packaging: Add source overriding "methods"

flake.nix

@@ -226,6 +226,27 @@
               LANG=C.UTF-8 ${pkgs.changelog-d}/bin/changelog-d ${./doc/manual/rl-next} >$out
             '';
           repl-completion = nixpkgsFor.${system}.native.callPackage ./tests/repl-completion.nix { };
+
+          /**
+            Checks for our packaging expressions.
+            This shouldn't build anything significant; just check that things
+            (including derivations) are _set up_ correctly.
+          */
+          packaging-overriding =
+            let
+              pkgs = nixpkgsFor.${system}.native;
+              nix = self.packages.${system}.nix;
+            in
+            assert (nix.appendPatches [ pkgs.emptyFile ]).libs.nix-util.src.patches == [ pkgs.emptyFile ];
+            # If this fails, something might be wrong with how we've wired the scope,
+            # or something could be broken in Nixpkgs.
+            pkgs.testers.testEqualContents {
+              assertion = "trivial patch does not change source contents";
+              expected = "${./.}";
+              actual =
+                # Same for all components; nix-util is an arbitrary pick
+                (nix.appendPatches [ pkgs.emptyFile ]).libs.nix-util.src;
+            };
         }
         // (lib.optionalAttrs (builtins.elem system linux64BitSystems)) {
           dockerImage = self.hydraJobs.dockerImage.${system};