Function syntax::diagnostics::plugin::expand_register_diagnostic

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_register_diagnostic<'cx>(
    ecx: &'cx mut ExtCtxt, 
    span: Span, 
    token_tree: &[TokenTree]
) -> Box<MacResult + 'cx>