For the past 20 months, President Trump has received free personal legal services from one of America’s highest-paid lawyers, who has traveled around the country and across the ocean to defend him in the special counsel’s inquiry and press Ukraine to investigate a political rival and unfounded conspiracy theories.

The lawyer, of course, is Rudolph W. Giuliani, but Mr. Trump did not mention Mr. Giuliani or his unpaid labor on the annual financial disclosure he filed in May, which requires that the value and source of gifts — including free legal work — be publicly listed.

That requirement is cut and dried, said Kathleen Clark, a law professor at Washington University in St. Louis. She cited guidance from the Office of Government Ethics, issued in November 2017, that states federal officials must disclose “gifts of legal defenses — in kind or by payment of the fees.”

“The purpose is to ensure the public has an opportunity to see whether there is any kind of corrupting influence,” said Ms. Clark, who has written on ethics issues involving government employees in need of lawyers.