Default Parameters & Optional Arguments in Rust

In Rust’s std docs, there is an example of using unwrap_or() to mimic a default parameter:

use std::fmt;

struct Vector2D {
    x: isize,
    y: isize,
}

impl fmt::Binary for Vector2D {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let magnitude = (self.x * self.x + self.y * self.y) as f64;
        let magnitude = magnitude.sqrt();

        let decimals = f.precision().unwrap_or(3);
        let string = format!("{:.*}", decimals, magnitude);
        f.pad_integral(true, "", &string)
    }
}

fn main() {
    let myvector = Vector2D { x: 3, y: 4 };
    println!("Hello {:10b}!", myvector); // defaults to 3 decimals
    println!("Hello {:10.2b}!", myvector);
}

## Hello      5.000!
## Hello       5.00!