message: syntax: decrease binary size
  Avoid depending on expression Debug stringifier in release builds.
  The code weighs over a kilobyte, which doesn't seem worth it for an unreachable.
diff: @@ -1227,7 +1227,7 @@ impl Parser {
  return Err(self.errat(
  chari, ErrorKind::UnclosedParen));
  }
  - e => unreachable!("{:?}", e),
  + _ => unreachable!(),
  }
  }
  Some(Build::Expr(e)) => { concat.push(e); }
  