space age - macros and Age<T>

lib.rs

use std::marker::PhantomData;

macro_rules! planet {
    ($planet: ident, $planetage: ident, $period: expr) => {
        pub struct $planet;
        impl Planet for $planet { fn period() -> f32 { $period } }
        pub type $planetage = Age<$planet>;
    };
}

pub trait Planet {
    fn period() -> f32;
}

planet!(Mercury, MercuryAge, 0.2408467);
planet!(Venus, VenusAge, 0.61519726);
planet!(Earth, EarthAge, 1.0);
planet!(Mars, MarsAge, 1.8808158);
planet!(Jupiter, JupiterAge, 11.862615);
planet!(Saturn, SaturnAge, 29.447498);
planet!(Uranus, UranusAge, 84.016846);
planet!(Neptune, NeptuneAge, 164.79132);

pub struct Age<T> {
    years: f32,
    planet: PhantomData<T>,
}

impl <T: Planet> From<u64> for Age<T> {
    fn from(seconds: u64) -> Self {
        Age {
            years: (seconds as f32) / 86400.0 / 365.25 / T::period(),
            planet: PhantomData,
        }
    }
}

impl <T> Age<T> {
    pub fn years(&self) -> f32 {
        self.years
    }
}