Merge pull request #292487 from jmbaur/nixos-cross-check

nixos/nixpkgs: fix determination for cross-compiled nixos system

nixos/modules/misc/nixpkgs.nix

@@ -208,7 +208,11 @@ in
       example = { system = "x86_64-linux"; };
       # Make sure that the final value has all fields for sake of other modules
       # referring to this.
-      apply = lib.systems.elaborate;
+      apply = inputBuildPlatform:
+        let elaborated = lib.systems.elaborate inputBuildPlatform;
+        in if lib.systems.equals elaborated cfg.hostPlatform
+          then cfg.hostPlatform  # make identical, so that `==` equality works; see https://github.com/NixOS/nixpkgs/issues/278001
+          else elaborated;
       defaultText = literalExpression
         ''config.nixpkgs.hostPlatform'';
       description = lib.mdDoc ''

nixos/modules/misc/nixpkgs/test.nix

@@ -12,6 +12,10 @@ let
     nixpkgs.hostPlatform = "aarch64-linux";
     nixpkgs.buildPlatform = "aarch64-darwin";
   };
+  withSameHostAndBuild = eval {
+    nixpkgs.hostPlatform = "aarch64-linux";
+    nixpkgs.buildPlatform = "aarch64-linux";
+  };
   ambiguous = {
     _file = "ambiguous.nix";
     nixpkgs.hostPlatform = "aarch64-linux";
@@ -81,6 +85,8 @@ lib.recurseIntoAttrs {
     assert withHost._module.args.pkgs.stdenv.buildPlatform.system == "aarch64-linux";
     assert withHostAndBuild._module.args.pkgs.stdenv.hostPlatform.system == "aarch64-linux";
     assert withHostAndBuild._module.args.pkgs.stdenv.buildPlatform.system == "aarch64-darwin";
+    assert withSameHostAndBuild.config.nixpkgs.buildPlatform == withSameHostAndBuild.config.nixpkgs.hostPlatform;
+    assert withSameHostAndBuild._module.args.pkgs.stdenv.buildPlatform == withSameHostAndBuild._module.args.pkgs.stdenv.hostPlatform;
     assert builtins.trace (lib.head (getErrors ambiguous))
       getErrors ambiguous ==
         [''