@Ralith, thanks for your PR! By analyzing the history of the files in this pull request, we identified @expipiplus1, @corngood and @lummax to be potential reviewers.

I am attempting to get the added patch upstreamed, so hopefully it will be unnecessary soon.

This will break nix-built packages that use Vulkan and run on non-NixOS in situations where the XDG vars are unset of unavailable for security reasons. This is a corner case of a corner case that we already don't support very well, so IMO it shouldn't block, but if anyone knows an easy way to omit the second element of cmakeFlags on non-NixOS, that would be an improvement.

There are still standard well-known paths like /etc/vulkan/icd.d and /usr/share/vulkan/icd.d that would be examined so usual distributions are fine -- I don't think de facto much distributions will rely on XDG paths for this. Anyway, NixOS and non-NixOS are undistinguishable in nixpkgs.

This change specifically disables the fallback search of /usr/share. AFAIK every distribution sets XDG_DATA_DIRS such that this will not be an issue most of the time, but the fallback is outright wrong on non-NixOS. Maybe that's a case for making such distinguishing possible.

I was under the impression that you don't remove SYSCONFDIR and EXTRASYSCONFDIR -- I don't see anything hinting at this in this patch, and current code looks fine too. Or have I been misreading something yet again? D:

SYSCONFDIR and EXTRASYSCONFDIR are responsible for searching /etc, not /usr/share.

Oh, isn't one of those pointed at /usr/share? I thought it to be so for some reason...

See https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/blob/master/CMakeLists.txt#L289

Yeah, already. Seems my memory is flaky from my old NIX_CFLAGS_COMPILE patch, so -- sorry for misleading again.

Can't we just set fallback to ${mesa.driverLink}/share:/usr/share? Should make no harm on NixOS and fix the problem.

Good idea! Done. I'd be happier with a conditional but this is easy and works.

Tested on a local system, verified that it works in absence of XDG_DATA_DIRS and XDG_CONFIG_DIRS. Merged in bfdfd6c, thanks and sorry for me being slow ^_^

The patch introduced in this PR has been upstreamed; the next release of vulkan-loader will no longer require either patch currently in use.