Okay, we may have been misleading in the title of this piece. It's not like Rihanna "forgot" her pants. Rihanna doesn't "forget" things. She opts to simply not participate in the concept of pants in this particular moment. And you know what? That is totally okay. Rihanna doesn't need to be confined by your socially constructed concepts of "pants" or "bottom halfs" or "trousers." She doesn't need trousers. Look, you know how when you have nothing to wear, you are like, "I'm just going to throw on this giant t-shirt and be done with it"? This is the RiRi version. Rihanna literally rolled up wearing a men's jacket three sizes too big, some sheer nylons, and still looks beyond stellar.
However, in case she suddenly decides she does need pants (because, like, wearing pants ought to be a choice, not a requirement) it appears Emma Watson brought some extra. Because they are literally wearing the same thing, Emma is just wearing pants, Rihanna is not. In fact, Emma has a lot going on in the pants department. (Or, as it is sometimes known, The Department Of Pants.) And that's cool. Being a woman in 2014 means maybe wearing two times the pants that you normally wear, or maybe wearing no pants at all.