There are particular methods that are unconstitutional:
Such is the general statement of that commentator, but he admits that in very atrocious crimes other circumstances of terror, pain, or disgrace were sometimes superadded. Cases mentioned by the author are, where the prisoner was drawn or dragged to the place of execution, in treason; or where he was embowelled alive, beheaded, and quartered, in high treason. Mention is also made of public dissection in murder, and burning alive in treason committed by a female. History confirms the truth of these atrocities, but the commentator states that the humanity of the nation by tacit consent allowed the mitigation of such parts of those judgments as savored of torture or cruelty, and he states that they were seldom strictly carried into effect. Examples of such legislation in the early history of the parent country are given by the annotator of the last edition of Archbold's Treatise. Arch. Crim. Pr. and Pl. (8th ed.) 584.
Many instances, says Chitty, have arisen in which the ignominious or more painful parts of the punishment of high treason have been remitted, until the result appears to be that the king, though he cannot vary the sentence so as to aggravate the punishment, may mitigate or remit a part of its severity. 1 Chitt. Cr. L. 787; 1 Hale, P. C. 370.
Difficulty would attend the effort to define with exactness [99 U.S. 130, 136] the extent of the constitutional provision which provides that cruel and unusual punishments shall not be inflicted; but it is safe to affirm that punishments of torture, such as those mentioned by the commentator referred to, and all others in the same line of unnecessary cruelty, are forbidden by that emendment to the Constitution. Cooley, Const. Lim. (4th ed.) 408; Wharton, Cr. L. (7th ed.), sect. 3405.
but they're not methods of execution anyone was actually using. I think it would take some really convoluted reasoning to declare that, say, lethal injection was cruel and unusual but hanging was not, but if the Supreme Court wanted to be perverse it could do that.