This is a really fascinating question. When I chatted with some of the Jane Street crew at NeurIPS they were adamant in the benefits of OCaml's Hindley-Milner type system.
I didn't ask, but am curious, how that compares to the type guarantees of Rust? Would moving to Rust cause them to lose that advantage of compile time error catching? I've never written a line of Rust (hopefully that changes soon) so I don't know, but am certainly interested.