{
  config,
  inputs,
  lib,
  pkgs,
  this,
  ...
}:
with lib;
let
  cfg = config.nixfiles.modules.common.nix;
in
{
  imports = with inputs.srvos.nixosModules; [
    mixins-nix-experimental
    mixins-trusted-nix-caches
  ];

  options.nixfiles.modules.common.nix.allowedUnfreePackages = mkOption {
    description = "A list of allowed unfree packages.";
    type = with types; listOf str;
    default = [ ];
  };

  config =
    let
      useNixpkgs =
        nixpkgs:
        import nixpkgs {
          inherit (config.nixpkgs) config;
          inherit (this) system;
        };

      pkgsLocal = useNixpkgs "${config.my.home}/src/nixpkgs"; # Impure!
      pkgsMaster = useNixpkgs inputs.nixpkgs-master;
      pkgsStable = useNixpkgs inputs.nixpkgs-stable;
      pkgsRev =
        rev: hash:
        useNixpkgs (
          pkgs.fetchFromGitHub {
            owner = "NixOS";
            repo = "nixpkgs";
            inherit rev hash;
          }
        );
      pkgsPr = pr: pkgsRev "refs/pull/${toString pr}/head";
    in
    {
      _module.args = {
        inherit
          pkgsLocal
          pkgsMaster
          pkgsStable
          pkgsRev
          pkgsPr
          ;
      };

      hm = {
        # Used primarily in conjunction with the "nixfiles" script.
        home.file.".nix-defexpr/default.nix".text =
          let
            hostname = strings.escapeNixIdentifier this.hostname;
          in
          optionalString this.isHeadful ''
            let
              self = builtins.getFlake "nixfiles";
              configurations = self.nixosConfigurations;
              local = configurations.${hostname};
            in rec {
              inherit self;
              inherit (self) inputs lib;
              inherit (lib) my;
              this = my.configurations.${hostname};
              inherit (local) config;
              inherit (local.config.system.build) toplevel vm vmWithBootLoader manual;
              pretty = expr: lib.trace (lib.generators.toPretty {} expr) {};
            } // configurations // local._module.args
          '';

        programs.bash.shellAliases.nix = "nix --verbose --print-build-logs";
      };

      nix =
        let
          notSelfInputs = filterAttrs (n: _: n != "self") inputs;
        in
        {
          daemonCPUSchedPolicy = "idle";
          daemonIOSchedClass = "idle";
          daemonIOSchedPriority = 7;

          settings = {
            keep-derivations = if this.isHeadful then "true" else "false";
            keep-outputs = if this.isHeadful then "true" else "false";

            warn-dirty = false;

            keep-going = true;

            substituters = [
              "https://azahi.cachix.org"
              "https://nix-community.cachix.org"
            ];

            trusted-substituters = [ "https://azahi.cachix.org" ];
            trusted-public-keys = [ "azahi.cachix.org-1:2bayb+iWYMAVw3ZdEpVg+NPOHCXncw7WMQ0ElX1GO3s=" ];

            trusted-users = [
              "root"
              my.username
            ];
          };

          nixPath = mapAttrsToList (n: v: "${n}=${v}") notSelfInputs ++ [
            "nixfiles=${config.my.home}/src/nixfiles"
          ];

          registry = mapAttrs (_: flake: { inherit flake; }) notSelfInputs // {
            nixfiles.flake = inputs.self;
          };
        };

      nixpkgs = {
        config.allowUnfreePredicate = p: elem (getName p) cfg.allowedUnfreePackages;

        overlays = with inputs; [
          self.overlays.default
          (_: prev: {
            # Global PR package overrides go here. Example:
            # ```
            # inherit (pkgsPr 309018 "sha256-x3ATxjrTVdaX5eo9P6pz+8/W6D2TNYzvjZpOBa3ZRI8=") endlessh-go;
            # ```

            spf-engine = prev.spf-engine.override {
              # FIXME https://nixpk.gs/pr-tracker.html?pr=321940
              inherit (pkgsMaster.python311.pkgs) pymilter;
            };
          })
        ];
      };

      environment = {
        localBinInPath = true;
        defaultPackages = [ ];
        systemPackages =
          with pkgs;
          optionals this.isHeadful [
            nix-tree
            nixfiles
          ];
        variables = {
          NIXFILES = "${config.my.home}/src/nixfiles";
          NIX_SHELL_PRESERVE_PROMPT = "1";
        };
      };

      system.stateVersion = this.stateVersion or trivial.release;
    };
}