It's about growing up/being sheltered/spoiled by parents/etc. She wears gas mask to protect her from bad things, everyone wants to keep their child away from harm after all. Everybody smiles at her in the video, as everyone smiles at little child. Girl achieves what she want (breakinng the discs with ball and getting the doll). Protected from harm, being adored and being "spoiled", as every child is. At the end she grows up and finds out she is alone. No people to smile at you, no one to help you. Life aint so easy you thought it was (dark alley in the video). You may not know what to do (girl hesitates). Rust usually means something like "getting bad", like steel is spoiled when it gets rusty or phrase- 'I'm a little bit rusty'. Being to coddled teaches you wrong that you always get everything you want so it may be shock when someone says no. That's for rust. Carnival is place where lots of thing happens. In video it's place where girl is spoiled, so Carnival of Rust. Makes lotta sense doesn't it? Xd.