Reapply "nixos/nixpkgs.config: make use of the module defined in config.nix"

This reverts commit a06a6caa6b.

nixos/modules/misc/nixpkgs.nix

@@ -2,51 +2,12 @@
   config,
   options,
   lib,
-  pkgs,
   ...
 }:
 let
   cfg = config.nixpkgs;
   opt = options.nixpkgs;
 
-  isConfig = x: builtins.isAttrs x || lib.isFunction x;
-
-  optCall = f: x: if lib.isFunction f then f x else f;
-
-  mergeConfig =
-    lhs_: rhs_:
-    let
-      lhs = optCall lhs_ { inherit lib pkgs; };
-      rhs = optCall rhs_ { inherit lib pkgs; };
-    in
-    lib.recursiveUpdate lhs rhs
-    // lib.optionalAttrs (lhs ? allowUnfreePackages) {
-      allowUnfreePackages = lhs.allowUnfreePackages ++ (lib.attrByPath [ "allowUnfreePackages" ] [ ] rhs);
-    }
-    // lib.optionalAttrs (lhs ? packageOverrides) {
-      packageOverrides =
-        pkgs:
-        optCall lhs.packageOverrides pkgs // optCall (lib.attrByPath [ "packageOverrides" ] { } rhs) pkgs;
-    }
-    // lib.optionalAttrs (lhs ? perlPackageOverrides) {
-      perlPackageOverrides =
-        pkgs:
-        optCall lhs.perlPackageOverrides pkgs
-        // optCall (lib.attrByPath [ "perlPackageOverrides" ] { } rhs) pkgs;
-    };
-
-  configType = lib.mkOptionType {
-    name = "nixpkgs-config";
-    description = "nixpkgs config";
-    check =
-      x:
-      let
-        traceXIfNot = c: if c x then true else lib.traceSeqN 1 x false;
-      in
-      traceXIfNot isConfig;
-    merge = args: lib.foldr (def: mergeConfig def.value) { };
-  };
-
   overlayType = lib.mkOptionType {
     name = "nixpkgs-overlay";
     description = "nixpkgs overlay";
@@ -73,6 +34,8 @@ let
     ++ lib.optional (opt.localSystem.highestPrio < (lib.mkOptionDefault { }).priority) opt.localSystem
     ++ lib.optional (opt.crossSystem.highestPrio < (lib.mkOptionDefault { }).priority) opt.crossSystem;
 
+  _configDefinitions = opt.config.definitionsWithLocations;
+
   defaultPkgs =
     if opt.hostPlatform.isDefined then
       let
@@ -90,14 +53,15 @@ let
       in
       import ../../.. (
         {
-          inherit (cfg) config overlays;
+          inherit _configDefinitions;
+          inherit (cfg) overlays;
         }
         // systemArgs
       )
     else
       import ../../.. {
+        inherit _configDefinitions;
         inherit (cfg)
-          config
           overlays
           localSystem
           crossSystem
@@ -165,7 +129,15 @@ in
       example = lib.literalExpression ''
         { allowBroken = true; allowUnfree = true; }
       '';
-      type = configType;
+      type = lib.types.deferredModuleWith {
+        staticModules = [
+          { _module.args.docPrefix = "https://nixos.org/manual/nixpkgs/unstable/"; }
+          ../../../pkgs/top-level/config.nix
+        ];
+      };
+      # Returns pkgs.config instead of nixpkgs.config
+      # This shadows the deferredModule to make it look like a submodule
+      apply = _: finalPkgs.config;
       description = ''
         Global configuration for Nixpkgs.
         The complete list of [Nixpkgs configuration options](https://nixos.org/manual/nixpkgs/unstable/#sec-config-options-reference) is in the [Nixpkgs manual section on global configuration](https://nixos.org/manual/nixpkgs/unstable/#chap-packageconfig).
@@ -406,7 +378,7 @@ in
           '';
         }
         {
-          assertion = opt.pkgs.isDefined -> cfg.config == { };
+          assertion = opt.pkgs.isDefined -> opt.config.highestPrio == (lib.mkOptionDefault null).priority;
           message = ''
             Your system configures nixpkgs with an externally created instance.
             `nixpkgs.config` options should be passed when creating the instance instead.

