The lazy collections are an interesting case, because as you build up a stack of transformations, the type itself grows in ways beyond simple A -> B type substi

The lazy collections are an interesting case, because as you build up a stack of transformations, the type itself grows in ways beyond simple A -> B type substitutions. That's a really good example of a case better served by GATs than HKTs -- they're not that kind of Functor to begin with. To be explicit, a lazy collection type in Rust is more than just `List<T>` -- it needs to carry an additional type describing the set of transformations being applied. You'd really have something closer to `List<F, T>`, and as you apply transformations, F grows while T is substituted. (You could avoid this by storing a trait object for your transformation stack instead of parametrizing over F, but half the point is that you can often flatten the whole stack down into highly-efficient generated code, and abstracting over the precise F hinders that flattenability.)