I'm not familiar but I think this paper describes how OxCaml works:

Oxidizing OCaml with Modal Memory Management - https://dl.acm.org/doi/10.1145/3674642

> We focus on three mode axes: affinity, uniqueness and locality. Modes are fully backwards compatible with existing OCaml code and can be completely inferred. Our work makes manual memory management in OCaml safe and convenient and charts a path towards bringing the benefits of Rust to OCaml.

https://github.com/oxcaml/oxcaml