unique vector patterns are no longer supported

Question

I realize Rust is in flux, but I'm trying to learn it anyway. I'm trying to understand how I would adapt the following example, which works with 0.9, into something similar that works with 0.10:

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

    let (first, last) = match argv {
        [_, first_arg, .., last_arg] => (first_arg, last_arg),
        _ => fail!("Error: At least two arguments expected.")
    };

    println!("The first argument was {:s}, \
              and the last argument was {:s}.", first, last); 
}

When I build this with 0.10, I get the following error:

error: couldn't read test.rc: no such file or directory (No such file or directory)
orflongpmacx8:rust pohl_longsine$ rustc test.rs
test.rs:9:9: 9:37 error: unique vector patterns are no longer supported
test.rs:9         [_, first_arg, .., last_arg] => (first_arg, last_arg),
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
error: aborting due to previous error

My question: is it still possible to use pattern matching on argv, but with different syntax, or is using a match statement on argv no longer possible at all? If it's the former, what would I have to change?

Answer 1

You can still match on slices &[T] and fixed length arrays [T, .. n]. so, in this case,

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

    let (first, last) = match argv.as_slice() {
        [_, ref first_arg, .., ref last_arg] => (first_arg, last_arg),
        _ => fail!("Error: At least two arguments expected.")
    };

    println!("The first argument was {:s}, \
              and the last argument was {:s}.", *first, *last); 
}

Note the addition of the refs. argv is ~[~str], i.e. the contents are owned strings ~str, which move ownership when passed by value as a [_, first_arg, .., last_arg] pattern would do. It's illegal to move ownership out from behind a borrowed pointer (like a slice &[~str]) so that pattern would be illegal. One can borrow into the slice (and any other pattern) using the ref keyword, making first and last both references of type &~str.

One might wonder why the *first and *last dereferences aren't trying to move a ~str out from behind a &~str, but it's because the println! macro is expanding to essentially &*first and &*last, which is fine.

(We could also write => (first_arg.as_slice(), last_arg.as_slice()) to borrow the two &~strs straight to string slices &str, which then means we don't need to dereference in the println!.)