Futures are really neat! Unfortunately, Rocket does not use a new enough version of Hyper to take advantage of futures (rocket uses 0.10, futures were introduced with 0.11). But since Async and Sync programming work so differently, it would require a re-architecture.
There are a number of frameworks that do support futures such as Hyper, Gotham, and Actix-Web. Right now, though, you shouldn't use diesel with any of them since diesel doesn't support async yet.