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 the upstream otpion-type is a bit more complex because it allows definitions to be either attrs or functions-to-attrs.

Directly based on https://github.com/NixOS/nixpkgs/blob/master/nixos/modules/misc/nixpkgs.nix

Follow up to #2740, #2670, #2430, and #2301
Partially split into #2835

Closes #2022
Closes #2437
Fixes #1784

Marking as a draft for now, still need to:

• add some test cases
• proof-read all nixpkgs.* option docs, new & existing

Future work

This will allow us to have a non-per-system replacement for makeNixvimWithModule and somewhat simplify the standalone template

In the future we will also be able to change the default value of nixpkgs.useGlobalPackages. I'm not 100% sure I know a good way to warn about that planned transition though.

[MattSturgeon]

Instead of a generic "tip", this could actually use a context option to dynamically produce platform-specific docs. This is possible because we actually show the per-platform options separately.

Probably out of scope for this PR though.

[MattSturgeon]

Trivial with the existing meta.config.name option that was added for the per-platform option-docs.

[MattSturgeon]

[khaneliman]

Added some comments to matrix that I'll move here:

… while calling the 'intersectAttrs' builtin
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/lib/customisation.nix:219:17:
          218|       # This includes automatic ones and ones passed explicitly
          219|       allArgs = intersectAttrs fargs autoArgs // args;
             |                 ^
          220|

       … in the right operand of the update (//) operator
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/pkgs/top-level/splice.nix:152:54:
          151|
          152|   newScope = extra: lib.callPackageWith (pkgsForCall // extra);
             |                                                      ^
          153|

       … in the left operand of the update (//) operator
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/lib/customisation.nix:649:32:
          648|       };
          649|       spliced = extra spliced0 // spliced0 // keep self;
             |                                ^
          650|       self = f self // {

       … from call site
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/lib/customisation.nix:649:17:
          648|       };
          649|       spliced = extra spliced0 // spliced0 // keep self;
             |                 ^
          650|       self = f self // {

       … while calling 'extra'
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/pkgs/top-level/darwin-packages.nix:48:11:
           47|   otherSplices = generateSplicesForMkScope "darwin";
           48|   extra = spliced: spliced.apple_sdk.frameworks;
             |           ^
           49|   f = lib.extends aliases (

       … while evaluating the attribute 'apple_sdk.frameworks'
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/pkgs/top-level/darwin-packages.nix:90:13:
           89|           inherit
           90|             apple_sdk
             |             ^
           91|             apple_sdk_10_12

       … while calling the 'throw' builtin
         at /nix/store/4hpdrd3qvj7nks3rrimqm2jdmcga8isc-source/pkgs/top-level/darwin-packages.nix:85:15:
           84|         .${stdenv.hostPlatform.darwinSdkVersion}
           85|           or (throw "Unsupported sdk: ${stdenv.hostPlatform.darwinSdkVersion}");
             |               ^
           86|

       error: Unsupported sdk: 11.3

The line of code is present in the sourced nixpkgs archive in the test

    # Test that a nixpkgs revision can be specified using `nixpkgs.source`
  (testModule "nixpkgs-source" (
    { pkgs, ... }:
    {     
      nixpkgs.source = builtins.fetchTarball {
        url = "https://github.com/NixOS/nixpkgs/archive/1807c2b91223227ad5599d7067a61665c52d1295.tar.gz";
        sha256 = "0xnj86751780hg1zhx9rjkbjr0sx0wps4wxz7zryvrj6hgwrng1z";
      };

      nixpkgs.hostPlatform = {
        inherit (stdenv.hostPlatform) system;
      };

      assertions = [
        {
          assertion = pkgs.lib.version == "24.11pre-git";
          message = ''
            Expected `pkgs.lib.version` to be "24.11pre-git", but found "${pkgs.lib.version}"
          '';
        }
      ];
    }
  ))

Is it a mismatch between the pkgs coming in for the test case and the nixpkgs.source being used in the test? Does it show an issue with the impelmentation? Or would that be an expected failure?

The earliest commit that would avoid this error:

      nixpkgs.source = builtins.fetchTarball {
        url = "https://github.com/NixOS/nixpkgs/archive/0f9067f865a22aa6c1fe94eed7f218287b3e9425.tar.gz";
        sha256 = "0bq40zax9s3hcw901qm91cxh7z6q5czvyz1814gvg3bgii0ndavv";
      };

[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.