Haskell type classes can not be implemented with zero runtime overhead in all cases, because it is possible to have an unbounded number of instances at runtime

Haskell type classes can not be implemented with zero runtime overhead in all cases, because it is possible to have an unbounded number of instances at runtime for a single type class. The Rust equivalent will just fail at compile time (of the trait implementation, not the trait definition) due to a recursion error.