immutable
    v.push(34);
    ^
note: previous borrow of `v` occurs here; the immutable borrow prevents
subsequent moves or mutable borrows of `v` until the borrow ends
for i in &v {
          ^
note: previous borrow ends here
for i in &v {
    println!(“{}”, i);
    v.push(34);
}
^

We can’t modify v because it’s borrowed by the loop.

use after free

References must not live longer than the resource they refer to. Rust will check the scopes of your references to ensure that this is true.

If Rust didn’t check this property, we could accidentally use a reference which was invalid. For example:

let y: &i32;
{
    let x = 5;
    y = &x;
}

println!("{}", y);

We get this error:

error: `x` does not live long enough
    y = &x;
         ^
note: reference must be valid for the block suffix following statement 0 at
2:16...
let y: &i32;
{
    let x = 5;
    y = &x;
}

note: ...but borrowed value is only valid for the block suffix following
statement 0 at 4:18
    let x = 5;
    y = &x;
}

In other words, y is only valid for the scope where x exists. As soon as x goes away, it becomes invalid to refer to it. As such, the error says that the borrow ‘doesn’t live long enough’ because it’s not valid for the right amount of time.

The same problem occurs when the reference is declared before the variable it refers to. This is because resources within the same scope are freed in the opposite order they were declared:

let y: &i32;
let x = 5;
y = &x;

println!("{}", y);