What is the test for a defendant to plead guilty under Rule 11 of the Criminal Code?

Under Rule 11(b)(2), "before accepting a plea of guilty or nolo contendere, the court must address the defendant personally in open court and determine that the plea is voluntary and did not result from force, threats, or promises (other than promises in a plea agreement)." In making a determination of voluntariness, the district court must weigh the defendant's competence and intelligence. Rule 11 requirements were specifically adopted "[t]o avoid having to speculate and engage in retrograde mind reading" with regard to an individual defendant's state of mind and circumstances at the time of the plea. United States v. Kennell , 15 F.3d 134, 137 (9th Cir. 1994). "[F]ailure to satisfy a core concern of Rule 11" affects substantial rights. See Pena , 314 F.3d at 115657.