If you play an AD mid vs AP and don't build Hexdrinker as your first item
You are not very smart.
You are not very smart.
If the lane is easy, there's no need for a hexdrinker. Just build ghostblade to snowball faster.
... and why?
your first item should be hex drinker in all cases though. the item is just THAT good.
Maw is a different story entirely. you want to go hexdrinker into boots into the dirk, and then from the dirk into a finished item depending on how the lane is going.
Unless you outrange them. 
Irelia's one that doesn't need hexdrinker.
Same with Wukong.
Just don't be an idiot and kills are almost guaranteed. Learning how to farm in a match-up that you're at a neutral disadvantage in is one of the skills required to be decent at this game.
If you play an AP mid vs AD and they don't build Hexdrinker as your first item
that means the champion you are playing is not
Veigar
its funny how when you play ap vs ad you don't need to buy zhonyas as your first item because its usually a skill match up. but i sure do love an anivia with half an item 100-0 me at level 7.
Or you are just ahead and can afford to ignore defenses.