How to create and use constants in Rust.

pi.rs

trait Pi {
    fn give_me_pi() -> f32;
}

struct PI();

impl Pi for PI {
    fn give_me_pi() -> f32 {
        3.14
    }
}

fn circumference<T: Pi>(radius: f32, _pi: T) -> f32 {
    2.0 * T::give_me_pi() * radius
}

fn main() {
    // A constant should not take any space.
    assert_eq!(0, std::mem::size_of::<PI>());

    let radius = 10.0;

    // prints radius: 10, circumference: 62.800003
    println!("radius: {}, circumference: {}", radius, circumference(radius, PI()));
}