Function syntax::ext::source_util::expand_line

Important traits for Box<R>

impl<R> Read for Box<R> where
    R: Read + ?Sized, impl<W> Write for Box<W> where
    W: Write + ?Sized, impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;

pub fn expand_line(
    cx: &mut ExtCtxt, 
    sp: Span, 
    tts: &[TokenTree]
) -> Box<MacResult + 'static>

line!(): expands to the current line number