cond_w_qquote.rs

#[plugin_registrar]
pub fn plugin_registrar(reg: &mut Registry) {
    reg.register_macro("cond", cond);
}

fn cond<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[TokenTree]) -> Box<base::MacResult + 'cx> {
    let output = cond_rec(TokenStream::from_tts(tts.clone().to_owned()));
    build_emitter(cx, sp, output)
}

fn cond_rec(input: TokenStream) -> TokenStream {
  if input.is_empty() { return qquote!(); }

  let next = input.slice(0..1);
  let rest = input.slice_from(1..);

  let clause : TokenStream = match next.maybe_delimited() {
    Some(ts) => ts,
    _ => panic!("Invalid input"),
  };

  if clause.len() < 2 { panic!("Invalid macro usage in cond: {:?}", clause) } // clause is ([test]) [rhs]

  let test: TokenStream = clause.slice(0..1);
  let rhs: TokenStream = clause.slice_from(1..);

  if ident_eq(&test[0], str_to_ident("else")) || rest.is_empty() {
    qquote!({unquote(rhs)})
  } else {
    qquote!({if unquote(test) { unquote(rhs) } else { cond!(unquote(rest)) } })
  }
}