modules/nixpkgs: complete the MVP

[MattSturgeon]

This PR finally gets the nixpkgs module to a fully functional state. If nixpkgs.pkgs is not defined, then an instance of nixpkgs is constructed from nixpkgs.source.

Constructing nixpkgs also requires knowing which system to target, so the hostPlatform and buildPlatform options are included.

I also included the nixpkgs.config option. This could just be attrsOf anything, but I've used the upstream option-type because it handles some special cases while merging definitions.

I've also added the non-standalone option nixpkgs.useGlobalPackages, which automatically sets nixpkgs.pkgs to the "host" or "global" packages; i.e. the pkgs arg in use by the outer module eval.

Most implementation is directly based on https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/misc/nixpkgs.nix

Follow up to:

• tests: fixes & refactoring for nixpkgs-module #2835
• tests/nixpkgs: move nixpkgs module test to dedicated drv #2740
• lib/modules: pass inputs.nixpkgs into evalNixvim #2670
• modules/nixpkgs: add overlays option #2430
• modules/nixpkgs: initial pkgs option; drop defaultPkgs specialArg #2301

Closes #2022
Closes #2437
Fixes #1784

Future work

system agnostic Nixvim factory

• Re-imagined standalone mode (nixvimConfig) #2210

Constructing pkgs within the module system allows us to have a system-agnostic replacement for makeNixvimWithModule and somewhat simplify the standalone template

Note: lib.nixvim.modules.evalNixvim is already system-agnostic. We simply need to provide documentation and/or new wrapper-functions for use by end-users.

Migrating useGlobalPackages default

• [nixpkgs] default to constructing our own instance #2839

I'm not 100% sure I know a good way to warn about that planned transition though. Maybe we'll just have to have an obnoxious but temporary warning shown to all users? Could still be missed by users who don't update very often.

[MattSturgeon]

Thanks for the help @khaneliman

I've reduced flakyness by testing with a mock nixpkgs source rather than downloading a real instance.

I've also narrowed the scope of the tests to focus on only the nixpkgs module and its dependencies. That way we aren't accidentally using the pkgs instance elsewhere in unrelated modules.

The testWrappers tests still test against a full nixvim configuration with all modules, due to them using the actual wrapper modules.

[GaetanLepage]

As far as I'm able to understand those changes (~70%), this looks good !
I trust you and @traxys on the details ;)

[MattSturgeon]

@mergify queue

[mergify]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at 5147429

[mergify]

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