LGTM. Did similar research to file the linked bug. Wikipedia has more primary sources that confirm the patent expiration.

I'm actually curious whether libtxc_dxtn is still needed for anything these days. I can't figure out what or how it would get loaded by anything (it's not a build time dependency, so presumably there is some kind of plugin system that would load it?) and it doesn't seem like the package is carried by the likes of Debian and others anymore.

In any case, I'm in strong favor of removing the option and either 1. getting rid of libtxc_dxtn; 2. just using the s3tc version by default. The current option is technical debt that ~nobody will use and can easily be implemented with local overrides if people really strongly care.

More digging: it seems like the only reason for libtxc_dxtn being useful was some wine-staging patch that could use it sometimes, and that patch was also picked up by Lutris.

This matches the build-time dependencies in nixpkgs. There could be other run-time dependencies, but I would expect the runtime path thing was also for wine-staging. Looking at the wine-staging docs from 2017:

Since 1.7.37 it is not necessary anymore to have this library available at build time, if it is called either libtxc_dxtn.so or libtxc_dxtn_s2tc.so.0. If your distro uses a library with a different name, you still need it. This change was done to simplify building Wine Staging on distros where libtxc_dxtn is not available in the official repositories.

However, wine-staging stopped using libtxc_dxtn around 2018: wine-staging/wine-staging#56

Gentoo went through the same deprecation process, though they had to keep the package for compatibility with old overlays: https://bugs.gentoo.org/654466

Let's just kill the package and all uses of it?

S3TC should now be built directly with mesa. This old split is obsolete, I believe, as well as S2TC.

Maybe we do need to tweak our mesa derivation, though. I don't even know off the top of my head how to test the support.

Release notes for 17.3.0 said:

libtxc_dxtn is now integrated into Mesa. GL_EXT_texture_compression_s3tc and GL_ANGLE_texture_compression_dxt are now always enabled on drivers that support them

I believe this NixOS option should be deprecated, and we could probably even remove expressions for the two libraries. I don't feel strongly about backporting to 20.03.

Agreed re: killing the packages; closing this PR in favour of that (might not get around to submitting a new one soon, so anyone else can feel free to open a replacement too).

Just sent one FYI.