nixpkgs module: Clean up platform options

- `localSystem` is added, it strictly supercedes system

 - `crossSystem`'s description mentions `localSystem` (and vice versa).

 - No more weird special casing I don't even understand

TEMP

lib/systems/parse.nix

@@ -201,7 +201,7 @@ rec {
 
   ################################################################################
 
-  types.system = mkOptionType {
+  types.parsedPlatform = mkOptionType {
     name = "system";
     description = "fully parsed representation of llvm- or nix-style platform tuple";
     merge = mergeOneOption;
@@ -215,7 +215,7 @@ rec {
   isSystem = isType "system";
 
   mkSystem = components:
-    assert types.system.check components;
+    assert types.parsedPlatform.check components;
     setType "system" components;
 
   mkSkeletonFromList = l: {

nixos/doc/manual/man-nixos-build-vms.xml

@@ -40,7 +40,7 @@ points to the generated virtual network.
   test1 = {pkgs, config, ...}:
     {
       services.openssh.enable = true;
-      nixpkgs.system = "i686-linux";
+      nixpkgs.localSystem.system = "i686-linux";
       deployment.targetHost = "test1.example.net";
 
       # Other NixOS options
@@ -51,7 +51,7 @@ points to the generated virtual network.
       services.openssh.enable = true;
       services.httpd.enable = true;
       environment.systemPackages = [ pkgs.lynx ];
-      nixpkgs.system = "x86_64-linux";
+      nixpkgs.localSystem.system = "x86_64-linux";
       deployment.targetHost = "test2.example.net";
 
       # Other NixOS options
@@ -66,7 +66,7 @@ In each NixOS configuration, two attributes have a special meaning.
 The <varname>deployment.targetHost</varname> specifies the address
 (domain name or IP address)
 of the system which is used by <command>ssh</command> to perform
-remote deployment operations. The <varname>nixpkgs.system</varname>
+remote deployment operations. The <varname>nixpkgs.localSystem.system</varname>
 attribute can be used to specify an architecture for the target machine,
 such as <varname>i686-linux</varname> which builds a 32-bit NixOS
 configuration. Omitting this property will build the configuration

nixos/lib/eval-config.nix

@@ -26,7 +26,7 @@
 , lib ? import ../../lib
 }:
 
-let extraArgs_ = extraArgs; pkgs_ = pkgs; system_ = system;
+let extraArgs_ = extraArgs; pkgs_ = pkgs;
     extraModules = let e = builtins.getEnv "NIXOS_EXTRA_MODULE_PATH";
                    in if e == "" then [] else [(import (builtins.toPath e))];
 in
@@ -36,7 +36,7 @@ let
     _file = ./eval-config.nix;
     key = _file;
     config = {
-      nixpkgs.system = lib.mkDefault system_;
+      nixpkgs.localSystem = lib.mkDefault { inherit system; };
       _module.args.pkgs = lib.mkIf (pkgs_ != null) (lib.mkForce pkgs_);
     };
   };

nixos/modules/misc/nixpkgs.nix

@@ -58,10 +58,13 @@ in
     pkgs = mkOption {
       defaultText = literalExample
         ''import "''${nixos}/.." {
-            inherit (config.nixpkgs) config overlays system;
+            inherit (config.nixpkgs) config overlays localSystem crossSystem;
           }
         '';
-      default = import ../../.. { inherit (cfg) config overlays system crossSystem; };
+      default = import ../../.. {
+        localSystem = { inherit (cfg) system; } // cfg.localSystem;
+        inherit (cfg) config overlays crossSystem;
+      };
       type = pkgsType;
       example = literalExample ''import <nixpkgs> {}'';
       description = ''
@@ -73,8 +76,9 @@ in
         relative to the location of this NixOS module, because
         NixOS and Nixpkgs are distributed together for consistency,
         so the <code>nixos</code> in the default value is in fact a
-        relative path. The <code>config</code>, <code>overlays</code>
-        and <code>system</code> come from this option's siblings.
+        relative path. The <code>config</code>, <code>overlays</code>,
+        <code>localSystem</code>, and <code>crossSystem</code> come
+        from this option's siblings.
 
