Not the person you replied to, but OCaml, Scala, and F# all suffer from the potential drawback of being functional-first, and thus difficult for people to adapt to. If you are someone who is an OO programmer and are considering Rust for its potential performance and different model, it would be hard for me to recommend a language whose model is as different as OCaml's. Learning Rust entails learning the borrow checker and unlearning some of your OO habits; learning OCaml requires a sea change in your whole mentality & approach to programming if you've never experienced a functional lang before.

Meanwhile, both D and Nim have the benefits of being procedurally grounded, and hence more familiar to someone coming from (say) C# or Java.

Having said that, I think both of those languages may suffer from a lack of tooling/library breadth & depth. Nim in particular seems really cool, but it seems to be struggling to catch enough attention & interest to build a larger community -- probably in large part because it targets many of the same people who are currently high on Rust.