Rust's Display Trait

Rust’s trait system is pretty cool, and one of the common traits is Display. Defining an implementation for this trait will allow you to use the standard formatter {} with your type, but the official example is a bit misleading.

use std::fmt;

struct Point {
    x: i32,
    y: i32,
}

impl fmt::Display for Point {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "({}, {})", self.x, self.y)
    }
}

fn main() {
    let origin = Point { x: 0, y: 0 };
    println!("The origin is: {origin}");
}

The origin is: (0, 0)

Well, that output is expected, but what if we add some padding? This should right align, using - as padding, and a width of 10 (reference).

fn main() {
    let origin = Point { x: 0, y: 0 };
    println!("The origin is: |{origin:->10}|");
}

The origin is: |(0, 0)|

Hm.. so format specifiers just get thrown away? It actually depends on how Display is implemented, which is why I say the example is misleading. Let’s fix it to respect formatting.

impl fmt::Display for Point {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        format!("({}, {})", self.x, self.y).fmt(f)
    }
}

The origin is: |----(0, 0)|

Yay!

What we’re doing here is generating a String and using its Display implementation to format it as expected. Note that we can also use any of the methods provided by Formatter if we want to pass formatting to our Point attributes too.

P.S. implementing Display allows you to call to_string() as well, which is pretty neat.