return "", -1
			}
			s, ok := stringConstantArg(pass, call, idx)
			if !ok {
				// The last argument before variadic args isn't a string.
				return "", -1
			}
			return s, idx
		}
	}

	// Cannot determine call's signature. Fall back to scanning for the first
	// string constant in the call.
	for idx := range call.Args {
		if s, ok := stringConstantArg(pass, call, idx); ok {
			return s, idx
		}