pkgs/top-level: propagate crossSystem0 via nixpkgsFun

This makes combinations work, where pkgsStatic is used on another
package set, for example:

- Keeps pkgsCross.mingwW64.pkgsStatic.stdenv.hostPlatform.config at
"x86_64-w64-mingw32"
- Keeps pkgsLLVM.pkgsStatic.stdenv.hostPlatform.useLLVM at "true"

Makes pkgsCross.mingwW64.pkgsStatic.stdenv build. Not sure whether it
actually builds static executables already, though.

Resolves NixOS#281596

pkgs/test/top-level/default.nix

@@ -118,8 +118,8 @@ lib.recurseIntoAttrs {
       # assert isComposable "pkgsZig";
       # TODO: attribute 'abi' missing
       # assert isComposable "pkgsMusl";
+      assert isComposable "pkgsStatic";
       # TODO: fails
-      # assert isComposable "pkgsStatic";
       # assert isComposable "pkgsi686Linux";
 
       # Special cases regarding buildPlatform vs hostPlatform

pkgs/top-level/default.nix

@@ -44,7 +44,7 @@
 
 let # Rename the function arguments
   config0 = config;
-  crossSystem0 = crossSystem;
+  crossSystem0 = if crossSystem == null then localSystem else crossSystem;
 
 in let
   lib = import ../../lib;
@@ -74,7 +74,7 @@ in let
   # inferred from the system double in `localSystem`.
   crossSystem =
     let system = lib.systems.elaborate crossSystem0; in
-    if crossSystem0 == null || lib.systems.equals system localSystem
+    if lib.systems.equals system localSystem
     then localSystem
     else system;
 
@@ -125,7 +125,7 @@ in let
     args // lib.optionalAttrs (newArgs ? "localSystem") {
       localSystem = newArgs.localSystem;
     } // lib.optionalAttrs (newArgs ? "crossSystem") {
-      crossSystem = newArgs.crossSystem;
+      crossSystem = lib.systems.asAttrs crossSystem0 // newArgs.crossSystem;
     } // lib.optionalAttrs (newArgs ? "overlays") {
       overlays = overlays ++ newArgs.overlays;
     });

pkgs/top-level/stage.nix

@@ -219,7 +219,7 @@ let
       # Bootstrap a cross stdenv using the LLVM toolchain.
       # This is currently not possible when compiling natively,
       # so we don't need to check hostPlatform != buildPlatform.
-      crossSystem = stdenv.targetPlatform // {
+      crossSystem = {
         useLLVM = true;
         linker = "lld";
       };
@@ -229,7 +229,7 @@ let
       # Bootstrap a cross stdenv using the Aro C compiler.
       # This is currently not possible when compiling natively,
       # so we don't need to check hostPlatform != buildPlatform.
-      crossSystem = stdenv.hostPlatform // {
+      crossSystem = {
         useArocc = true;
         linker = "lld";
       };
@@ -239,7 +239,7 @@ let
       # Bootstrap a cross stdenv using the Zig toolchain.
       # This is currently not possible when compiling natively,
       # so we don't need to check hostPlatform != buildPlatform.
-      crossSystem = stdenv.hostPlatform // {
+      crossSystem = {
         useZig = true;
         linker = "lld";
       };
@@ -290,13 +290,10 @@ let
     pkgsStatic = createPackageSet "pkgsStatic" ({
       crossSystem = {
         isStatic = true;
-        config = lib.systems.parse.tripleFromSystem (
-          if stdenv.hostPlatform.isLinux
-          then makeMuslParsedPlatform stdenv.hostPlatform.parsed
-          else stdenv.hostPlatform.parsed
-        );
-        gcc = lib.optionalAttrs (stdenv.hostPlatform.system == "powerpc64-linux") { abi = "elfv2"; } //
-          stdenv.hostPlatform.gcc or {};
+      } // lib.optionalAttrs stdenv.hostPlatform.isLinux {
+        config = lib.systems.parse.tripleFromSystem (makeMuslParsedPlatform stdenv.hostPlatform.parsed);
+      } // lib.optionalAttrs (stdenv.hostPlatform.system == "powerpc64-linux") {
+        gcc = { abi = "elfv2"; } // stdenv.hostPlatform.gcc or {};
       };
     });