It's been a while since I played with OCaml, but I'm pretty sure that modules are stateless. A quick googling shows that you can use global state, but it's not the same as one instance of state per one instance of struct as in Rust.
They are often used without mutable state, but the language doesn't make them so.
See eg https://realworldocaml.org/v1/en/html/first-class-modules.ht...