[Hirrolot]

HKTs in Haskell are quite castrated, too. Haskell only lets you define HKTs via data type declarations, while, in general, HKTs can be considered just functions "one level up" -- instead of operating on the term-level, they operate on the level of types. The type checker thus becomes undecidable for a Turing-complete language, since you need to be able to perform arbitrary computation on types during type checking. Not a huge problem for Rust though, since its type checker is already undecidable.