use std::io;

trait WriteExt {
    fn write_ext(&self) { println!("I implement WriteExt"); }
}

impl<W: io::Write> WriteExt for W {
}

fn test_fn<W: WriteExt>(mut w: W) {
    w.write_ext();
    test_fn(&mut w);
}

fn main() {
    println!("Hello, world!");
}