ゼロコピーなのでは！？

Cargo.toml

[package]
name = "rust_de_zundoko"
version = "0.1.0"

[dependencies]
rand="0.3.0"

zundoko.rs

extern crate rand;

use rand::Rng;

struct Zundoko {
    // ライフタイムを明記してみる
    zun: &'static str,
    doko: &'static str,
    kiyoshi: &'static str,
}

impl Zundoko {
    fn new(zun: &'static str, doko: &'static str, kiyoshi: &'static str) -> Zundoko {
        Zundoko { 
            zun: zun,
            doko: doko,
            kiyoshi: kiyoshi,
        }
    }

    fn go(&self) {
        let mut zun_counter = 0;

        loop{
            // &で借用する気持ちを表明
            let zundoko = if rand::thread_rng().gen() {&self.zun} else {&self.doko};
            println!("{}", zundoko);

            if zundoko == &self.zun {
                zun_counter += 1;
            } else if zun_counter > 4 {
                println!("{}", self.kiyoshi);
                break;
            } else {
                zun_counter = 0;
            }
        }

    }
}

fn main() {
    Zundoko::new("ズン", "ドコ", "キ・ヨ・シ！！").go();
}