struct matching problem (compile with rustc structmatch.rs --test)

structmatch.rs

extern mod std; // else --test goes wonky

extern mod rparse; // https://github.com/jesse99/rparse or: cargo install rparse

use rparse::{Parser, Combinators, ParseFailed, match0 };

pub fn space() -> Parser<@~str> { match0(|ch| ws_char.contains_char(ch)) }
const ws_char: &str = &" \t\r\n\x0A";
               
#[cfg(test)]
mod test_preliminaries {

    // This shows ParseFailed is correctly imported.
    fn mk_failed() -> ParseFailed {
	ParseFailed{file: @~"", line: 1, col: 2, mesg: @~"hello"}
    }

    #[test]
    fn match_problemr() {
        let actual = space()
            .parse(@~"f", @"$( x \u0000 is not allowed $)");
        match actual {
	  Ok(_) => fail,
	  Err(ParseFailed{file: f, line: y, col: x, mesg: txt}) => assert x == 6,
          // The with the line below in place of the one above, this compiles.
	  //Err(_) => ()
        }
    }
}

er... compiles, rather