Do macros not expand interpolated token trees?

Question

Why does this:

macro_rules! a_macro {
    ($($a:tt)+) => ($($a)+);
}   

fn main() {
    let x:u32 = 1;
    let y:u32 = a_macro!(-x);
}

fail to compile with

<anon>:2:23: 2:25 error: unexpected token: `an interpolated tt`
<anon>:2     ($($a:tt)+) => ($($a)+);
                               ^~
playpen: application terminated with error code 101

Answer 1

The why is: it's not implemented yet. This is a known limitation (as of Rust 1.0). tt arguments to macros are useful, but they must always be forwarded to macros when used.