Oct 06 2024

Rust has two main string types: String and &str. Sometimes, people argue that these two types make Rust code difficult to write, because you have to think about which one you should be using in a given situation. My experience of writing Rust is that I don’t really think about this very much, and this post is about some rules of thumb that you can use to be like me.

Level 1: Don’t think about it at all

The very first thing you can do is follow the simplest rule:

Always use String, never use &str.

That looks like this:

struct Person {
    name: String,
}

fn first_word(words: String) -> String {
    words
        .split_whitespace()
        .next()
        .expect("words should not be empty")
        .to_string()
}

This style means you may need to add .to_string() or .clone() for things to work sometimes:

fn main() {
    let sentence = "Hello, world!";

    println!("{}", first_word(sentence.to_string()));

    let owned = String::from("A string");

    // if we don't clone here, we can't use owned the second time
    println!("{}", first_word(owned.clone()));
    println!("{}", first_word(owned));
}

But that’s okay, the compiler will let you know when you need to:

error[E0382]: use of moved value: `owned`
  --> src/main.rs:21:31
   |
18 |     let owned = String::from("A string");
   |         ----- move occurs because `owned` has type `String`, which does not implement the `Copy` trait
19 |
20 |     println!("{}", first_word(owned));
   |                               ----- value moved here
21 |     println!("{}", first_word(owned));
   |                               ^^^^^ value used here after move
   |
note: consider changing this parameter type in function `first_word` to borrow instead if owning the value isn't necessary
  --> src/main.rs:5:22
   |
5  | fn first_word(words: String) -> String {
   |    ----------        ^^^^^^ this parameter takes ownership of the value
   |    |
   |    in this function
help: consider cloning the value if the performance cost is acceptable
   |
20 |     println!("{}", first_word(owned.clone()));
   |                                    ++++++++

Hey, compiler, that’s a great idea. Let’s move on to level 2.

Level 2: prefer &str for function parameters

A rule that’s a little better is this one:

Always use String in structs, and for functions, use &str for parameters and String types for return values.

This is what the compiler error for level 1 was suggesting we do instead of .clone.

That results in code that looks like this:

struct Person {
    name: String,
}

fn first_word(words: &str) -> String {
    words
        .split_whitespace()
        .next()
        .expect("words should not be empty")
        .to_string()
}

fn main() {
    let sentence = "Hello, world!";

    println!("{}", first_word(sentence));

    let owned = String::from("A string");

    println!("{}", first_word(&owned));
    println!("{}", first_word(&owned));
}

We’re now doing much less copying. We do need to add a & on to String values that we wish to pass into first_word, but that’s not too bad, the compiler will help us when we forget:

error[E0308]: mismatched types
  --> src/main.rs:20:31
   |
20 |     println!("{}", first_word(owned));
   |                    ---------- ^^^^^ expected `&str`, found `String`
   |                    |
   |                    arguments to this function are incorrect
   |
note: function defined here
  --> src/main.rs:5:4
   |
5  | fn first_word(words: &str) -> String {
   |    ^^^^^^^^^^ -----------
help: consider borrowing here
   |
20 |     println!("{}", first_word(&owned));
   |                               +

Following this rule will get you through 95% of situations successfully. Yes, that number was found via the very scientific process of “I made it up, but it feels correct after writing Rust for the last twelve years.”

For 4% of that last 5%, we can go to level 3:

Level 3: return &str sometimes

Here’s a slightly more advanced rule for certain circumstances:

Always use String in structs, and for functions, use &str for parameters. If the return type of your function is derived from an argument and isn’t mutated by the body, return &str. If you run into any trouble here, return String instead.

That would look like this:

struct Person {
    name: String,
}

// we're returning a substring of words, so &str is appropriate
fn first_word(words: &str) -> &str {
    words
        .split_whitespace()
        .next()
        .expect("words should not be empty")
}

fn main() {
    let sentence = "Hello, world!";

    println!("{}", first_word(sentence));

    let owned = String::from("A string");

    println!("{}", first_word(&owned));
    println!("{}", first_word(&owned));
}

This lets us remove a copy, we no longer have a .to_string in the body of first_word.

Sometimes, we can’t do that though:

// because we are going to uppercase the first word, our return type can't
// be &str anymore, because we aren't actually returning a substring: we are
// creating our own new string.
fn first_word_uppercase(words: &str) -> String {
    words
        .split_whitespace()
        .next()
        .expect("words should not be empty")
        .to_uppercase()
}

fn main() {
    let sentence = "Hello, world!";

    println!("{}", first_word_uppercase(sentence));

    let owned = String::from("A string");

    println!("{}", first_word_uppercase(&owned));
    println!("{}", first_word_uppercase(&owned));
}

How do you know that this is the case? Well, in this specific case, to_uppercase already returns a String. So that’s a great hint. If we tried to return a &str, we’d get an error:

// this can't work
fn first_word_uppercase(words: &str) -> &str {
    &words
        .split_whitespace()
        .next()
        .expect("words should not be empty")
        .to_uppercase()
}

would give us

error[E0515]: cannot return reference to temporary value
  --> src/main.rs:7:5
   |
7  |        &words
   |  ______^-
   | | ______|
8  | ||         .split_whitespace()
9  | ||         .next()
10 | ||         .expect("words should not be empty")
11 | ||         .to_uppercase()
   | ||                       ^
   | ||_______________________|
   |  |_______________________returns a reference to data owned by the current function
   |                          temporary value created here

And that’s really it. Following this rule will get you through virtually every scenario where you need to wonder about String and &str. With some practice, you’ll internalize these rules, and when you feel comfortable with a level, you can go up to the next one.

What about that last 1% though? Well, there is a next level…

Level 4: When to use &str in a struct

Here’s the rule for level 4:

Should you use a &str in a struct? If you’re asking that question, use String. When you need to use &str in a struct, you’ll know.

Storing references in structs is useful, for sure, and it’s good that Rust supports it. But you’re only going to need it in fairly specific scenarios, and if you feel like you’re worring about String vs &str, you’re just not in the position to be worrying about the complexity of storing a &str in a struct yet.

In fact, some people believe in this rule so strongly that they’re working on a language where storing references in structs isn’t even possible, and it’s a language I’ve found very interesting lately: Hylo. They go a bit farther than that, even: in Hylo, you think of everything as being values, rather than having references at all. They think that you can write meaningful programs with this model. But this isn’t a post about Hylo. I’ll write one of those eventually. My point is just that you can really, truly, get away with not storing &strs in structs for a lot of useful Rust programs. So it’s not really worth spending mental energy on, until you determine that you must do so. That is the case when you’ve profiled your program and determined that copying strings to and from your struct is a big enough issue to bother with lifetimes.

I hope you’ve found this helpful, and if you have any other rules of thumb like this, I’d love to hear about them!

Here’s my post about this post on BlueSky: