fix: nixpkgs instead of inputs.nixpkgs should be used in mkConfig

[gabyx]

I was stumbling accross the inputs.nixpkgs use in mkConfig and it seemed to me that it would make more sense to use the nixpkgs input from devenv's flake instead of what the user inputs to mkConfig { inputs = {...} }.

Maybe I am missing the point of using inputs.nixpkgs which the users provided inside mkConfig 😄
Shouldn't the devenv use its rolling version evalModules etc... with what it was tested to be more stable?

Currently mkConfig needs inputs.self, inputs.devenv and inputs.nixpkgs. I think one can get rid of inputs.nixpkgs and replace it with nixpkgs.

[gabyx]

addendum: Als there is the question why not defaulting the pkgs to pkgs ? nixpkgs.legacyPackages.${system} which takes the correct inputs as devenv was tested with.
But then I guess the mkConfig needs a system paramter which is .... argh.....

[sandydoo]

In other words, we always had an implicit follows because were always using the root flake's nixpkgs input.

The concern I have is what happens when a user deliberately passes in a nixpkgs input? Instead of ignoring the devenv flake's nixpkgs input, we're now ignoring the argument.

I think the best approach here is to have a fallback. Use the flake's nixpkgs input by default. This makes nixpkgs an optional input in mkConfig. If we get a nixpkgs in inputs, then it takes precedence.

[gabyx]

@sandydoo : I will introduce the fallback for the backwards compatibility:
My question is just, do you really want this this backwards compatibility or is it not just better to use always nixpkgs.

Pro: No fallback:

• Because its only used to evaluate the configuration and that functionality is bound to the version of devenv.
• The user already passes pkgs so it will construct it from devenv/rolling if one wants to be up-to-date with testing.

Contra: Fallback:

• The user should be able to choose which nixpkgs.lib.evalModules function gets used. Is this really the case? So you could have devenv on verison X and use version Z (Z > X) for the NixOS module system to do the eval?
  How would that work, cause devenvs module code in version X might not be compatible with the passed nixpkgs on verison Z.

With the Pro argument: the example in the docs is directly correct:

{
  inputs = {
    devenv.url = "github:cachix/devenv";
    nixpkgs.url = "github:cachix/devenv-nixpkgs/rolling"; # The packages against devenv was tested.
  };

  nixConfig = {
    extra-trusted-public-keys = "devenv.cachix.org-1:w1cLUi8dv3hnoSPGAuibQv+f9TZLr6cv/Hm9XgU50cw=";
    extra-substituters = "https://devenv.cachix.org";
  };

  outputs = { self, nixpkgs, devenv, ... } @ inputs:
    let
      system = "x86_64-linux";
      pkgs = nixpkgs.legacyPackages.${system};
    in
    {
      packages.${system}.devenv-up = self.devShells.${system}.default.config.procfileScript;
      packages.${system}.devenv-test = self.devShells.${system}.default.config.test;

      devShells.${system}.default = devenv.lib.mkShell {
        inherit inputs pkgs;
        modules = [
          ({lib, pkgs, config, ... }: {
          
            # Important: `lib` is the nixpkgs lib with what devenv was tested.
            # You do not need `devenv.inputs.nixpkgs.follows = "nixpkgs"` anymore above.
            
            # This is your devenv configuration
            packages = [ pkgs.hello ];

            enterShell = ''
              hello
            '';

            processes.run.exec = "hello";
          })
        ];
      };
    };
}

[gabyx]

@sandydoo : I tried to improve the pkgs and inputs arguments. Check the last commit, isn't this solution better?
So by default we use the stuff with what devenv was tested with.