10b88f8fa2
Turn custom code classes in docs into warning By habit, since it was a new feature gate, I added a check which emitted an error in case the new syntax was used. However, since rustdoc tags parser was accepting *everything*, using the "new" syntax should never ever emit errors. It now emits a warning. Follow-up of #110800. cc `@Manishearth` r? `@notriddle`