my first rust macro

macro.rs

  fn when(&mut self, v:VID, val:NID, nid:NID)->NID {
    macro_rules! op {
      [not $x:ident] => {{ let x1 = self.when(v, val, $x); self.not(x1) }};
      [$f:ident $x:ident $y:ident] => {{
        let x1 = self.when(v, val, $x);
        let y1 = self.when(v, val, $y);
        self.$f(x1,y1) }}}
    if v >= self.vars.len() { nid }
    else { match self[nid] {
      Op::Var(x) if x==v => val,
      Op::O | Op::I | Op::Var(_) => nid,
      Op::Not(x)    => op![not x],
      Op::And(x, y) => op![and x y],
      Op::Xor(x, y) => op![xor x y],
      other => { println!("unhandled match: {:?}", other); nid }}}}