Advantages of Using `passthru.tests` for Package Testing

[]{#var-meta-tests-packages}  Besides tests provided by upstream, that you run in the [`checkPhase`](#ssec-check-phase), you may want to define tests derivations in the `passthru.tests` attribute, which won't change the build. `passthru.tests` have several advantages over running tests during any of the [standard phases](#sec-stdenv-phases): - They access the package as consumers would, independently from the environment in which it was built - They can be run and debugged without rebuilding the package, which is useful if that takes a long time - They don't add overhead to each build, as opposed checks added to the [`installCheckPhase`](#ssec-installCheck-phase), such as [`versionCheckHook`](#versioncheckhook). It is also possible to use `passthru.tests` to test the version with [`testVersion`](#tester-testVersion), but since that is pretty trivial and recommended thing to do, we recommend using [`versionCheckHook`](#versioncheckhook) for that, which has the following advantages over `passthru.tests`: - If the `versionCheckPhase` (the phase defined by [`versionCheckHook`](#versioncheckhook)) fails, it triggers a failure which can't be ignored if you use the package, or if you find out about it in a [`nixpkgs-review`](https://github.com/Mic92/nixpkgs-review) report. - Sometimes packages become silently broken - meaning they fail to launch but their build passes because they don't perform any tests in the `checkPhase`. If you use this tool infrequently, such a silent breakage may rot in your system / profile configuration, and you will not notice the failure until you will want to use this package. Testing such basic functionality ensures you have to deal with the failure when you update your system / profile. - When you open a PR, [ofborg](https://github.com/NixOS/ofborg)'s CI _will_ run `passthru.tests` of [packages that are directly changed by your PR (according to your commits' messages)](https://github.com/NixOS/ofborg?tab=readme-ov-file#automatic-building), but if you'd want to use the [`@ofborg build`](https://github.com/NixOS/ofborg?tab=readme-ov-file#build) command for dependent packages, you won't have to specify in addition the `.tests` attribute of the packages you want to build, and no body will be able to avoid these tests.

Using `passthru.tests` for package testing offers several advantages over running tests during standard phases. It allows you to test the package as a consumer would, independently of the build environment, debug without rebuilding, and avoid build overhead. While `passthru.tests` can be used to test package versions with `testVersion`, using `versionCheckHook` is recommended due to its ability to trigger failures that cannot be ignored and to detect silent breakages. Additionally, `versionCheckPhase` failures can be detected during package updates or when opening pull requests.