vec!() does not handle a trailing comma

[lifthrasiir]

fn main() {
    println!("{}", vec!(1,2,3,).len());
}

gives the following error unlike the old syntax (~[1,2,3,]):

t.rs:2:30: 2:31 error: unexpected end of macro invocation
t.rs:2     println!("{}", vec!(1,2,3,).len());
                                    ^