A (Rust) example of why HKTs would be useful is if you have a user-defined container that want's to abstract between having an Rc or an Arc, so that people can

A (Rust) example of why HKTs would be useful is if you have a user-defined container that want's to abstract between having an Rc or an Arc, so that people can pay or not the cost of thread safety. A more immediate example of where these are useful are in async Rust: being able to represent HKTs is necessary to allow a trait to hold an async fn, which gets desugared to a function returning an associated type that implements Future<Output = T>. This is being worked on.