struct Container { pub foo: Box<Foo> }

struct Bar { a: u32 }

trait Foo {
    fn into_container(self: Box<Self>) -> Container where Self: Sized + 'static { Container { foo: self }}
}

impl Foo for Bar {}

fn main() {
    let foo: Box<Foo> = Box::new(Bar{ a: 5 });
    foo.into_container();
}