trait Widget {}

#[derive(Debug)]
struct A;
impl Widget for A {}

struct B<C: List>(C);
impl<C: List> Widget for B<C> {}

trait List {}
#[derive(Debug)]
struct Cons<A: Widget, B: List>(A, B);
impl<A: Widget, B: List> List for Cons<A, B> {}
#[derive(Debug)]
struct Nil;
impl List for Nil {}

macro_rules! children {
    ($head:expr) => { Cons($head, Nil) };
    ($head:expr, $($tail:expr),+) => { Cons($head, children![$($tail),+]) };
}

fn main() {
    B(Cons(A, Nil));
    dbg!(children![A, A, A]);
}