use std::fmt::Display;

trait W {
    fn write(&mut self, s: &str);

    fn write_fmt<F: Display>(&mut self, f: &F) where Self: Sized {
        let s = f.to_string();
        self.write(&s);
    }
}

trait Combo: W {
    fn write_fmt_erased(&mut self, f: &Display);
}

impl<T: ?Sized + W> W for Box<T> {
    fn write(&mut self, s: &str) {
        (**self).write(s);
    }
}

struct Moo;

impl W for Moo {
    fn write(&mut self, s: &str) {
        println!("moo: {}", s);
    }

    fn write_fmt<F: Display>(&mut self, f: &F) {
        println!("MOOOOOOO: {}", f);
    }
}

impl Combo for Moo {
    fn write_fmt_erased(&mut self, f: &Display) {
        println!("erased ->");
        self.write_fmt(&f);
    }
}

struct Cage {
    w: Box<Combo>,
}

impl W for Cage {
    fn write(&mut self, s: &str) {
        println!("caged!");
        self.w.write(s);
    }

    fn write_fmt<F: Display>(&mut self, f: &F) {
        println!("caged!");
        self.w.write_fmt_erased(f);
    }
}

fn main() {
    let mut moo = Moo;
    moo.write("hello");
    moo.write_fmt(&"sup");

    let mut cage = Cage { w: Box::new(moo) };
    cage.write("hello");
    cage.write_fmt(&"sup");
}