The OCaml module system doesn't exist in Rust, and is key to being able to fling around large abstractions with very little runtime cost.

It's worth bearing in mind that OCaml is really two distinct languages: the core ML-style language (which is concise due to type inference, and I think what the above poster is referring to by "pretty functional-programmey"), and the module language (which is verbose and signatures explicitly written down with no inference).

The latter is really the key to constructing large scale systems like Mirage, since they allow fragments of OCaml code to be mapped onto everything from Xen unikernels to JavaScript. Modules can be higher-order (that is, depend on other modules). You can read more about it at https://realworldocaml.org/v1/en/html/functors.html

It's also not yet clear how well Rust's ownership types will scale in bigger systems, but I'm sure we'll find out as the language stabilises. OCaml is a stable, fast and mature implementation of ML that has been around for years.