Slugs do in fact eat weeds to some extent, but usually not much. If matters were otherwise, the weeds soon would stop being weeds; they would be slug. Weeds largely survive because they are poisonous and nasty to pests.

It is easier to spray an eco-friendly slug-repellant onto your plants, than to make weeds tasty and safe.Some of the poisons of weeds are in fact suitable. A particularly good one is caffeine. Slugs and snails hate it, and if they crawl over it they turn back and even die if they absorb more than a tiny bit.

Used coffee grounds (NOT (yuch!) decaf!) are an effective anti-snail  mulch.

Jon