pkgs/test/config-nix-unit.nix

@@ -0,0 +1,117 @@
+# Tests for nixpkgs config forwarding from NixOS modules.
+#
+# Run with:
+#   nix-unit pkgs/test/config-nix-unit.nix
+# or
+#   nix-build -A tests.config-nix-unit
+#
+{
+  nixpkgsPath ? ../..,
+  pkgs ? import nixpkgsPath { },
+}:
+let
+  lib = pkgs.lib;
+
+  # Test helper
+  evalNixos =
+    modules:
+    import (nixpkgsPath + "/nixos/lib/eval-config.nix") {
+      modules = [ { nixpkgs.hostPlatform = "x86_64-linux"; } ] ++ modules;
+    };
+in
+{
+  # Basic: a single config option is forwarded correctly.
+  testSingleConfigOption = {
+    expr = (evalNixos [ { nixpkgs.config.allowUnfree = true; } ]).config.nixpkgs.config.allowUnfree;
+    expected = true;
+  };
+
+  # Multiple config definitions from separate modules are merged.
+  testMultipleModulesMerge = {
+    expr =
+      let
+        eval = evalNixos [
+          { nixpkgs.config.allowUnfree = true; }
+          { nixpkgs.config.allowBroken = true; }
+        ];
+      in
+      {
+        inherit (eval.config.nixpkgs.config) allowUnfree allowBroken;
+      };
+    expected = {
+      allowUnfree = true;
+      allowBroken = true;
+    };
+  };
+
+  # mkForce works. Also covers other properties
+  testMkForce = {
+    expr =
+      (evalNixos [
+        { nixpkgs.config.allowUnfree = true; }
+        { nixpkgs.config.allowUnfree = lib.mkForce false; }
+      ]).config.nixpkgs.config.allowUnfree;
+    expected = false;
+  };
+
+  testDefaults = {
+    expr = (evalNixos [ ]).config.nixpkgs.config.allowUnfree;
+    expected = false;
+  };
+
+  # Standalone nixpkgs (i.e. import <nixpkgs> { ... })
+  testStandaloneConfig = {
+    expr = (import nixpkgsPath { config.allowUnfree = true; }).config.allowUnfree;
+    expected = true;
+  };
+
+  # Standalone nixpkgs with a function (i.e. import <nixpkgs> ({pkgs, lib, ...}: { ... })
+  testStandaloneConfigFunctionPkgs = {
+    expr =
+      (import nixpkgsPath {
+        config =
+          { pkgs, lib, ... }:
+          {
+            allowUnfree = lib.isAttrs pkgs;
+          };
+      }).config.allowUnfree;
+    expected = true;
+  };
+
+  # NixOS module sets nixpkgs.config as a function
+  testNixosConfigFunction = {
+    expr =
+      (evalNixos [
+        {
+          nixpkgs.config =
+            { lib, ... }:
+            {
+              allowUnfree = lib.isFunction lib.id;
+            };
+        }
+      ]).config.nixpkgs.config.allowUnfree;
+    expected = true;
+  };
+
+  # Passing both config and _configDefinitions is not allowed
+  testConfigAndDefinitionsMutuallyExclusive = {
+    expr =
+      (import nixpkgsPath {
+        config = {
+          allowUnfree = true;
+        };
+        _configDefinitions = [
+          {
+            file = "test";
+            value = {
+              allowBroken = true;
+            };
+          }
+        ];
+      }).config.allowUnfree;
+    expectedError = {
+      type = "ThrownError";
+      msg = ".*_configDefinitions.*internal.*must not be combined.*";
+    };
+  };
+}

pkgs/test/default.nix

@@ -128,6 +128,21 @@ in
 
   config = callPackage ./config.nix { };
 
+  # Technically nix-unit binds to a fixed nix version
+  # We have tests in lib to test the module system itself against different nix-versions
+  # Based on this assumption (transitivity of correctness) this test should therefore also cover all tested nix-versions
+  config-nix-unit =
+    pkgs.runCommand "config-nix-unit"
+      {
+        nativeBuildInputs = [ pkgs.nix-unit ];
+      }
+      ''
+        export HOME=$TMPDIR
+        nix-unit --eval-store "$HOME" ${./config-nix-unit.nix} \
+          --arg nixpkgsPath "${../..}"
+        mkdir $out
+      '';
+
   top-level = callPackage ./top-level { };
 
   haskell = callPackage ./haskell { };

pkgs/top-level/config.nix

@@ -6,7 +6,12 @@
 #     nix-build -A tests.config
 #
 
-{ config, lib, ... }:
+{
+  config,
+  lib,
+  docPrefix,
+  ...
+}:
 
 let
   inherit (lib)
@@ -115,13 +120,13 @@ let
     gitConfig = mkOption {
       type = types.attrsOf (types.attrsOf types.anything);
       description = ''
-        The default [git configuration](https://git-scm.com/docs/git-config#_variables) for all [`pkgs.fetchgit`](#fetchgit) calls.
+        The default [git configuration](https://git-scm.com/docs/git-config#_variables) for all [`pkgs.fetchgit`](${docPrefix}#fetchgit) calls.
 
         Among many other potential uses, this can be used to override URLs to point to local mirrors.
 
         Changing this will not cause any rebuilds because `pkgs.fetchgit` produces a [fixed-output derivation](https://nix.dev/manual/nix/stable/glossary.html?highlight=fixed-output%20derivation#gloss-fixed-output-derivation).
 
-        To set the configuration file directly, use the [`gitConfigFile`](#opt-gitConfigFile) option instead.
+        To set the configuration file directly, use the [`gitConfigFile`](${docPrefix}#opt-gitConfigFile) option instead.
 
         To set the configuration file for individual calls, use `fetchgit { gitConfigFile = "..."; }`.
       '';
@@ -135,9 +140,9 @@ let
     gitConfigFile = mkOption {
       type = types.nullOr types.path;
       description = ''
-        A path to a [git configuration](https://git-scm.com/docs/git-config#_variables) file, to be used for all [`pkgs.fetchgit`](#fetchgit) calls.
+        A path to a [git configuration](https://git-scm.com/docs/git-config#_variables) file, to be used for all [`pkgs.fetchgit`](${docPrefix}#fetchgit) calls.
 
