Add moduleLocation to mkFlake argument

dev/tests/eval-tests.nix

@@ -26,6 +26,22 @@ rec {
       systems = [ ];
     };
 
+  emptyExposeArgs = mkFlake
+    { inputs.self = { outPath = "the self outpath"; }; }
+    ({ config, moduleLocation, ... }: {
+      flake = {
+        inherit moduleLocation;
+      };
+    });
+
+  emptyExposeArgsNoSelf = mkFlake
+    { inputs.self = throw "self won't be available in case of some errors"; }
+    ({ config, moduleLocation, ... }: {
+      flake = {
+        inherit moduleLocation;
+      };
+    });
+
   example1 = mkFlake
     { inputs.self = { }; }
     {
@@ -162,6 +178,8 @@ rec {
 
     assert packagesNonStrictInDevShells.packages.a.default == pkg "a" "hello";
 
+    assert emptyExposeArgs.moduleLocation == "the self outpath/flake.nix";
+
     ok;
 
   result = runTests "ok";

extras/flakeModules.nix

@@ -1,4 +1,4 @@
-{ self, lib, flake-parts-lib, ... }:
+{ self, lib, flake-parts-lib, moduleLocation, ... }:
 let
   inherit (lib)
     mapAttrs
@@ -13,8 +13,8 @@ let
     type = types.lazyAttrsOf types.deferredModule;
     default = { };
     apply = mapAttrs (k: v: {
-      _file = "${toString self.outPath}/flake.nix#flakeModules.${k}";
-      key = "${toString self.outPath}/flake.nix#flakeModules.${k}";
+      _file = "${toString moduleLocation}#flakeModules.${k}";
+      key = "${toString moduleLocation}#flakeModules.${k}";
       imports = [ v ];
     });
     description = ''

lib.nix

@@ -81,7 +81,24 @@ let
 
           ${errorExample}
         '')
+      , moduleLocation ? "${self.outPath}/flake.nix"
       }:
+      let
+        module = lib.setDefaultModuleLocation errorLocation module;
+        inputsPos = builtins.unsafeGetAttrPos "inputs" args;
+        errorLocation =
+          # Best case: user makes it explicit
+          args.moduleLocation or (
+            # Slightly worse: Nix does not technically commit to unsafeGetAttrPos semantics
+            if inputsPos != null
+            then inputsPos.file
+            # Slightly worse: self may not be valid when an error occurs
+            else if args?inputs.self.outPath
+            then args.inputs.self.outPath + "/flake.nix"
+            # Fallback
+            else "<mkFlake argument>"
+          );
+      in
       throwIf
         (!args?self && !args?inputs) ''
         When invoking flake-parts, you must pass in the flake output arguments.
@@ -100,7 +117,7 @@ let
         (module:
         lib.evalModules {
           specialArgs = {
-            inherit self flake-parts-lib;
+            inherit self flake-parts-lib moduleLocation;
             inputs = args.inputs or /* legacy, warned above */ self.inputs;
           } // specialArgs;
           modules = [ ./all-modules.nix module ];
@@ -120,12 +137,7 @@ let
 
     mkFlake = args: module:
       let
-        loc =
-          if args?inputs.self.outPath
-          then args.inputs.self.outPath + "/flake.nix"
-          else "<mkFlake argument>";
-        mod = lib.setDefaultModuleLocation loc module;
-        eval = flake-parts-lib.evalFlakeModule args mod;
+        eval = flake-parts-lib.evalFlakeModule args module;
       in
       eval.config.flake;
 

modules/nixosModules.nix

@@ -1,4 +1,4 @@
-{ self, lib, flake-parts-lib, ... }:
+{ self, lib, flake-parts-lib, moduleLocation, ... }:
 let
   inherit (lib)
     mapAttrs
@@ -15,7 +15,7 @@ in
       nixosModules = mkOption {
         type = types.lazyAttrsOf types.unspecified;
         default = { };
-        apply = mapAttrs (k: v: { _file = "${toString self.outPath}/flake.nix#nixosModules.${k}"; imports = [ v ]; });
+        apply = mapAttrs (k: v: { _file = "${toString moduleLocation}#nixosModules.${k}"; imports = [ v ]; });
         description = ''
           NixOS modules.