comprehensive_rust/src/main.rs

fn main() {
    let mut rect = Rectangle {
        width: 10,
        height: 5,
    };
    println!("old area: {}", rect.area());
    rect.inc_width(5);
    println!("new area: {}", rect.area());
}

struct Rectangle {
    width: u32,
    height: u32,
}

impl Rectangle {
    fn area(&self) -> u32 {
        return self.width * self.height;
    }

    fn inc_width(&mut self, delta: u32) {
        self.width += delta;
    }
}