rust generic impl error

use std;
import std::io::*;

iface ninja_str {
    fn to_str() -> str;
}

impl of ninja_str for str {
    fn to_str() -> str { self }
}

impl of ninja_str for [str] {
    fn to_str() -> str {
        alt vec::len(self) {
          0u { "" }
          x {
            vec::foldl(self[0], vec::slice(self, 1u, x)) {|iter, y|
                iter + ", " + y
            }
          }
        }
    }
}

// this works fine
fn print<T: ninja_str>(str: T) {
    println(str.to_str());
}

impl ninja_file for writer {
    // this causes 'error: internal error in instantiate'
    fn build<T: ninja_str>(str: T) {
        self.write(str::bytes(str.to_str()));
    }
}

fn main() {
    //let n = result::get(mk_file_writer("build.ninja", [create]));
    //n.build("hello, world" as ninja_str);
    print("hello, world");
}