use std::fmt::Debug;

// This won't work:
// struct Node<?> { this: usize, next: Option<&? mut Node<?>> }

trait Chain: Debug {
    fn out(&mut self) -> Option<(&mut usize, &mut dyn Chain)>;
}

impl Chain for () {
    fn out(&mut self) -> Option<(&mut usize, &mut dyn Chain)> {
        None
    }
}

impl Chain for (usize, &mut dyn Chain) {
    fn out(&mut self) -> Option<(&mut usize, &mut dyn Chain)> {
        Some((&mut self.0, self.1))
    }
}

fn func(n: usize, chain: &mut dyn Chain) {
    if n == 10 {
        *chain.out().unwrap().1.out().unwrap().0 += 100;
        dbg!(&chain);
    } else {
        func(n + 1, &mut (n, chain));
    }
}

fn main() {
    func(0, &mut ());
}