nixos: XDG Base Directory Specification environment variables

[SandaruKasa]

Closes #224525.

The spec: https://specifications.freedesktop.org/basedir-spec/latest/

This PR introduces an xdg.directories.enable option, which automatically sets up the following environment variables:

• $XDG_CACHE_HOME
• $XDG_CONFIG_HOME
• $XDG_DATA_HOME
• $XDG_STATE_HOME

Their default values are taken from the spec, but can be overridden using config.xdg.directories.cache-home & so on.

Newbie question:

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • NixOS tests in nixos/tests.
  • Package tests at passthru.tests.
  • Tests in lib/tests or pkgs/test for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See nixpkgs-review usage.
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits CONTRIBUTING.md, pkgs/README.md, maintainers/README.md and other READMEs.

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[SandaruKasa]

nixpkgs-review result

Generated using nixpkgs-review.

Command: nixpkgs-review pr 432111
Commit: 48646a39daabd672ac66e74beb8403088d9ab5ab

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x86_64-linux

⏩ 2 packages blacklisted:

• nixos-install-tools
• tests.nixos-functions.nixos-test

✅ 5 packages built:

• tests.devShellTools.nixos
• tests.testers.lycheeLinkCheck.network
• tests.testers.nixosTest-example
• tests.testers.runNixOSTest-example (tests.testers.runNixOSTest-extendNixOS)
• tests.trivial-builders.references

[cafkafk]

Cool idea, I'm however wondering how controversial this change would be, or if it would be considered something that should just be done through home manager. I'm personally not sure where I land on that question, but I get the sense that it would be a good idea to get a slight amount of consensus, so that people aren't surprised this was done.

[cafkafk]

Changed default from true to lib.versionAtLeast config.system.stateVersion "25.11"

This may be a good start, but I think it would also be wise to add this change to the release notes as well, just so people will know that this behavior was added.

[SandaruKasa]

Changed default from true to lib.versionAtLeast config.system.stateVersion "25.11"

This may be a good start, but I think it would also be wise to add this change to the release notes as well, just so people will know that this behavior was added.

Done. Also rebased on top of master.