test.rs

enum Expression {
    Symbol(~str),
    Compound(~str, ~[Expression])
}

fn standard_form(expr: &Expression) -> ~str {
    match (*expr) {
        Compound(~"List", ref items) => ~"list",
        
        Compound(~"Blank", []) => ~"_",
        
        _ => ~"default"
    }
}

fn main() {
    let x = ~Symbol(~"x");
    println(standard_form(x));
}

// error: internal compiler error: sequence_element_type called on non-sequence value
// This message reflects a bug in the Rust compiler. 
// We would appreciate a bug report: https://github.com/mozilla/rust/wiki/HOWTO-submit-a-Rust-bug-report
// task 'rustc' failed at 'explicit failure', /var/tmp/portage/dev-lang/rust-9999/work/rust-9999/src/libsyntax/diagnostic.rs:102
// task '<main>' failed at 'explicit failure', /var/tmp/portage/dev-lang/rust-9999/work/rust-9999/src/librustc/lib.rs:394