Not even in nightly yet - follow https://github.com/rust-lang/rust/pull/35091 for updates (we're trying to land it this week).

It is, however, it's not currently actually using that syntax in the code: http://alexcrichton.com/futures-rs/futures/trait.Future.html

So, this will work on stable. It was presented that way to not get into the whole details about the whole impl trait shenanigans in general.

Ah, great news!