expansion error

gistfile1.rs

/*
<std macros>:2:34: 4:6 warning: unused result which must be used, #[warn(unused_must_use)] on by default
<std macros>:2     ($dst:expr, $($arg:tt)*) => ({
<std macros>:3         format_args_method!($dst, write_fmt, $($arg)*)
<std macros>:4     })
<std macros>:1:1: 5:2 note: in expansion of write!
<std macros>:3:9: 3:51 note: expansion site
<std macros>:1:1: 5:2 note: in expansion of writeln!
/src/main.rs:70:9: 70:54 note: expansion site
*/

fn write_file(buckets: &TreeMap<String, uint>, filename: &str) {
    let mut file = File::create(&Path::new(filename));
    for (bucket, value) in buckets.iter() {
        writeln!(&mut file, "{}\t{}", bucket, value);
    }
}