Challenge 2: Adding a Constructor — Possible Solution
====================================================================

struct Circle {
    radius: f64,
}

impl Circle {
    fn new(radius: f64) -> Circle {
        Circle { radius }
    }

    fn area(&self) -> f64 {
        3.14159 * self.radius * self.radius
    }
}

fn main() {
    let c = Circle::new(4.0);
    println!("Area: {}", c.area());
}

WHY THIS WORKS AS AN ANSWER
------------------------------
fn new(radius: f64) -> Circle has NO self parameter, which is exactly
what makes it an ASSOCIATED FUNCTION rather than a method — it can't
be called on an existing instance (there isn't one yet, which is the
whole point of a constructor), only via Circle::new(...), using the ::
namespacing syntax this chapter introduced.

Inside new, Circle { radius } uses Rust's field-init shorthand — since
the parameter is already named radius, matching the struct's own field
name, it doesn't need to be written as radius: radius.

Circle::new(4.0) replaces the earlier struct-literal syntax
(Circle { radius: 4.0 }) with a proper constructor call — functionally
equivalent here, but the associated-function form becomes valuable
once a constructor needs to do more than a plain literal could (validation,
computing a derived field, etc.), which is exactly why Rust code
almost always reaches for a ::new() constructor by convention, mirroring
the exact pattern String::from(...) already demonstrated back in
Chapter 1.