In my case, Cache is not aware of Entry. Cache is very abstract. But I see the point.
I never noticed the .extend method before. I could use that to get rid of some .append(&mut vec) calls, which I find a bit ugly. But will shortly have a question in that matter, but that goes to a new topic.
Anyway, my Cache isn't really a collection I guess… It doesn't hold the items that I'm going to process… well, implicitly yes, but more out of necessity not because it's the main purpose of the Cache.