macro_rules! impl_xy {
    // `()` indicates that the macro takes no argument.
    ($t:ty) => (
        impl Point for $t {
            fn x(&self) -> f64 {
                self.x
            }
            fn y(&self) -> f64 {
                self.y
            }
        }
    )
}

pub trait Point {
    fn x(&self) -> f64;
    fn y(&self) -> f64;
}

struct A {
    x: f64,
    y: f64,
}

impl_xy!(A);

struct B {
    x: f64,
    y: f64,
    z: f64,
}
impl_xy!(B);


fn get_x<T: Point>(p: &T) -> f64 {
    p.x()
}

fn main() {
    let a = A { x: 1.0, y: 2.0 };
    let b = B { x: 3.0, y: 4.0, z: 5.0 };

    println!("{}, {}", get_x(&a), get_x(&b));
}