From 172ac3d46df733fde8516f477c7b08fd1f56fac0 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 20 Jan 2022 14:53:35 +0100
Subject: [PATCH 1/3] nixos/system/build: Extract

Extracting system.build into its own module allows other modules
that do not depend on e.g. toplevel to be evaluated, used and
tested in isolation.
---
 nixos/modules/system/activation/top-level.nix | 10 +---------
 nixos/modules/system/build.nix                | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+), 9 deletions(-)
 create mode 100644 nixos/modules/system/build.nix

diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
index 1c588ff969184..e10d668ad2cf6 100644
--- a/nixos/modules/system/activation/top-level.nix
+++ b/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; }; }";
diff --git a/nixos/modules/system/build.nix b/nixos/modules/system/build.nix
new file mode 100644
index 0000000000000..a8fc5dc78be5f
--- /dev/null
+++ b/nixos/modules/system/build.nix
@@ -0,0 +1,18 @@
+{ lib, ... }:
+let
+  inherit (lib) mkOption types;
+in
+{
+  options = {
+
+    system.build = mkOption {
+      internal = true;
+      default = {};
+      type = types.lazyAttrsOf types.unspecified;
+      description = ''
+        Attribute set of derivations used to set up the system.
+      '';
+    };
+
+  };
+}

From 50e48a57c51252c5b1207101889e8fd29cb35f41 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 20 Jan 2022 15:04:08 +0100
Subject: [PATCH 2/3] nixos: Make system.build a submodule with freeformType

This allows the values below it to be specified as options, while
remaining compatible with existing code.
---
 nixos/modules/system/build.nix | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/nixos/modules/system/build.nix b/nixos/modules/system/build.nix
index a8fc5dc78be5f..f7e2c8dfb1a44 100644
--- a/nixos/modules/system/build.nix
+++ b/nixos/modules/system/build.nix
@@ -6,12 +6,15 @@ in
   options = {
 
     system.build = mkOption {
-      internal = true;
       default = {};
-      type = types.lazyAttrsOf types.unspecified;
       description = ''
         Attribute set of derivations used to set up the system.
       '';
+      type = types.submoduleWith {
+        modules = [{
+          freeformType = types.lazyAttrsOf types.unspecified;
+        }];
+      };
     };
 
   };

From 28e8aeb34dc236db77405fbaeee99b29d6b3ec81 Mon Sep 17 00:00:00 2001
From: Robert Hensing <robert@roberthensing.nl>
Date: Thu, 20 Jan 2022 15:05:45 +0100
Subject: [PATCH 3/3] nixos: Document system.build.toplevel

---
 nixos/modules/system/activation/top-level.nix | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/nixos/modules/system/activation/top-level.nix b/nixos/modules/system/activation/top-level.nix
index e10d668ad2cf6..eae4b8c4e65ab 100644
--- a/nixos/modules/system/activation/top-level.nix
+++ b/nixos/modules/system/activation/top-level.nix
@@ -293,6 +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.
+      '';
+    };
+
   };