message: Better error message for unsupported "C" abi args
diff: @@ -49,7 +49,7 @@ fn get_pass_mode<'a, 'tcx: 'a>(
  PassMode::ByVal(ret_ty)
  } else {
  if abi == Abi::C {
  - unimplemented!("Non scalars are not yet supported for \"C\" abi");
  + unimpl!("Non scalars are not yet supported for \"C\" abi ({:?}) is_return: {:?}", ty, is_return);
  }
  PassMode::ByRef
  }
  