ScalaPB VS ScalaCheck

I've had great experience with ScalaPB and even better with fs2-grpc (which builds on top of ScalaPB).

One way I've solved this is to use Protobuf contracts to serialize json with ScalaPB. They have a way implementing oneof that keeps each object individual and separate. Even if you don't decide to use PB, you can take some inspiration from the implementation.

You could try out protocol buffers. Not sure how stable it is, but I noticed they have a Scala3 version of their runtime library. (Disclaimer: I have not tried it out myself)

Regarding numbers 2 and 3, I believe you are describing "property-based testing"[0]. A Scala version of this is ScalaCheck and can be found here[1].

There appears to be at least one Rust library which claims to provide same, but I am not a Rust developer so cannot recommend any for fitness of purpose.

0 - https://hypothesis.works/articles/what-is-property-based-tes...

1 - https://github.com/typelevel/scalacheck/blob/main/doc/UserGu...

And the readmes and tutorials written for the major libraries are some of the best, and most concise out there. Seriously, even the old stuff, like scalacheck's guide is really good. Compare that to hypothesis. The hypothesis uses readthedocs, but other than the window dressing, much better. Try to find similar concise guides for major Java projects outside of spring boot. They're all external to the projects, published on Baeldung.

Write your test as properties that must be held, and let the framework generate random values and apply the function to thus values and compare the output by those properties. Take a look to ScalaCheck for that approach.