> Idiomatic rust avoids heap allocations, but it very much encourages immutable stack allocation...something that OCaml also tries to encourage.

No doubt, but most of your OCaml data structures will, in a typical program, still be on the heap, while idiomatic Rust will avoid Box whenever possible.

Apart from that, I know about the links between the two (and I think the blend between the OCaml/Java (for the generics) and Ruby syntax (lambdas) works really well in practice. But I think the underlying philosophy, and the way of structuring code are really different. This is a much more fundamental concern than surface-level considerations.