We now check the #[rustc_autodiff] attributes which we generated from the #[autodiff(…)]
macros. There are two forms. The pure one without args to mark primal functions (the functions
being differentiated). The other form is #[rustc_autodiff(Mode, ActivityList)] on top of the
placeholder functions. We wrote the rustc_autodiff attributes ourself, so this should never
panic, unless we introduced a bug when parsing the autodiff macro.