i'm not aware of any legislation requiring disclosure per se. on the contrary, for many, many years hospitals have been told by their risk managers and attorneys that they should never let on that they know anything about any adverse effect (unless it's easily discerned by lay persons, like an instrument left inside at surgery, a wrong-side surgery, or a test that was ordered for someone else; and then, they should not discuss it with the patient/family...so let the lawsuits begin). and since most legislatures are populated largely by attorneys, i'm not expecting that they would advocate for such legislation.
however, more recent studies have indicated that when the adverse event is disclosed to the patient by the hospital (not necessarily by the nurse or physician, but they are usually involved in the meeting) as soon as it is discovered, an apology given, an explanation of how the mistake has resulted in a change in whatever made it occur, and any reparations made (for instance, if the error resulted in extra days in the hospital, this care is not billed), most people accept that and no legal action is forthcoming.
while i am sure this upsets the lawyers, it's good news for those who believe in people's basic good nature. i mean, if somebody came to you and apologized, said "i screwed up, i'll make it up to you, and i'll do my very best to be sure it will never happen again," wouldn't that be enough? turns out, for most folks, it is. in this increasingly litigious world, that's heartening.