-        This overrides the [`gitConfig`](#opt-gitConfig) option, see its documentation for more details.
+        This overrides the [`gitConfig`](${docPrefix}#opt-gitConfig) option, see its documentation for more details.
       '';
       default =
         if config.gitConfig != { } then
@@ -155,7 +160,7 @@ let
 
         For example, an override like `"registry.npmjs.org" = "my-mirror.local/registry.npmjs.org"` will replace a URL like `https://registry.npmjs.org/foo.tar.gz` with `https://my-mirror.local/registry.npmjs.org/foo.tar.gz`.
 
-        To set the string directly, see [`npmRegistryOverridesString`](#opt-npmRegistryOverridesString).
+        To set the string directly, see [`npmRegistryOverridesString`](${docPrefix}#opt-npmRegistryOverridesString).
       '';
       default = { };
       example = {
@@ -174,7 +179,7 @@ let
       description = ''
         A string containing a string with a JSON representation of npm registry overrides for `fetchNpmDeps`.
 
-        This overrides the [`npmRegistryOverrides`](#opt-npmRegistryOverrides) option, see its documentation for more details.
+        This overrides the [`npmRegistryOverrides`](${docPrefix}#opt-npmRegistryOverrides) option, see its documentation for more details.
       '';
       default = builtins.toJSON config.npmRegistryOverrides;
     };
@@ -412,7 +417,7 @@ let
       type = types.listOf types.str;
       default = [ "https://tarballs.nixos.org" ];
       description = ''
-        The set of content-addressed/hashed mirror URLs used by [`pkgs.fetchurl`](#sec-pkgs-fetchers-fetchurl).
+        The set of content-addressed/hashed mirror URLs used by [`pkgs.fetchurl`](${docPrefix}#sec-pkgs-fetchers-fetchurl).
         In case `pkgs.fetchurl` can't download from the given URLs,
         it will try the hashed mirrors based on the expected output hash.
 
@@ -466,11 +471,27 @@ let
         Silence the warning for the upcoming deprecation of the
         `x86_64-darwin` platform in Nixpkgs 26.11.
 
-        See the [release notes](#x86_64-darwin-26.05) for more
+        See the [release notes](${docPrefix}#x86_64-darwin-26.05) for more
         information.
       '';
     };
 
+    packageOverrides = mkOption {
+      type = types.functionTo types.attrs;
+      default = pkgs: { };
+      description = ''
+        A function to replace or add packages in `pkgs` expects an attrset to be returned when called.
+      '';
+    };
+
+    perlPackageOverrides = mkOption {
+      type = types.functionTo types.attrs;
+      default = pkgs: { };
+      description = ''
+        The same as `packageOverrides` but for packages in the perl package set.
+      '';
+    };
+
     problems = (import ../stdenv/generic/problems.nix { inherit lib; }).configOptions;
   };
 
@@ -494,6 +515,7 @@ in
   inherit options;
 
   config = {
+    _module.args.docPrefix = lib.mkDefault "";
     warnings =
       optionals config.warnUndeclaredOptions (
         mapAttrsToList (k: v: "undeclared Nixpkgs option set: config.${k}") config._undeclared or { }

pkgs/top-level/default.nix

@@ -64,6 +64,11 @@ in
   # list it returns.
   stdenvStages ? import ../stdenv,
 
+  # Temporary parameter to unify nixpkgs/pkgs evaluation
+  # Internal, do not use this manually!
+  # Will be removed again within the next releases
+  _configDefinitions ? null,
+
   # Ignore unexpected args.
   ...
 }@args:
@@ -109,7 +114,13 @@ let
         then
           x86_64DarwinDeprecationWarning
         else
-          x: x
+          x:
+          x throwIfNot (lib.all lib.isFunction crossOverlays)
+            "All crossOverlays passed to nixpkgs must be functions."
+      )
+      (
+        throwIfNot (_configDefinitions == null || config0 == { })
+          "The `_configDefinitions` argument is an internal interface and must not be combined with `config`."
       );
 
   localSystem = lib.systems.elaborate args.localSystem;
@@ -134,20 +145,24 @@ let
 
   # Allow both:
   # { /* the config */ } and
-  # { pkgs, ... } : { /* the config */ }
+  # { lib, pkgs, ... } : { /* the config */ }
   config1 = if lib.isFunction config0 then config0 { inherit lib pkgs; } else config0;
 
   configEval = lib.evalModules {
     modules = [
       ./config.nix
-      (
-        { options, ... }:
-        {
-          _file = "nixpkgs.config";
-          config = config1;
-        }
-      )
-    ];
+    ]
+    ++ (
+      if _configDefinitions != null then
+        map (def: lib.modules.setDefaultModuleLocation def.file def.value) _configDefinitions
+      else
+        [
+          {
+            _file = "nixpkgs.config";
+            config = config1;
+          }
+        ]
+    );
     class = "nixpkgsConfig";
   };