foreigner_class!(class Foo {
    self_type Foo;
    constructor Foo::new(_: i32) -> Foo;
    method Foo::f(&self, _: i32, _: i32) -> i32;
});

foreigner_class!(class Boo {
    self_type Boo;

    constructor Boo::new(_: i32, _: usize) -> Boo;
    method Boo::f1(&self, _: &mut [Foo]) -> &[u32];
    method Boo::f2(&self, _: &[Foo]) -> &[u32];
});