// Important: traits are of unknown size, and so must be passed around
// as a reference
trait Shape {
    fn area(&self) -> f32;
}
// As seen here.
struct ShapeCollection<'a> {
    shapes: Vec<&'a Shape>
}

// This function needs to return a ShapeCollection, but since the
// data structure requires references to work, it seems impossible!
fn make_shapes<'a>(needRectangle: bool, multiplier: f32) -> ShapeCollection<'a> {
    let rect = Rectangle {
        width: 42.0 * multiplier,
        height: 24.0 * multiplier
    };
    let circle = Circle {
        radius: 42.0 * multiplier
    };

    match needRectangle {
        true => ShapeCollection {
            shapes: vec![&rect, &circle]
        },
        false => ShapeCollection {
            shapes: vec![&circle]
        },
    }
}
// ^ This function won't compile because rect and circle do not
//   life long enough, and the compiler dies at this point

// Impls if you're interested / want to compile, but not that important
struct Rectangle {
    width: f32,
    height: f32
}
impl Shape for Rectangle {
    fn area(&self) -> f32 {
        self.width * self.height
    }
}
struct Circle {
    radius: f32
}
impl Shape for Circle {
    fn area(&self) -> f32 {
        (std::f32::consts::PI * self.radius).powf(2f32)
    }
}

enum Shape2 {
    Rectangle { width: f32, height: f32 },
    Circle { radius: f32 }
}
fn area(shape: Shape2) -> f32 {
    match shape {
        Shape2::Rectangle {width, height} => width * height,
        Shape2::Circle  {radius} => (std::f32::consts::PI * radius).powf(2f32)
    }
}
struct Shape2Collection {
    shapes: Vec<Shape2>
}

fn make_shapes2(needRectangle: bool, multiplier: f32) -> Shape2Collection {
    let rect = Shape2::Rectangle {
        width: 42.0 * multiplier,
        height: 24.0 * multiplier
    };
    let circle = Shape2::Circle {
        radius: 42.0 * multiplier
    };

    match needRectangle {
        true => Shape2Collection {
            shapes: vec![rect, circle]
        },
        false => Shape2Collection {
            shapes: vec![circle]
        },
    }
}