Other things I've done in Elixir that are way easier: implemented a multiverse registry that tracks supervised transient gen_statems in segregated test contexts (so a test only sees gen_statems that have been started in their caller context). Elixir has an opinionated way of tracking caller and ancestors in the supervision and temporal history that are crazy powerful. Thanks to erlang term format, it's also easy to send that information out of the vm (for example into chromedriver and back into the vm via http headers). It's become an accepted standard in the community, and the web drivers, database adapters, and dependency injection/mocking libraries consistently use them.

Implemented a virtualization library that seamlessly launches virtual machines with a unified abstraction between locally launched vms and remotely launched vms (there's things that elixir has that make this easier and compile-time checked). It's also got 100% code coverage thanks to Elixir's powerful dependency injection and abstraction capabilities.

Implemented compile-time checking of configuration settings that makes 100% sure I don't deploy broken configs to prod; they get stopped at the release stage. But also they get actively code highlighted at the TOML level in vscode, so I don't even have to wait to generate a release. It's dead easy to emit custom file/line information in Elixir's CompileError exception.