extendr

Rust lets you define custom types by grouping related values together using the struct keyword. This is similar in spirit to a named list in R, but with fixed fields and a declared type. Struct names use PascalCase, while field names use snake_case:

struct Person {
    name: String,
    age: i32,
}

A struct is created using “literal” syntax, where we write the struct name followed by curly brackets containing each field and its value:

let me = Person { name: "Josiah".to_string(), age: 29 };

Derive

By default, a new struct has no built-in behavior. It does not even have a defined behavior for printing. So, right now, if you try to print the Person struct, you will get an error.

src/main.rs

struct Person {
    name: String,
    age: i32
}

fn main() {
    let me = Person { name: "Josiah".to_string(), age: 29 };
    println!("{me:?}");
}

error[E0277]: `Person` doesn't implement `Debug`
 --> src/main.rs:8:15
  |
8 |     println!("{me:?}");
  |               ^^^^^^ `Person` cannot be formatted using `{:?}` because it doesn't implement `Debug`
  |
  = help: the trait `Debug` is not implemented for `Person`
  = note: add `#[derive(Debug)]` to `Person` or manually `impl Debug for Person`
  = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)
help: consider annotating `Person` with `#[derive(Debug)]`
  |
1 + #[derive(Debug)]
2 | struct Person {
  |

For more information about this error, try `rustc --explain E0277`.

Here again we see a great demonstration of Rust’s very informative error messages. In this case, it also suggests a solution. We should derive the Debug trait. What is a trait exactly? In Rust, traits are similar to R’s S3 generics — they define a set of methods that a type can support. In this case, the Debug trait allows for printing the content of a struct to stdout using the special :? syntax in println!() or the dbg!() macro.

For very simple behaviors like printing, it is possible to derive the behavior automatically from the struct definition itself. In fact, this is what the error message above is telling us to do. It tells us to use another type of macro, specifically the derive macro, placing it above the struct definition and telling it what specific behavior to derive, in this case, Debug.

src/main.rs

#[derive(Debug)]
struct Person {
    name: String,
    age: i32
}

fn main() {
    let me = Person { name: "Josiah".to_string(), age: 29 };
    println!("{me:?}");
}

Person { name: "Josiah", age: 29 }

Fields

Fields in a struct can be accessed with dot notation: me.name, me.age. But beware that ownership rules apply here, too. Accessing a field by moving it out of the struct partially moves the struct itself, making the whole struct unusable afterward:

src/main.rs

#[derive(Debug)]
struct Person {
    name: String,
    age: i32,
}

fn main() {
    let me = Person { name: "Josiah".to_string(), age: 29 };
    let name = me.name; // ⬅️ name moved out
    println!("{me:?}"); // ❌ error: partial move
}

error[E0382]: borrow of partially moved value: `me`
  --> src/main.rs:10:16
   |
 9 |     let name = me.name; // name moved out
   |                ------- value partially moved here
10 |     println!("{me:?}"); // error: partial move
   |                ^^ value borrowed here after partial move
   |
   = note: partial move occurs because `me.name` has type `String`, which does not implement the `Copy` trait
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0382`.

To read a field without consuming it, borrow it instead by referencing the struct.

src/main.rs

#[derive(Debug)]
struct Person {
    name: String,
    age: i32,
}

fn main() {
    let me = Person { name: "Josiah".to_string(), age: 29 };
    let name = &me.name;
    println!("{me:?}");
}

Person { name: "Josiah", age: 29 }

Destructuring

Rust lets you unpack all fields of a struct in a single let binding. This is called destructuring and uses the syntax let StructName { field1, field2 } = variable:

src/main.rs

fn main() {
    let me = Person { name: "Josiah".to_string(), age: 29 };
    let Person { name, age } = me;
    println!("{name} is {age} years old");
}

Josiah is 29 years old

Destructuring is useful when you need several fields at once and want to avoid repeated .field access. The struct is consumed in the process, so the original variable is no longer available.