nix: Add registry option

modules/environment/nix.nix

@@ -46,6 +46,65 @@ in
         '';
       };
 
+      registry = mkOption {
+        type = types.attrsOf (types.submodule (
+          let
+            referenceAttrs = with types; attrsOf (oneOf [
+              str
+              int
+              bool
+              package
+            ]);
+          in
+          { config, name, ... }:
+          {
+            options = {
+              from = mkOption {
+                type = referenceAttrs;
+                example = { type = "indirect"; id = "nixpkgs"; };
+                description = "The flake reference to be rewritten.";
+              };
+              to = mkOption {
+                type = referenceAttrs;
+                example = { type = "github"; owner = "my-org"; repo = "my-nixpkgs"; };
+                description = "The flake reference <option>from</option> is rewritten to.";
+              };
+              flake = mkOption {
+                type = types.nullOr types.attrs;
+                default = null;
+                example = literalExpression "nixpkgs";
+                description = ''
+                  The flake input <option>from</option> is rewritten to.
+                '';
+              };
+              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) (mkDefault
+                {
+                  type = "path";
+                  path = config.flake.outPath;
+                } // filterAttrs
+                (n: _: n == "lastModified" || n == "rev" || n == "revCount" || n == "narHash")
+                config.flake);
+            };
+          }
+        ));
+        default = { };
+        description = ''
+          A system-wide flake registry.
+        '';
+      };
+
       substituters = mkOption {
         type = types.listOf types.str;
         default = [];
@@ -86,6 +145,11 @@ in
           trusted-public-keys = ${concatStringsSep " " cfg.trustedPublicKeys}
           ${cfg.extraConfig}
         '';
+
+        "nix/registry.json".text = builtins.toJSON {
+          version = 2;
+          flakes = mapAttrsToList (n: v: { inherit (v) from to exact; }) cfg.registry;
+        };
       };
 
       nix = {