syntactic broccoli. The idea behind 'sugar' is 'tasty but inessential', so the strict opposite would be something essential but not so tasty. But maybe 'synta

syntactic broccoli. The idea behind 'sugar' is 'tasty but inessential', so the strict opposite would be something essential but not so tasty. But maybe 'syntactic thorns' correspond better to the sense you intend to express.