Understanding Rust Strings and str inside structs.

Rust is a challenging language to work in coming from a more dynamic language like Javascript or Ruby. Here's a short breakdown of a compiler error message that stumped me for a bit.

Background

First some background about Rust. Rust is a strongly-typed language which means variables and return values from functions usually require a type annotation that looks like this:

let favorite_number: u32 = 12;

The type annotation on the variable favorite_number is u32. This should look familiar to anyone who has used a strongly typed language before.

A struct in Rust is a way for the user to define any "data type". Here's an example:

struct Config {
  name: String,
  id: u32
}

Here is how we would do this in Javascript:

const config = {
  name: "Adam",
  id: 4
}

Notice how in Javascript we use an Object named config to hold some config for our application. In Rust we create a struct called Config and define the various fields we need. Rust is strongly-typed so even the fields in the Config struct are type-annotated. Let's move to where I got tripped up.

Stumped

I created a Vector (a mutable Array data structure) that contained a couple of instances of a struct. It looked something like this:

struct Example {
    id: String
}

fn main() {
    let example: Vec<Example> = vec![Example{id: "one".to_string()}, Example{id: "two".to_string()}];
    println!("{:?}", example);
}

So far so good. However I now wanted to map over the collection and create a new collection that just contained the id. Here's what I tried and what failed:

struct Example {
    id: String
}

fn main() {
    let example: Vec<Example> = vec![Example{id: "one".to_string()}, Example{id: "two".to_string()}];
    let map_fail: Vec<String> = example.iter().map(|item| item.id).collect();
    println!("{:?}", map_fail);
}

Rust playground

Understanding the problem

What's happening here? Well the compiler message is instructive:

error[E0507]: cannot move out of `item.id` which is behind a shared reference
  --> src/main.rs:10:59
   |
10 |     let map_fail: Vec<String> = example.iter().map(|item| item.id).collect();
   |                                                           ^^^^^^^ move occurs because `item.id` has type `std::string::String`, which does not implement the `Copy` trait

error: aborting due to previous error

For more information about this error, try `rustc --explain E0507`.

In Rust there is no Garbage collection (automatic memory management). Instead Rust requires you the programmer to indicate to the Rust compiler what variables have ownership over the value they represent. If that ownership moves in an invalid way, Rust will throw a compile error to prevent a memory error at runtime. The benefit in not having Garbage Collection is two-fold:

  1. Faster code without a GC process.
  2. Whole classes of bugs are impossible due to Rust's ownership rules.

In our error case, mapping over our vector "moves" the data stored at id to a new Vector. This shift to a new Vector means Rust can no longer guarantee the safety of that portion of memory. Let's look at the documentation around this error code from the compiler.

The explainer page for this error gives us three options:

  1. Try to avoid moving the variable.
  2. Somehow reclaim the ownership.
  3. Implement the Copy trait on the type.

#3 sounded like too much for this specific case. #2 also seemed like the wrong approach because I didn't want to change the ownership, I just wanted a copy or reference to it. #1 was the right answer but how do I avoid moving the variable?

The Solution

This is where it's important to understand the two basic types for strings in Rust, String and str. The doc page for String states (I added the underlines for emphasis):

The String type is the most common string type that has ownership over the contents of the string. It has a close relationship with its borrowed counterpart, the primitive str.

Aha, so we're dealing with a String here and it sounds like we need a str. A quick glance at the String doc page again reveals just the function we need: to_str. to_str gives us a reference called a slice to the value of a String. Armed with this new knowledge, here's the solution to this compiler error:

struct Example {
    id: String
}

fn main() {
    let example: Vec<Example> = vec![Example{id: "one".to_string()}, Example{id: "two".to_string()}];
    println!("{:?}", example);

    let map_str: Vec<&str> = example.iter().map(|item| item.id.as_str()).collect();
    println!("{:?}", map_str);
}

Rust playground

Fin

Rust has proven challenging to learn but I think the journey and difficulty is worth it. Ownership, the type-system, and the excellent performance continue to impress me. I hope this post encourages folks to give Rust a try! I'm happy to answer any questions via the comments or twitter or wherever else.

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