nixos: move default module location logic to `eval-config.nix`

flake.nix

@@ -21,35 +21,13 @@
 
         nixos = import ./nixos/lib { lib = final; };
 
-        nixosSystem = { modules, ... } @ args:
+        nixosSystem = args:
           import ./nixos/lib/eval-config.nix (args // {
-            modules =
-              let
-                moduleDeclarationFile =
-                  let
-                    # Even though `modules` is a mandatory argument for `nixosSystem`, it doesn't
-                    # mean that the evaluator always keeps track of its position. If there
-                    # are too many levels of indirection, the position gets lost at some point.
-                    intermediatePos = builtins.unsafeGetAttrPos "modules" args;
-                  in
-                    if intermediatePos == null then null else intermediatePos.file;
-
-                # Add the invoking file as error message location for modules
-                # that don't have their own locations; presumably inline modules.
-                addModuleDeclarationFile =
-                  m: if moduleDeclarationFile == null then m else {
-                    _file = moduleDeclarationFile;
-                    imports = [ m ];
-                  };
-
-              in
-              map addModuleDeclarationFile modules ++ [
-                {
-                  system.nixos.versionSuffix =
-                    ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
-                  system.nixos.revision = final.mkIf (self ? rev) self.rev;
-                }
-              ];
+            modules = args.modules ++ [ {
+              system.nixos.versionSuffix =
+                ".${final.substring 0 8 (self.lastModifiedDate or self.lastModified or "19700101")}.${self.shortRev or "dirty"}";
+              system.nixos.revision = final.mkIf (self ? rev) self.rev;
+            } ];
           });
       });
 

nixos/lib/eval-config.nix

@@ -21,6 +21,7 @@ evalConfigArgs@
 , # !!! See comment about args in lib/modules.nix
   specialArgs ? {}
 , modules
+, modulesLocation ? (builtins.unsafeGetAttrPos "modules" evalConfigArgs).file or null
 , # !!! See comment about check in lib/modules.nix
   check ? true
 , prefix ? []
@@ -74,7 +75,18 @@ let
         _module.check = lib.mkDefault check;
       };
     };
-  allUserModules = modules ++ legacyModules;
+
+  allUserModules =
+    let
+      # Add the invoking file (or specified modulesLocation) as error message location
+      # for modules that don't have their own locations; presumably inline modules.
+      locatedModules =
+        if modulesLocation == null then
+          modules
+        else
+          map (lib.setDefaultModuleLocation modulesLocation) modules;
+    in
+      locatedModules ++ legacyModules;
 
   noUserModules = evalModulesMinimal ({
     inherit prefix specialArgs;