Excellent and detailed post!
Up until now I've been focusing on Julia and Swift for projects that can use a new language, as the Rust pain/productivity ratio has looked too high.
Perhaps things are changing...correctness is certainly a worthy goal!
I'm studying rust. In the past, I've used C when I wanted or needed to get close to hardware; even short snatches of assembler. I hope to never again have to write a non-trivial C program and make it thread-safe. Rust looks promising. The pain/productivity of rust should be high; it's trying to let you work as low-level as C, while helping you achieve a "single mutator at a time" safety model. I think of Julia and swift as higher-level languages, more domain-specific. Different tools for different problem domains.
Well...my particular interest in these types of languages is primarily in the realm of soft real-time simulation, specifically various kinematic simulations.
Of the three, only Julia is garbage collected, and (unlike some other GC languages) it's fairly easy to not exercise the collector. I'm encouraged that will continue to be the case, since there's an organization using it for robotics, which is implicitly a hard real time use case.
Julia, Swift and Rust are all clearly general purpose languages. Swift is unabashedly general purpose, while Julia and Rust each have a primary niche - math/science and systems, respectively. All three use the excellent LLVM infrastructure.
Aside from determinism (which mainly requires pre-allocating nearly everything), my primary requirements are expressiveness/productivity, readability, and efficient runtime performance.
All three languages produce highly optimized code, and Rust probably has the edge as far as efficiency goes - but it clearly loses on the first two criteria, at least to Julia. If one needs access to machine level functionality in Julia, there's an extremely efficient C FFI, so mixing Rust and Julia (for instance) would be painless if needed.
https://github.com/dyu/ffi-overhead
It's a great time to be a software developer, and things will only get better as languages and tooling continue to improve!