1 files changed, 73 insertions, 0 deletions
diff --git a/nixos/lib/eval-config.nix b/nixos/lib/eval-config.nix
new file mode 100644
index 00000000000..47e7d1a0eaf
--- /dev/null
+++ b/nixos/lib/eval-config.nix
@@ -0,0 +1,73 @@
+# From an end-user configuration file (`configuration'), build a NixOS
+# configuration object (`config') from which we can retrieve option
+# values.
+
+{ system ? builtins.currentSystem
+, pkgs ? null
+, baseModules ? import ../modules/module-list.nix
+, extraArgs ? {}
+, modules
+, nixpkgs ? <nixpkgs>
+}:
+
+let extraArgs_ = extraArgs; pkgs_ = pkgs; system_ = system; in
+
+rec {
+
+  # These are the NixOS modules that constitute the system configuration.
+  configComponents = modules ++ baseModules;
+
+  # Merge the option definitions in all modules, forming the full
+  # system configuration.  It's not checked for undeclared options.
+  systemModule =
+    pkgs.lib.fixMergeModules configComponents extraArgs;
+
+  optionDefinitions = systemModule.config;
+  optionDeclarations = systemModule.options;
+  inherit (systemModule) options;
+
+  # These are the extra arguments passed to every module.  In
+  # particular, Nixpkgs is passed through the "pkgs" argument.
+  extraArgs = extraArgs_ // {
+    inherit pkgs modules baseModules;
+    modulesPath = ../modules;
+    pkgs_i686 = import nixpkgs { system = "i686-linux"; };
+    utils = import ./utils.nix pkgs;
+  };
+
+  # Import Nixpkgs, allowing the NixOS option nixpkgs.config to
+  # specify the Nixpkgs configuration (e.g., to set package options
+  # such as firefox.enableGeckoMediaPlayer, or to apply global
+  # overrides such as changing GCC throughout the system), and the
+  # option nixpkgs.system to override the platform type.  This is
+  # tricky, because we have to prevent an infinite recursion: "pkgs"
+  # is passed as an argument to NixOS modules, but the value of "pkgs"
+  # depends on config.nixpkgs.config, which we get from the modules.
+  # So we call ourselves here with "pkgs" explicitly set to an
+  # instance that doesn't depend on nixpkgs.config.
+  pkgs =
+    if pkgs_ != null
+    then pkgs_
+    else import nixpkgs (
+      let
+        system = if nixpkgsOptions.system != "" then nixpkgsOptions.system else system_;
+        nixpkgsOptions = (import ./eval-config.nix {
+          inherit system extraArgs modules;
+          # For efficiency, leave out most NixOS modules; they don't
+          # define nixpkgs.config, so it's pointless to evaluate them.
+          baseModules = [ ../modules/misc/nixpkgs.nix ];
+          pkgs = import nixpkgs { system = system_; config = {}; };
+        }).optionDefinitions.nixpkgs;
+      in
+      {
+        inherit system;
+        inherit (nixpkgsOptions) config;
+      });
+
+  # Optionally check wether all config values have corresponding
+  # option declarations.
+  config =
+    let doCheck = optionDefinitions.environment.checkConfigurationOptions; in
+    assert doCheck -> pkgs.lib.checkModule "" systemModule;
+    systemModule.config;
+}