The menhir parser generator is extremely good and doesn't quite have an equivalent in the Rust world.

But other than that, in no particular order:

- While it has its downsides, OCaml's module system (and functors especially) is extremely powerful and allows for all sorts of higher-kinded abstractions that are currently out of reach of Rust's type system, especially when combined with first-class modules

- No monorphisation means more reasonable compile times

- Some of the advanced type system features (GADTs, extensible types, polymorphic variants) are really quite nice

(but really, menhir is great, and I miss it whenever I need to parse things in another language)