I think one of the core challenges is how traits play with HKTs. You can only allow for the defining of HKed type parameters and type arguments so many ways, but it becomes more complicated when dealing with the resolution of traits around HKTs. In Haskell for ex. there is no `Self` so the type class can easily be dispatched on a higher kinded type without needing a receiver type. In Rust `Self` must currently be fully applied as it is resolved from a value directly: `value.trait_method` and we pick our instance based on value's type. If we want to implement Functor (Mappable or w/e) we need to come up with another approach. I've thought about a few but they seem to play best with multi-dispatch + associated types + where clauses. I'm hoping to finish the RFC once all these features land and I can actually hack a prototype, instead of scribbling on paper.

Awesome - looking forward to seeing that. :)