No one would think, for example, that when you buy a Noble Brigand with Haggler in play you can first trash your opponent's Gold, then gain a card via Haggler, and then gain the trashed Gold.
Obligatory challenge to find a situation where it would matter
Kingdom: Highway, Market Square, Haggler, Watchtower, Knights, Noble Brigand, Ironmonger, Market, and 2 other cards
I play three Highways, 3 Markets, and Haggler. I play Gold. I have Market Square and Watchtower (and an Estate) in hand. I have $8, 4 buys, and everything costs $3 less. I buy Noble Brigand, trashing my opponent's gold. I reveal and discard Market Square, gaining and topdecking a Gold. Haggler gains a Market Square, topdecking it. I gain my opponent's gold, topdecking it. The top of my deck is Gold, Market Square, Gold.(Without resolving the Haggler in the middle, this would not be possible.)
Then I buy Gold, gaining Haggler, Gold, gaining Ironmonger, and Sir Martin, topdecking all 5 cards. Haggler gains me a Curse from Sir Martin, which I trash with Watchtower. Now the top of my deck, in order, is
Sir Martin, Ironmonger, Gold, Haggler, Gold, Gold, Market Square, Gold
My next turn, I draw Sir Martin, Ironmonger, Gold, Haggler, Gold. I want to get an extra action to play both my terminals. This is only possible because I put Market square in between the two Golds, so Ironmonger draws the first Gold, and reveals the Market Square.
EDIT: Market Square doesn't care when I make my opponent trash cards.