overlays/selfExpr: allow providing options

pkgs/overlays/selfExpr.nix

@@ -1,21 +1,43 @@
 { nixpkgsPath ? <nixpkgs> }:
 
+let
+  defaultNixpkgsPath = nixpkgsPath;
+in
+
 self: super: {
   selfExpr = let
     config = builtins.removeAttrs self.config [ "_undeclared" ];
     configJson = builtins.toJSON config;
-  in
-    self.writeTextFile {
-    name = "nixpkgs-self";
-    destination = "/default.nix";
-    text = ''
+
+    getSelfExpr = {
+      useConfig ? true,
+      nixpkgsPath ? defaultNixpkgsPath,
+      ...
+    }: let
+      configText = ''
+          config = (builtins.fromJSON '''
+            ${configJson}
+          ''') // args.config or {};
+      '';
+
+      removedAttrNames = self.lib.optional useConfig "config";
+      removedAttrNamesText = builtins.toJSON removedAttrNames;
+    in ''
       { ... } @ args:
 
       import ${builtins.toString nixpkgsPath} {
-        config = (builtins.fromJSON '''
-          ${configJson}
-        ''') // args.config or {};
-      } // builtins.removeAttrs args [ "config" ]
+        ${self.lib.optionalString useConfig configText}
+      } // builtins.removeAttrs args (builtins.fromJSON '''
+        ${removedAttrNamesText}
+      ''')
     '';
-  };
+
+    mkNixpkgsChannel = args: self.writeTextFile {
+      name = args.name or "nixpkgs-self";
+      destination = "/default.nix";
+      text = getSelfExpr args;
+    } // {
+      __functor = _: args: mkNixpkgsChannel args;
+    };
+  in mkNixpkgsChannel {};
 }