When is entropic ward considered "used"?

Question

Entropic Ward is a patron of the old god feature that allows the warlock to impose disadvantage on an attack against him as a reaction. If the target misses, the warlock can give himself advantage when attacking that creature until the end of his next turn.

The PHB says that (pg. 110)

Once you use this feature, you can't use it again until you finish a short or long rest.

When is it considered used? Can I only impose disadvantage with the hopes of a miss once per short or long rest, or is it only expended once I gain advantage?

My gut tells me the former, but I wanted to make sure.

Answer 1

A wider quotation makes the intent quite clear:

When a creature makes an attack roll against you, you can use your reaction to impose disadvantage on that roll. If the attack misses you, your next attack roll against the creature has advantage if you make it before the end of your next turn.

Once you use this feature, you can’t use it again until you finish a short or long rest.

You "use" this feature when you "use your reaction to impose disadvantage"; at that point you have received a distinct benefit (disadvantage on the attack). What follows are further consequences of the "use" - you get advantage if they miss but not if they hit.

Answer 2

By my reading it would indeed be the former. Entropic ward is a defensive feature with a conditional buff on the probable counterattack.