Consts in structs, enums, and traits

Structs, enums, and traits definitions can also be provided with constant field members. They can be used in cases where you need to share a constant among them. Take, for example, a scenario where we have a Circle trait that's is meant to be implemented by different circular shape types. We can add a PI constant to the Circle trait, which can be shared by any type that has an area property and relies on value of PI for calculating the area:

// trait_constants.rs

trait Circular {
    const PI: f64 = 3.14;
    fn area(&self) -> f64;
}

struct Circle {
    rad: f64
}

impl Circular for Circle {
    fn area(&self) -> f64 {
        Circle::PI * self.rad * self.rad
    }
}

fn main() {
    let c_one = Circle { rad: 4.2 };
    let c_two = Circle { rad: 75.2 };
    println!("Area of circle one: {}", c_one.area());
    println!("Area...