Rust workaround to implement factory-pattern

gistfile1.txt

trait Construct {
    fn new(_hint: Option<Self>) -> Box<Construct>;
}

struct Foo { x: int }
struct Bar { y: uint }

impl Construct for Foo {
    fn new(_: Option<Foo>) -> Box<Construct> {
        box Foo { x: 42 } as Box<Construct>
    }
}

impl Construct for Bar {
    fn new(_: Option<Bar>) -> Box<Construct> {
        box Bar { y: 24 } as Box<Construct>
    }
}

fn main() {
    let x: Box<Construct> = Construct::new(None::<Foo>);
}

Another snippet

struct A;
struct B;

trait O { }

impl O for A { }
impl O for B { }

fn main() {
let os: [Box, ..2] = [box() A as Box, box() B as Box];
}