cross: fix enableParallelBuilding to work

flake.nix

@@ -94,17 +94,7 @@
       evalHost = { name, local, target }: nixpkgs.lib.nixosSystem {
         system = target;
         modules = [
-          {
-            nixpkgs = (if (local != null) then {
-              buildPlatform = local;
-            } else {}) // {
-              # TODO: does the earlier `system` arg to nixosSystem make its way here?
-              hostPlatform.system = target;
-            };
-            # nixpkgs.buildPlatform = local;  # set by instantiate.nix instead
-            # nixpkgs.config.replaceStdenv = { pkgs }: pkgs.ccacheStdenv;
-          }
-          (import ./hosts/instantiate.nix { hostName = name; })
+          (import ./hosts/instantiate.nix { localSystem = local; hostName = name; })
           self.nixosModules.default
           self.nixosModules.passthru
           {
@@ -113,6 +103,12 @@
               self.overlays.sane-all
             ];
           }
+          ({ lib, ... }: {
+            # TODO: does the earlier `system` arg to nixosSystem make its way here?
+            nixpkgs.hostPlatform.system = target;
+            # nixpkgs.buildPlatform = local;  # set by instantiate.nix instead
+            # nixpkgs.config.replaceStdenv = { pkgs }: pkgs.ccacheStdenv;
+          })
         ];
       };
     in {

hosts/instantiate.nix

@@ -1,10 +1,10 @@
 # trampoline from flake.nix into the specific host definition, while doing a tiny bit of common setup
 
 # args from flake-level `import`
-{ hostName }:
+{ hostName, localSystem }:
 
 # module args
-{ ... }:
+{ lib, ... }:
 
 {
   imports = [
@@ -14,4 +14,5 @@
   ];
 
   networking.hostName = hostName;
+  nixpkgs.buildPlatform = lib.mkIf (localSystem != null) localSystem;
 }

nixpatches/flake.nix

@@ -15,15 +15,9 @@
       patchedFlakeFor = system: import "${patchedPkgsFor system}/flake.nix";
       patchedFlakeOutputsFor = system:
         (patchedFlakeFor system).outputs { inherit self; };
-
-      extractBuildPlatform = nixosSystemArgs:
-        let
-          firstMod = builtins.head nixosSystemArgs.modules;
-        in
-          firstMod.nixpkgs.buildPlatform or nixosSystemArgs.system;
     in
     {
-      lib.nixosSystem = args: (patchedFlakeOutputsFor (extractBuildPlatform args)).lib.nixosSystem args;
+      lib.nixosSystem = args: (patchedFlakeOutputsFor args.system).lib.nixosSystem args;
 
       legacyPackages = builtins.mapAttrs
         (system: _:

overlays/cross.nix

@@ -89,7 +89,7 @@ let
     # so hack its components into something which *does* handle cross compilation
     lib.overrideDerivation binfmtDeriv ({ builder, args, ... }: {
       builder = "${final.buildPackages.bash}/bin/sh";
-      args = ["-e" (final.buildPackages.vmTools.vmRunCommand qemuCommandLinux)];
+      args = ["-e" (final.vmTools.vmRunCommand qemuCommandLinux)];
       # orig{Builder,Args} gets used by the vmRunCommand script:
       origBuilder = builder;
       origArgs = args;