nix: introduce nix.registry options

modules/module-list.nix

@@ -40,6 +40,7 @@
   ./time
   ./networking
   ./nix
+  ./nix/flakes.nix
   ./nix/linux-builder.nix
   ./nix/nix-darwin.nix
   ./nix/nixpkgs.nix

modules/nix/flakes.nix

@@ -0,0 +1,97 @@
+/*
+  Manages the flake registry.
+
+  Based on <nixpkgs>/nixos/modules/config/nix-flakes.nix
+ */
+{ config, lib, ... }:
+let
+  inherit (lib)
+    filterAttrs
+    literalExpression
+    mapAttrsToList
+    mkDefault
+    mkIf
+    mkOption
+    types
+    ;
+
+  cfg = config.nix;
+
+in
+{
+  meta.maintainers = with lib.maintainers; [
+    Scrumplex
+  ];
+
+  options = {
+    nix = {
+      registry = mkOption {
+        type = types.attrsOf (types.submodule (
+          let
+            referenceAttrs = with types; attrsOf (oneOf [
+              str
+              int
+              bool
+              path
+              package
+            ]);
+          in
+          { config, name, ... }:
+          {
+            options = {
+              from = mkOption {
+                type = referenceAttrs;
+                example = { type = "indirect"; id = "nixpkgs"; };
+                description = lib.mdDoc "The flake reference to be rewritten.";
+              };
+              to = mkOption {
+                type = referenceAttrs;
+                example = { type = "github"; owner = "my-org"; repo = "my-nixpkgs"; };
+                description = lib.mdDoc "The flake reference {option}`from` is rewritten to.";
+              };
+              flake = mkOption {
+                type = types.nullOr types.attrs;
+                default = null;
+                example = literalExpression "nixpkgs";
+                description = lib.mdDoc ''
+                  The flake input {option}`from` is rewritten to.
+                '';
+              };
+              exact = mkOption {
+                type = types.bool;
+                default = true;
+                description = lib.mdDoc ''
+                  Whether the {option}`from` reference needs to match exactly. If set,
+                  a {option}`from` reference like `nixpkgs` does not
+                  match with a reference like `nixpkgs/nixos-20.03`.
+                '';
+              };
+            };
+            config = {
+              from = mkDefault { type = "indirect"; id = name; };
+              to = mkIf (config.flake != null) (mkDefault (
+                {
+                  type = "path";
+                  path = config.flake.outPath;
+                } // filterAttrs
+                  (n: _: n == "lastModified" || n == "rev" || n == "revCount" || n == "narHash")
+                  config.flake
+              ));
+            };
+          }
+        ));
+        default = { };
+        description = lib.mdDoc ''
+          A system-wide flake registry.
+        '';
+      };
+    };
+  };
+
+  config = mkIf cfg.enable {
+    environment.etc."nix/registry.json".text = builtins.toJSON {
+      version = 2;
+      flakes = mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
+    };
+  };
+}