As a testament to how terrible this rule is, not even this is the right one. The rule is, when you cast a spell (including cantrips) with your bonus action you can’t cast any other spells except a cantrip with a casting time of one action on the same turn. So casting Shillelagh stops you from casting leveled spells and (although I’m not sure why you would want to) from using your action to start or continue casting a cantrip like Mending, because it has a cast time of 1 minute (Aka 10 actions, aka not one action).