Document system.build.toplevel

nixos/modules/system/activation/top-level.nix

@@ -139,21 +139,13 @@ in
 
 {
   imports = [
+    ../build.nix
     (mkRemovedOptionModule [ "nesting" "clone" ] "Use `specialisation.«name» = { inheritParentConfig = true; configuration = { ... }; }` instead.")
     (mkRemovedOptionModule [ "nesting" "children" ] "Use `specialisation.«name».configuration = { ... }` instead.")
   ];
 
   options = {
 
-    system.build = mkOption {
-      internal = true;
-      default = {};
-      type = types.lazyAttrsOf types.unspecified;
-      description = ''
-        Attribute set of derivations used to setup the system.
-      '';
-    };
-
     specialisation = mkOption {
       default = {};
       example = lib.literalExpression "{ fewJobsManyCores.configuration = { nix.buildCores = 0; nix.maxJobs = 1; }; }";
@@ -301,6 +293,16 @@ in
       '';
     };
 
+    system.build.toplevel = mkOption {
+      type = types.package;
+      readOnly = true;
+      description = ''
+        This option contains the store path that typically represents a NixOS system.
+
+        You can read this path in a custom deployment tool for example.
+      '';
+    };
+
   };
 
 

nixos/modules/system/build.nix

@@ -0,0 +1,21 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options = {
+
+    system.build = mkOption {
+      default = {};
+      description = ''
+        Attribute set of derivations used to set up the system.
+      '';
+      type = types.submoduleWith {
+        modules = [{
+          freeformType = types.lazyAttrsOf types.unspecified;
+        }];
+      };
+    };
+
+  };
+}