Challenge 1: A Shape Enum With match — Possible Solution
====================================================================

enum Shape {
    Circle(f64),
    Rectangle(f64, f64),
}

fn area(shape: &Shape) -> f64 {
    match shape {
        Shape::Circle(radius) => 3.14159 * radius * radius,
        Shape::Rectangle(width, height) => width * height,
    }
}

fn main() {
    let c = Shape::Circle(4.0);
    let r = Shape::Rectangle(3.0, 5.0);

    println!("Circle area: {}", area(&c));
    println!("Rectangle area: {}", area(&r));
}

WHY THIS WORKS AS AN ANSWER
------------------------------
Shape::Circle(f64) and Shape::Rectangle(f64, f64) are two variants
carrying genuinely DIFFERENT data — one number vs. two — exactly the
"variants can carry their own shape of data" capability this chapter
introduced, something Go's iota constants have no equivalent for.

The match expression is EXHAUSTIVE: both variants have an arm, so the
compiler accepts it without needing a catch-all _. Each arm
DESTRUCTURES the variant's data directly — Shape::Circle(radius) binds
the single f64 to radius, and Shape::Rectangle(width, height) binds
both f64s at once — making each arm's calculation immediately usable
with no manual unwrapping step.

Calling area(&c) and area(&r) borrows each shape (Chapter 4's &),
letting the function compute an area without taking ownership of
either shape value.