Elaborate localSystem and crossSystem in a consistent manner

lib/systems/default.nix

@@ -2,4 +2,22 @@ rec {
   doubles = import ./doubles.nix;
   parse = import ./parse.nix;
   platforms = import ./platforms.nix;
+
+  # Elaborate a `localSystem` or `crossSystem` so that it contains everything
+  # necessary.
+  #
+  # `parsed` is inferred from args, both because there are two options with one
+  # clearly prefered, and to prevent cycles. A simpler fixed point where the RHS
+  # always just used `final.*` would fail on both counts.
+  elaborate = args: let
+    final = {
+      # Prefer to parse `config` as it is strictly more informative.
+      parsed = parse.mkSystemFromString (if args ? config then args.config else args.system);
+      # Either of these can be losslessly-extracted from `parsed` iff parsing succeeds.
+      system = parse.doubleFromSystem final.parsed;
+      config = parse.tripleFromSystem final.parsed;
+      # Just a guess, based on `system`
+      platform = platforms.selectBySystem final.system;
+    } // args;
+  in final;
 }

lib/systems/platforms.nix

@@ -474,7 +474,7 @@ rec {
     };
   };
 
-  selectPlatformBySystem = system: {
+  selectBySystem = system: {
       "i686-linux" = pc32;
       "x86_64-linux" = pc64;
       "armv5tel-linux" = sheevaplug;

pkgs/top-level/default.nix

@@ -40,6 +40,7 @@
 
 let # Rename the function arguments
   configExpr = config;
+  crossSystem0 = crossSystem;
 
 in let
   lib = import ../../lib;
@@ -52,12 +53,15 @@ in let
     then configExpr { inherit pkgs; }
     else configExpr;
 
-  # Allow setting the platform in the config file. Otherwise, let's use a
-  # reasonable default.
-  localSystem =
-    { platform = lib.systems.platforms.selectPlatformBySystem args.localSystem.system; }
-    // builtins.intersectAttrs { platform = null; } config
-    // args.localSystem;
+  # From a minimum of `system` or `config` (actually a target triple, *not*
+  # nixpkgs configuration), infer the other one and platform as needed.
+  localSystem = lib.systems.elaborate (
+    # Allow setting the platform in the config file. This take precedence over
+    # the inferred platform, but not over an explicitly passed-in onw.
+    builtins.intersectAttrs { platform = null; } config
+    // args.localSystem);
+
+  crossSystem = lib.mapNullable lib.systems.elaborate crossSystem0;
 
   # A few packages make a new package set to draw their dependencies from.
   # (Currently to get a cross tool chain, or forced-i686 package.) Rather than