nix: add module (#2623)

modules/misc/nix.nix

@@ -0,0 +1,86 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.nix;
+
+in {
+  options.nix = {
+    registry = mkOption {
+      type = types.attrsOf (types.submodule (let
+        inputAttrs = types.attrsOf
+          (types.oneOf [ types.str types.int types.bool types.package ]);
+      in { config, name, ... }: {
+        options = {
+          from = mkOption {
+            type = inputAttrs;
+            example = {
+              type = "indirect";
+              id = "nixpkgs";
+            };
+            description = "The flake reference to be rewritten.";
+          };
+          to = mkOption {
+            type = inputAttrs;
+            example = {
+              type = "github";
+              owner = "my-org";
+              repo = "my-nixpkgs";
+            };
+            description =
+              "The flake reference to which <option>from></option> is to be rewritten.";
+          };
+          flake = mkOption {
+            type = types.nullOr types.attrs;
+            default = null;
+            example = literalExpression "nixpkgs";
+            description = ''
+              The flake input to which <option>from></option> is to be rewritten.
+            '';
+          };
+          exact = mkOption {
+            type = types.bool;
+            default = true;
+            description = ''
+              Whether the <option>from</option> reference needs to match exactly. If set,
+              a <option>from</option> reference like <literal>nixpkgs</literal> does not
+              match with a reference like <literal>nixpkgs/nixos-20.03</literal>.
+            '';
+          };
+        };
+        config = {
+          from = mkDefault {
+            type = "indirect";
+            id = name;
+          };
+          to = mkIf (config.flake != null) ({
+            type = "path";
+            path = config.flake.outPath;
+          } // lib.filterAttrs (n: v:
+            n == "lastModified" || n == "rev" || n == "revCount" || n
+            == "narHash") config.flake);
+        };
+      }));
+      default = { };
+      description = ''
+        User level flake registry.
+      '';
+    };
+    registryVersion = mkOption {
+      type = types.int;
+      default = 2;
+      internal = true;
+      description = "The flake registry format version.";
+    };
+  };
+
+  config = mkIf (cfg.registry != { }) {
+    xdg.configFile."nix/registry.json".text = builtins.toJSON {
+      version = cfg.registryVersion;
+      flakes =
+        mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
+    };
+  };
+}

modules/modules.nix

@@ -257,6 +257,7 @@ let
     ./xcursor.nix
     ./xresources.nix
     ./xsession.nix
+    ./misc/nix.nix
     (pkgs.path + "/nixos/modules/misc/assertions.nix")
     (pkgs.path + "/nixos/modules/misc/meta.nix")
   ] ++ optional useNixpkgsModule ./misc/nixpkgs.nix