[rabidferret]

Just because a type can be named doesn't mean that returning the named type properly expresses your intent.

The name of said type can get real hairy real fast, as well.

[Manishearth]

Right, but in most cases returning a named type is fine. Usually with futures/iterators I've seen that you end up returning a new custom type that implements the trait, and often it's okay to just name it.

[rabidferret]

I think that's only the case when you are specifically trying to avoid boxing.