main.rs

struct Entity {
    shape: Shape
}

enum Shape {
    Square (u32),
    Rectangle { width: u32, height: u32 },
}

trait Area {
    fn get_area(&self) -> u32;
}

impl Area for Entity {
    fn get_area(&self) -> u32 {
        match self.shape {
            Shape::Rectangle { width, height } => width * height,
            Shape::Square (width) => width * width,
        }
    }
}

fn main() {
    let a = Entity {
        shape: Shape::Rectangle { width: 3, height: 2 }
    };

    let b = Entity {
        shape: Shape::Square(3)
    };

    println!("A: {}", a.get_area());
    println!("B: {}", b.get_area());
}

Warnings

struct is never constructed: `Entity`
enum is never used: `Shape`