nixos/qt: set QT_PLUGIN_PATH and QML2_IMPORT_PATH when enabled

[lilyinstarlight]

Description of changes

This is needed for installed Qt plugins to be found by Qt6 (and Qt5 seemed to possibly be working by accident rather than intent, so I set those paths here as well)

I can confirm this lets me use adwaita on qgnomeplatform with Qt6 apps

Fixes #210891

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 23.05 Release Notes (or backporting 22.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
  • (Release notes changes) Ran nixos/doc/manual/md-to-db.sh to update generated release notes
• Fits CONTRIBUTING.md.

[NickCao]

The path should actually be added to environment.profileRelativeSessionVariables, so that plugins installed in user profiles also works. But I was reluctant to do this as it introduces much impurities to nixos. Hope someone could come up with a better idea.

[lilyinstarlight]

The path should actually be added to environment.profileRelativeSessionVariables, so that plugins installed in user profiles also works. But I was reluctant to do this as it introduces much impurities to nixos. Hope someone could come up with a better idea.

Yeah I originally had written this to set the values directly from the packages to "${drv}/${qt.qtbase.qtPluginPath}" to QT_PLUGIN_PATH, but many other NixOS modules seemed to set stuff like that var to /run/current-system/sw. And setting it like this would make it work better for config switches without having to relog/reboot so I changed it to this before opening the PR

Also qt5 was already pulling from PATH impurely to search /run/current-system/sw (and that seems to have been the only reason qt5 styles were ever applied to begin with)

Those issues aside, I do not feel strongly if you or anyone else has a better idea to implement this or would prefer something else

[NickCao]

I'm happy to merge this, just update it to use environment.profileRelativeSessionVariables as in nixos/modules/programs/environment.nix

[aviallon]

This causes issues when a package uses another Qt library version, as when you pull one specific package from nixos-unstable.

[lilyinstarlight]

This causes issues when a package uses another Qt library version, as when you pull one specific package from nixos-unstable.

Is this an issue distinct from #227795? If so, can you open a new issue and describe what you are experiencing?

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/gpt4all-nix-derivation/27744/3

[Misterio77]

I wonder if this should be added even when qt.enable is false? Adding qt6 plugins to my environment should make them work, similarly to qt{4,5} and GTK{2,3,4}.

My usecase is adding qt6 plugins (e.g. #245571) to my home-manager config (using qt.* options), while my NixOS configuration does not have anything qt related enabled.

[aviallon]

@lilyinstarlight Yes, it is different. It also happens when one application / dependency uses a patched Qt, especially if using a derivation from nixpkgs-unstable on a nixos-23.05 system