Validating compression systems is usually really straightforward. There are 3 layers - decode known values from compressed files (or encode, same), round trip without any alterations, and fuzzing with arbitrary binaries

Because it's a defined format there can be binary exact comparisons between the input and output files - we already have an oracle in the form of proper RAR format software, so if they are identical, you don't need to look further for that specific case.

You can see a version of this that I did quite similarly, for postgresql wire format, here: https://github.com/pgdogdev/pgdog/tree/main/integration/sql

It validates that sql with the same setup, teardown, and test results in perfectly exact compatibility between raw postgresql as the control and various configurations of PgDog, with both the text format and binary format, so ultimately a 6-way multivariate test that should always result in binary-exact results.

Right, that's very different from "using it" and it's also different from "Have an LLM generate code that compiles".