gistfile1.rs

struct Env {
    out: ~str,
}

impl Env {
    fn write(&self, msg: ~str) {
        self.out += msg;
    }
}

fn callback(env: @Env) {
    env.write(~"hi");
}

fn main() {
    let pattern = ~"a.*?";
    let env = @Env {out: ~""};

    callback(env);

    io::println(fmt!("%s -> %s", pattern, env.out));
}

// test.rs:7:8: 7:16 error: assigning to immutable field
// test.rs:7         self.out += msg;
//                   ^~~~~~~~
// error: aborting due to previous error

gistfile2.rs

struct Env {
    out: ~str,
}

impl Env {
    //fn write(&self, msg: ~str) {
    fn write(&mut self, msg: ~str) {
        self.out += msg;
    }
}

//fn callback(env: @Env) {
fn callback(env: @mut Env) {
    env.write(~"hi");
}

fn main() {
    let pattern = ~"a.*?";
    //let env = @Env {out: ~""};
    let env = @mut Env {out: ~""};

    callback(env);

    //io::println(fmt!("%s -> %s", pattern, env.out));
    io::println(fmt!("%s -> %s", pattern, copy env.out));
}