         This option can be used by applications like NixOps to increase
         the performance of evaluation, or to create packages that depend
@@ -130,13 +134,40 @@ in
       '';
     };
 
+    localSystem = mkOption {
+      type = types.attrs; # TODO utilize lib.systems.parsedPlatform
+      default = { system = builtins.currentSystem; };
+      example = { system = "aarch64-linux"; config = "aarch64-unknown-linux-gnu"; };
+      defaultText = literalExample
+        ''(import "''${nixos}/../lib").lib.systems.examples.aarch64-multiplatform'';
+      description = ''
+        Specifies the platform on which NixOS should be built. When
+        <code>nixpkgs.crossSystem</code> is unset, it also specifies
+        the platform <emphasis>for</emphasis> which NixOS should be
+        built.  If this option is unset, it defaults to the platform
+        type of the machine where evaluation happens. Specifying this
+        option is useful when doing distributed multi-platform
+        deployment, or when building virtual machines. See its
+        description in the Nixpkgs manual for more details.
+
+        Ignored when <code>nixpkgs.pkgs</code> is set.
+      '';
+    };
+
     crossSystem = mkOption {
-      type = types.nullOr types.attrs;
+      type = types.nullOr types.attrs; # TODO utilize lib.systems.parsedPlatform
       default = null;
+      example = { system = "aarch64-linux"; config = "aarch64-unknown-linux-gnu"; };
+      defaultText = literalExample
+        ''(import "''${nixos}/../lib").lib.systems.examples.aarch64-multiplatform'';
       description = ''
-        The description of the system we're cross-compiling to, or null
-        if this isn't a cross-compile. See the description of the
-        crossSystem argument in the nixpkgs manual.
+        Specifies the platform for which NixOS should be
+        built. Specify this only if it is different from
+        <code>nixpkgs.localSystem</code>, the platform
+        <emphasis>on</emphasis> which NixOS should be built. In other
+        words, specify this to cross-compile NixOS. Otherwise it
+        should be set as null, the default. See its description in the
+        Nixpkgs manual for more details.
 
         Ignored when <code>nixpkgs.pkgs</code> is set.
       '';
@@ -146,10 +177,20 @@ in
       type = types.str;
       example = "i686-linux";
       description = ''
-        Specifies the Nix platform type for which NixOS should be built.
-        If unset, it defaults to the platform type of your host system.
-        Specifying this option is useful when doing distributed
-        multi-platform deployment, or when building virtual machines.
+        Specifies the Nix platform type on which NixOS should be built.
+        It is better to specify <code>nixpkgs.localSystem</code> instead.
+        <programlisting>
+        {
+          nixpkgs.system = ..;
+        }
+        </programlisting>
+        is the same as
+        <programlisting>
+        {
+          nixpkgs.localSystem.system = ..;
+        }
+        </programlisting>
+        See <code>nixpkgs.localSystem</code> for more information.
 
         Ignored when <code>nixpkgs.pkgs</code> is set.
       '';

nixos/modules/services/misc/dysnomia.nix

@@ -158,7 +158,7 @@ in
 
     services.dysnomia.properties = {
       hostname = config.networking.hostName;
-      system = if config.nixpkgs.system == "" then builtins.currentSystem else config.nixpkgs.system;
+      inherit (config.nixpkgs.localSystem) system;
 
       supportedTypes = (import "${pkgs.stdenv.mkDerivation {
         name = "supportedtypes";

nixos/modules/services/misc/nixos-manual.nix

@@ -23,7 +23,7 @@ let
     options =
       let
         scrubbedEval = evalModules {
-          modules = [ { nixpkgs.system = config.nixpkgs.system; } ] ++ baseModules;
+          modules = [ { nixpkgs.localSystem = config.nixpkgs.localSystem; } ] ++ baseModules;
           args = (config._module.args) // { modules = [ ]; };
           specialArgs = { pkgs = scrubDerivations "pkgs" pkgs; };
         };

nixos/modules/virtualisation/containers.nix

@@ -112,7 +112,7 @@ let
 
       # If the host is 64-bit and the container is 32-bit, add a
       # --personality flag.
-      ${optionalString (config.nixpkgs.system == "x86_64-linux") ''
+      ${optionalString (config.nixpkgs.localSystem.system == "x86_64-linux") ''
         if [ "$(< ''${SYSTEM_PATH:-/nix/var/nix/profiles/per-container/$INSTANCE/system}/system)" = i686-linux ]; then
           extraFlags+=" --personality=x86"
         fi
@@ -255,7 +255,7 @@ let
   };
 
 
-  system = config.nixpkgs.system;
+  system = config.nixpkgs.localSystem.system;
 
   bindMountOpts = { name, config, ... }: {
 

pkgs/stdenv/generic/check-meta.nix

@@ -144,7 +144,7 @@ let
     license = either (listOf lib.types.attrs) (either lib.types.attrs str);
     maintainers = listOf (attrsOf str);
     priority = int;
-    platforms = listOf (either str lib.systems.parsed.types.system);
+    platforms = listOf (either str lib.systems.parsedPlatform.types.system);
     hydraPlatforms = listOf str;
     broken = bool;