std::getopt::opts_str fails for arguments other than the first one

[bruceiv]

I'm new to Rust, so it's possible I'm doing something wrong, but I can't get std::getopt::opts_str to work for any argument but the first one given (I'm running Rust 0.6). I've tried some other permutations, but here's a minimal test case:

extern mod std;
use std::getopts::*;

fn main() {
    let args = os::args();

    let opts = ~[
        optopt("a"), optopt("arg")
    ];

    let matches = match getopts(vec::tail(args), opts) {
        result::Ok(m)  => { m }
        result::Err(f) => { io::println(fail_str(f)); return }
    };

    if opts_present(&matches, [~"a", ~"arg"]) {
        io::println(fmt!("Argument `%s`", opts_str(&matches, [~"a", ~"arg"])));
        return;
    }
}

and here's the output of some invocations (the expected result would be for the second line to output the same as first):

% rustc test.rs
% ./test -a foo   
Argument `foo`
% ./test --arg foo
rust: task failed at 'index out of bounds: the len is 0 but the index is 0', /home/aaron/Downloads/rust-0.6/src/libstd/getopts.rs:360

[emberian]

Updated testcase:

extern mod extra;
use extra::getopts::*;

fn main() {
    let args = std::os::args();

    let opts = ~[
        optopt("a"), optopt("arg")
    ];

    let matches = match getopts(args.tail(), opts) {
        Ok(m)  => m,
        Err(f) => fail!(fail_str(f))
    };

    if opts_present(&matches, [~"a", ~"arg"]) {
        println(fmt!("Argument `%s`", opts_str(&matches, [~"a", ~"arg"])));
        return;
    }
}

I think this is a bug in getopts, not positive.