What is the ruling on saying "If the forbidden were permissible, I would do it," such as "If alcohol were permissible, I would drink it," or "If adultery were permissible, I would commit it"?

There is no blame in saying, "Had it not been for the Sharia, I would have done such-and-such," because it means that the prohibition of the Sharia is what prevented the commission of the act. Similar expressions have been reported from some Companions, such as the statement of Abu Ayyub al-Ansari: "By Him who sent him with the truth, were it not for Islam, I would have struck you with the sword," and the statement of Qais bin Sa'd: "Were it not for Islam, I would have plotted a scheme that the Arabs could not endure." The natural inclination towards disobedience is not blameworthy if one is unable to remove it, and Allah praised those who restrain the soul from its desires.