code.rs

    current! {
        FOO: bar
        => {
            print_text();
            print_text();
        }
    }

error.txt

<current macros>:8:41: 8:43 error: expected ident, found =>
<current macros>:8             current!{ $($TAIL: $tail),* => $f:expr }
                                                           ^~

macro.rs

#[macro_export]
macro_rules! current {
    (=> $f:expr) => {{
        $f
    }};
    ($HEAD:ident: $head:ident $(,$TAIL:ident: $tail:ident)* => $f:expr) => {{
        let $head = RefCell::new($head);
        $HEAD.set(&$head, || {
            current!{ $($TAIL: $tail),* => $f:expr }
        })
    }};
}