circular reasoning is a potential source of unsoundness

I think it would be unsound if you could say "impl Debug for Link where Link: Debug {}", but that's not allowed, and the impl thing that is allowed is really saying: this type has Debug, and I'll tell you how, but I need to be able to call its methods if so. It's more of an induction step than circular reasoning: an object of this type has this trait if any other objects of this type that it references have the trait, and any other obligations are met.