trait Foo
{

}

struct Bar
{
    capacity_hint : usize
}

impl Foo for Bar
{

}

fn main()
{
    let mut factories : Vec<Box<Fn() -> Box<Foo>>> = Vec::new();
    let capacity_hint : usize = 10;

    factories.push(Box::new(||
    {
        Box::new(Bar {capacity_hint })
    }));
}