Scala Test-State VS ScalaCheck

It actually been rock-solid for many, many years now! I've built quite a few things with it and introduced it to teams who picked it up very quickly and with no major problems. I've never had any production problems with it EVER and I've been living and breathing it pretty much every day for ages now. It's very, very unit testable (oh! check out Scala Test State if you're interested in really powerful and concise Scala.JS testing) so it's nearly trivial to have to very well tested webapp, including mouse clicks, key presses, whatever you want. Awesome awesome stuff and the Scala.JS team deserve some kind of tech Nobel prize imo. It's been a game changer for me.

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.