err := firstError(input)
		if err == nil {
			t.Errorf("%s: got no error", test.error)
			continue
		}
		if !strings.Contains(err.Error(), test.error) {
			t.Errorf("got error %q expected %q", err.Error(), test.error)
		}
	}
}

// firstError returns the first error value triggered by the input.
func firstError(input *Input) (err error) {
	panicOnError = true