[Manishearth]

I'm not really sure if that advice is accurate. For both iterators and futures, in most cases you will be returning a concrete type which can be named. You only really need to box it if you have a closure involved (and if you have a complicated adapter chain it becomes cleaner to box/impl trait it)

E.g. in this case there's no specific compulsion to use a trait object.

I wouldn't consider this to be a rule of thumb; it's more like "if you have trouble naming the return type try using Box<Trait>"

[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.