message: Do not panic in parse_arg
  Summary: Return an error with span.
diff: @@ -388,7 +388,14 @@ fn parse_arg(x: FnArg) -> syn::Result<StarArg> {
  default: ident.subpat.map(|x| *x.1),
  source: StarArgSource::Unknown,
  }),
  - arg => panic!("Unexpected argument, {:?}", arg),
  + FnArg::Typed(PatType { .. }) => Err(syn::Error::new(
  + span,
  + "Function parameter pattern must be identifier",
  + )),
  + FnArg::Receiver(..) => Err(syn::Error::new(
  + span,
  + "Function cannot have `self` parameters",
  + )),
  }
  }
  