test_associated_types.hs

{-# LANGUAGE FlexibleContexts, GADTs, TypeFamilies #-}
class T a where
  type Config a
  getConfig :: a -> Config a

data D where
  T :: T a => a -> D

test_associated_types.rs

trait T {
    type Config;
    fn getConfig(&self) -> Self::Config;
}

enum D {
    T(Box<dyn T>)
}
/*
error[E0191]: the value of the associated type `Config` (from trait `T`) must be specified
 --> test_associated_types.rs:7:15
  |
2 |     type Config;
  |     ------------ `Config` defined here
...
7 |     T(Box<dyn T>)
  |               ^ help: specify the associated type: `T<Config = Type>`
*/