nixos/nixpkgs: make nixpkgs.config mergeable

lib/tests/modules.sh

@@ -348,6 +348,10 @@ checkConfigOutput 'ok' config.freeformItems.foo.bar ./adhoc-freeformType-survive
 # because of an `extendModules` bug, issue 168767.
 checkConfigOutput '^1$' config.sub.specialisation.value ./extendModules-168767-imports.nix
 
+checkConfigOutput '^24$' config.result.test ./override-fn-type.nix
+checkConfigOutput '^25$' config.result.test2 ./override-fn-type.nix
+checkConfigOutput '^1$' config.result.test3 ./override-fn-type.nix
+
 cat <<EOF
 ====== module tests ======
 $pass Pass

lib/tests/modules/override-fn-type.nix

@@ -0,0 +1,34 @@
+{ lib, config, ... }: with lib; {
+  options.packageOverrides = mkOption {
+    default = const {};
+    type = types.overrideFnType;
+  };
+  options.result = mkOption {
+    type = types.attrs;
+  };
+  config = mkMerge [
+    {
+      packageOverrides = pkgs: {
+        test = pkgs.num + 1;
+      };
+    }
+    {
+      packageOverrides = pkgs: {
+        test2 = pkgs.test + 1;
+      };
+    }
+    {
+      packageOverrides = mkAfter (pkgs: {
+        test3 = 1;
+      });
+    }
+    {
+      packageOverrides = pkgs: {
+        test3 = 2;
+      };
+    }
+    {
+      result = config.packageOverrides { num = 23; };
+    }
+  ];
+}

lib/types.nix

@@ -4,6 +4,7 @@
 
 let
   inherit (lib)
+    const
     elem
     flip
     isAttrs
@@ -884,6 +885,28 @@ rec {
     # Augment the given type with an additional type check function.
     addCheck = elemType: check: elemType // { check = x: elemType.check x && check x; };
 
+    # EXPERIMENTAL
+    # Known issue:
+    # Overrides composition does not compose symmetrically, exposing the arbitrary
+    # nature of module `imports` ordering in a potentially harmful manner.
+    # TODO
+    #  - Restrict override composition by `throw`-ing when any two layers contain the
+    #    same attribute name, which may create an `imports` ordering dependency.
+    #    (a layer is the result of applying one or more override functions)
+    #  - Apply this restriction only to unordered layers, which means layers that
+    #    have the same `mkOrder` number.
+    #  - Document this type in the NixOS manual
+    overrideFnType = mkOptionType {
+      name = "overrides";
+      check = builtins.isFunction;
+      merge = const
+        (foldl'
+          (final: override: pkgs:
+            let
+              final' = final pkgs;
+            in final' // (override.value (pkgs // final')))
+          (const {}));
+    };
   };
 };
 

nixos/modules/misc/nixpkgs.nix

@@ -14,34 +14,6 @@ let
     then f x
     else f;
 
-  mergeConfig = lhs_: rhs_:
-    let
-      lhs = optCall lhs_ { inherit pkgs; };
-      rhs = optCall rhs_ { inherit pkgs; };
-    in
-    recursiveUpdate lhs rhs //
-    optionalAttrs (lhs ? packageOverrides) {
-      packageOverrides = pkgs:
-        optCall lhs.packageOverrides pkgs //
-        optCall (attrByPath ["packageOverrides"] ({}) rhs) pkgs;
-    } //
-    optionalAttrs (lhs ? perlPackageOverrides) {
-      perlPackageOverrides = pkgs:
-        optCall lhs.perlPackageOverrides pkgs //
-        optCall (attrByPath ["perlPackageOverrides"] ({}) rhs) pkgs;
-    };
-
-  configType = 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: foldr (def: mergeConfig def.value) {};
-  };
-
   overlayType = mkOptionType {
     name = "nixpkgs-overlay";
     description = "nixpkgs overlay";
@@ -156,7 +128,7 @@ in
         ''
           { allowBroken = true; allowUnfree = true; }
         '';
-      type = configType;
+      type = types.submodule ../../../pkgs/top-level/config.nix;
       description = lib.mdDoc ''
         The configuration of the Nix Packages collection.  (For
         details, see the Nixpkgs documentation.)  It allows you to set

pkgs/top-level/config.nix

@@ -131,10 +131,26 @@ let
     checkMeta = mkOption {
       type = types.bool;
       default = false;
-      description = ''
+      description = lib.mdDoc ''
         Whether to check that the `meta` attribute of derivations are correct during evaluation time.
       '';
     };
+
+    packageOverrides = mkOption {
+      default = const {};
+      type = types.overrideFnType;
+      description = lib.mdDoc ''
+        **Not recommended.** Use `overlays` instead!
+      '';
+    };
+
+    perlPackageOverrides = mkOption {
+      default = const {};
+      type = types.overrideFnType;
+      description = lib.mdDoc ''
+        A function that takes the final set of Perl packages and returns an attribute set of packages to add or change.
+      '';
+    };
   };
 
 in {