#[derive(Debug)]
enum Shape { Circle { r: f64 }, Rect { w: f64, h: f64 }, Dot }

fn main() {

let mut packed: Vec<Shape> = Vec::new(); // name only; Rust elements are fixed-size
packed.push(Shape::Circle { r: 1.0 });
packed.push(Shape::Dot);
packed.push(Shape::Rect { w: 2.0, h: 3.0 });
for s in &mut packed {
    if let Shape::Rect { w, .. } = s { *w += 1.0; }
}

assert!(matches!(packed[2],Shape::Rect {w:3.0,h:3.0}));assert!(matches!(packed[1],Shape::Dot));
}