It's going to end up being a key part of the future inheritance proposal, but that's not a part of this RFC at all.

  > I would prefer if it were simpler.

If you have thoughts about how to simplify it, please get involved in that thread! We don't actively try to introduce complexity, but sometimes, features are just inherently complex. There's also the case that sometimes features are easier to use than they are to define. I think this will be one of those kinds of features. At its core it just means that if you have a Vec<i32> and you also have

    impl<T> Foo for Vec<T>

you'll be able to define an additional

    impl Foo for Vec<i32>

and your Vec<i32> will use that one instead of the more general Vec<T> one.