We've been rewriting a big Scala code base to Rust now since January and the team is advancing in a nice pace.
Our plan was:
- Take one part of the system to rewrite over JNA
- Run a massive amount of Scala tests against the new code
- When green, take another part
- Meanwhile the other part of the team writes the user-facing code base, connecting it to the new Rust crates
- The test suit works also over an integration layer, so the other team can test their code and the backend with the same test suite
We're quite far already. Originally I was the only person with Rust experience. Now there's five of us.
Why are you migrating from Scala?
For speeding up other-language libraries, D has a -betterC mode, which prevents you from using the subset of the language and the libraries (standard & user-defined) that relies on GC. The remaining language is a very clean C that simply works on the other language's GC'd memory (using the other language's C interface), and can use stack allocation or any heap allocation strategy of your choice for its working set (reference counting ala C++ shared_ptr could be the obvious choice, but it is your party).
For other language applications, it is a valid option to speed up the entire application by writing it in D, as it has "all" the features of those other languages + all the convenience that is afforded by a GC + threads if you don't want a multi-process design. I quote "all" because I mean useful things like blocks/closures, generic data structures, etc. - of course, neither Rust nor D give you runtime devices like monkey-patching/meta-class hackery/prototype changes.