Add minimal NixOS entrypoint

flake.nix

@@ -18,6 +18,9 @@
     in
     {
       lib = lib.extend (final: prev: {
+
+        nixos = import ./nixos/lib { lib = final; };
+
         nixosSystem = { modules, ... } @ args:
           import ./nixos/lib/eval-config.nix (args // {
             modules =

nixos/lib/default.nix

@@ -0,0 +1,33 @@
+let
+  # The warning is in a top-level let binding so it is only printed once.
+  minimalModulesWarning = warn "lib.nixos.evalModules is experimental and subject to change. See nixos/lib/default.nix" null;
+  inherit (nonExtendedLib) warn;
+  nonExtendedLib = import ../../lib;
+in
+{ # Optional. Allows an extended `lib` to be used instead of the regular Nixpkgs lib.
+  lib ? nonExtendedLib,
+
+  # Feature flags allow you to opt in to unfinished code. These may change some
+  # behavior or disable warnings.
+  featureFlags ? {},
+
+  # This file itself is rather new, so we accept unknown parameters to be forward
+  # compatible. This is generally not recommended, because typos go undetected.
+  ...
+}:
+let
+  seqIf = cond: if cond then builtins.seq else a: b: b;
+  # If cond, force `a` before returning any attr
+  seqAttrsIf = cond: a: lib.mapAttrs (_: v: seqIf cond a v);
+
+  eval-config-minimal = import ./eval-config-minimal.nix { inherit lib; };
+in
+/*
+  This attribute set appears as lib.nixos in the flake, or can be imported
+  using a binding like `nixosLib = import (nixpkgs + "/nixos/lib") { }`.
+*/
+{
+  inherit (seqAttrsIf (!featureFlags?minimalModules) minimalModulesWarning eval-config-minimal)
+    evalModules
+    ;
+}

nixos/lib/eval-config-minimal.nix

@@ -0,0 +1,49 @@
+
+# DO NOT IMPORT. Use nixpkgsFlake.lib.nixos, or import (nixpkgs + "/nixos/lib")
+{ lib }: # read -^
+
+let
+
+  /*
+    Invoke NixOS. Unlike traditional NixOS, this does not include all modules.
+    Any such modules have to be explicitly added via the `modules` parameter,
+    or imported using `imports` in a module.
+
+    A minimal module list improves NixOS evaluation performance and allows
+    modules to be independently usable, supporting new use cases.
+
+    Parameters:
+
+      modules:        A list of modules that constitute the configuration.
+
+      specialArgs:    An attribute set of module arguments. Unlike
+                      `config._module.args`, these are available for use in
+                      `imports`.
+                      `config._module.args` should be preferred when possible.
+
+    Return:
+
+      An attribute set containing `config.system.build.toplevel` among other
+      attributes. See `lib.evalModules` in the Nixpkgs library.
+
+   */
+  evalModules = {
+    prefix ? [],
+    modules ? [],
+    specialArgs ? {},
+  }:
+  # NOTE: Regular NixOS currently does use this function! Don't break it!
+  #       Ideally we don't diverge, unless we learn that we should.
+  #       In other words, only the public interface of nixos.evalModules
+  #       is experimental.
+  lib.evalModules {
+    inherit prefix modules;
+    specialArgs = {
+      modulesPath = builtins.toString ../modules;
+    } // specialArgs;
+  };
+
+in
+{
+  inherit evalModules;
+}

nixos/lib/eval-config.nix

@@ -33,6 +33,12 @@ let pkgs_ = pkgs;
 in
 
 let
+  evalModulesMinimal = (import ./default.nix {
+    inherit lib;
+    # Implicit use of feature is noted in implementation.
+    featureFlags.minimalModules = { };
+  }).evalModules;
+
   pkgsModule = rec {
     _file = ./eval-config.nix;
     key = _file;
@@ -70,11 +76,9 @@ let
     };
   allUserModules = modules ++ legacyModules;
 
-  noUserModules = lib.evalModules ({
-    inherit prefix;
+  noUserModules = evalModulesMinimal ({
+    inherit prefix specialArgs;
     modules = baseModules ++ extraModules ++ [ pkgsModule modulesModule ];
-    specialArgs =
-      { modulesPath = builtins.toString ../modules; } // specialArgs;
   });
 
   # Extra arguments that are useful for constructing a similar configuration.

nixos/modules/misc/nixpkgs.nix

@@ -64,6 +64,11 @@ let
 in
 
 {
+  imports = [
+    ./assertions.nix
+    ./meta.nix
+  ];
+
   options.nixpkgs = {
 
     pkgs = mkOption {

nixos/modules/misc/nixpkgs/test.nix

@@ -0,0 +1,8 @@
+{ evalMinimalConfig, pkgs, lib, stdenv }:
+lib.recurseIntoAttrs {
+  invokeNixpkgsSimple =
+    (evalMinimalConfig ({ config, modulesPath, ... }: {
+      imports = [ (modulesPath + "/misc/nixpkgs.nix") ];
+      nixpkgs.system = stdenv.hostPlatform.system;
+    }))._module.args.pkgs.hello;
+}

nixos/tests/all-tests.nix

@@ -19,6 +19,13 @@ let
   handleTestOn = systems: path: args:
     if elem system systems then handleTest path args
     else {};
+
+  nixosLib = import ../lib {
+    # Experimental features need testing too, but there's no point in warning
+    # about it, so we enable the feature flag.
+    featureFlags.minimalModules = {};
+  };
+  evalMinimalConfig = module: nixosLib.evalModules { modules = [ module ]; };
 in
 {
   _3proxy = handleTest ./3proxy.nix {};
@@ -327,6 +334,7 @@ in
   nix-serve-ssh = handleTest ./nix-serve-ssh.nix {};
   nixops = handleTest ./nixops/default.nix {};
   nixos-generate-config = handleTest ./nixos-generate-config.nix {};
+  nixpkgs = pkgs.callPackage ../modules/misc/nixpkgs/test.nix { inherit evalMinimalConfig; };
   node-red = handleTest ./node-red.nix {};
   nomad = handleTest ./nomad.nix {};
   novacomd = handleTestOn ["x86_64-linux"] ./novacomd.nix {};