Checking a flake in recursive Nix

flake.lock

{
  "nodes": {
    "flake-utils": {
      "locked": {
        "lastModified": 1614513358,
        "narHash": "sha256-LakhOx3S1dRjnh0b5Dg3mbZyH0ToC9I8Y2wKSkBaTzU=",
        "owner": "numtide",
        "repo": "flake-utils",
        "rev": "5466c5bbece17adaab2d82fae80b46e807611bf3",
        "type": "github"
      },
      "original": {
        "owner": "numtide",
        "repo": "flake-utils",
        "type": "github"
      }
    },
    "nixpkgs": {
      "locked": {
        "lastModified": 1622966049,
        "narHash": "sha256-6g+28v94ISkVk9TBSsITVOnB2slK8plieWPIF2jo/l0=",
        "owner": "nixos",
        "repo": "nixpkgs",
        "rev": "fbfb79400a08bf754e32b4d4fc3f7d8f8055cf94",
        "type": "github"
      },
      "original": {
        "owner": "nixos",
        "ref": "nixos-unstable",
        "repo": "nixpkgs",
        "type": "github"
      }
    },
    "root": {
      "inputs": {
        "nixpkgs": "nixpkgs",
        "rust-overlay": "rust-overlay"
      }
    },
    "rust-overlay": {
      "inputs": {
        "flake-utils": "flake-utils",
        "nixpkgs": [
          "nixpkgs"
        ]
      },
      "locked": {
        "lastModified": 1623378256,
        "narHash": "sha256-nlSwmn7ywh02EOkr+pnrq3yiD/Wq1GjAirEuHQLqbss=",
        "owner": "oxalica",
        "repo": "rust-overlay",
        "rev": "45fbdefb8cdd5f4e8e28e27ae1f2a90241e66529",
        "type": "github"
      },
      "original": {
        "owner": "oxalica",
        "repo": "rust-overlay",
        "type": "github"
      }
    }
  },
  "root": "root",
  "version": 7
}

flake.nix

{
  description = "I'M FRED POO";

  inputs = {
    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";

    rust-overlay.url = "github:oxalica/rust-overlay";
    rust-overlay.inputs.nixpkgs.follows = "nixpkgs";
  };

  outputs = inputs @ { self, nixpkgs, ... }:
    let
      supportedSystems = [
        "x86_64-linux"
      ];

      overlays = [
        inputs.rust-overlay.overlay
      ];

      perSystem = nixpkgs.lib.genAttrs supportedSystems;
      pkgsPerSystem = f: perSystem (system: f (import nixpkgs { inherit system overlays; }));
      appsPerSystem = names: perSystem (system: nixpkgs.lib.genAttrs names (name: {
        type = "app";
        program = "${self.packages."${system}"."${name}"}/bin/${name}";
      }));
    in
    {
      #$ nix run nixpkgs.nixUnstable -c nix develop
      devShell = pkgsPerSystem (pkgs: pkgs.mkShell {
        buildInputs = [
          pkgs.rust-bin.stable.latest.default
        ];
      });

      checks = pkgsPerSystem (pkgs: {
        hello = pkgs.hello.overrideAttrs (_: { name = "hello-rebuild"; });
      });

      packages = pkgsPerSystem (pkgs: {
        fredpoo = pkgs.runCommand "fredpoo"
          {
            requiredSystemFeatures = [ "recursive-nix" ];

            buildInputs = [ pkgs.nixUnstable ];
            NIX_PATH = "nixpkgs=${nixpkgs}";
            NIX_CONFIG = "experimental-features = nix-command flakes";

            flake = self;

            # Inject the sources of transitive inputs in the context
            transitive =
              let
                inherit (builtins) substring;
                inherit (nixpkgs.lib) attrValues concatMapStrings;
                f = inputs:
                  concatMapStrings
                    (input: substring 0 0 input.outPath + f input.inputs)
                    (attrValues inputs);
              in
              f self.inputs;
          }
          ''
            # Shows the list of sources that were in context (supposedly)
            nix --offline path-info --all | grep 'source$'

            nix --show-trace --offline flake metadata $flake
            nix --show-trace --offline flake check $flake

            # touch $out
          '';

        build-fredpoo = pkgs.writeScriptBin "build-fredpoo" ''
          ${pkgs.nixUnstable}/bin/nix \
            --experimental-features 'nix-command flakes recursive-nix' \
            build ${self}#fredpoo

          ${pkgs.nixUnstable}/bin/nix log ${self}#fredpoo
        '';
      });

      apps = appsPerSystem [ "build-fredpoo" ];

      defaultApp = perSystem (system: self.apps."${system}".build-fredpoo);
    };
}