Set module class to "homeManager"

[roberth]

Motivation

Enable a basic type check for module imports, so that a simple error appears when e.g. a NixOS module is loaded into Home Manager.

While currently almost no modules are "typed" like this yet, this will increase to a non-zero fraction, so supporting this feature will be beneficial.

It is important for applications (including Home Manager) to initiate this improvement, by choosing a name for class, as we can not expect others to guess the name and hope for the best.

Description

This enables a module system feature documented here: https://nixos.org/manual/nixpkgs/stable/index.html#module-system-lib-evalModules-param-class

For example, it allows a mistake to be caught, which is loading a NixOS module into home-manager. This only works when the offending module declares what it's for with a _class attribute.

It is not expected that users declare the _type, because the payoff is small. It is only expected to be set by generic code, such as functions or libraries that help with the "publishing" of modules (e.g. flake-parts, flake-utils).

The class feature has been available in the module system since NixOS/nixpkgs#197547, merged May 6, 2023. It has been part of all releases since 23.05-beta. The last NixOS release that did not support it has been end-of-life for close to a year now.

Example:

(lib.homeManagerConfiguration {
  pkgs = nixpkgs.legacyPackages.x86_64-linux;
  modules = [{ _class = "nixos"; imports = [ ./foo.nix ]; }];
}).activation-script

Corresponding error:

error: The module <unknown-file> was imported into homeManager instead of nixos.

(<unknown-file> can be improved by also setting _file, if known;
a much older feature)

Checklist

• Change is backwards compatible.

  • Down to Nixpkgs lib 23.05.

• Code formatted with ./format.

• Code tested through nix-shell --pure tests -A run.all or nix develop --ignore-environment .#all using Flakes.

  • You've made me grep -v OK. Would appreciate a summary of the test result at the end.

• Test cases updated/added. See example.

  • Yes, assuming your entrypoint (homeManagerConfiguration or modules/default.nix) is tested.
  • The class feature itself is tested upstream and it's a simple feature, for which I (personally) would not require a test.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • N/A Added myself as module maintainer. See example.

Maintainer CC

[rycee]

Thanks! Looks great. Rebased to master in 26e72d8.

[roberth]

Ok, great, thanks!