What features does SML or OCaml's type system have that Rust's doesn't?
Haskell has higher-kinded type parameters and Idris has dependent types. But SML and OCaml don't even have typeclasses at all, much less higher kinds.
Furthermore, what makes Rust's type system interesting isn't just its use of regions (which the ML Kit used); it's regions combined with inherited mutability plus the borrow check to maintain soundness.
Haskell has higher-kinded type parameters and Idris has dependent types. But SML and OCaml don't even have typeclasses at all, much less higher kinds.
Furthermore, what makes Rust's type system interesting isn't just its use of regions (which the ML Kit used); it's regions combined with inherited mutability plus the borrow check to maintain soundness.