overlay: make finalNix depend on the nix version being used by a package

This should address both of the following (were 35 is more general and
36 a specific case)

- lix-project/nixos-module#35
- lix-project/nixos-module#36

flake.lock

@@ -5,11 +5,11 @@
         "systems": "systems"
       },
       "locked": {
-        "lastModified": 1710146030,
+        "lastModified": 1726560853,
-        "narHash": "sha256-SZ5L6eA7HJ/nmkzGG7/ISclqe6oZdOZTNoesiInkXPQ=",
+        "narHash": "sha256-X6rJYSESBVr3hBoH0WbKE5KvhPU5bloyZ2L4K60/fPQ=",
         "owner": "numtide",
         "repo": "flake-utils",
-        "rev": "b1d9ab70662946ef0850d488da1c9019f3a9752a",
+        "rev": "c1dfcf08411b08f6b8615f7d8971a2bfa81d5e8a",
         "type": "github"
       },
       "original": {
@@ -36,11 +36,11 @@
     "lix": {
       "flake": false,
       "locked": {
-        "lastModified": 1723511168,
+        "lastModified": 1726590994,
-        "narHash": "sha256-XDcqLVPcsMhORerIPuQ1XNALtDvG6QRA2dKyNrccXyg=",
+        "narHash": "sha256-CrvIEzBzvvfE7jGIXBv6hSYDxv4eYeHWAwVho5WrF48=",
-        "rev": "f9a3bf6ccccf8ac6b1604c1a2980e3a565ae4f44",
+        "rev": "8ab5743904a06c78153281bf61b3aa8aa451a489",
         "type": "tarball",
-        "url": "https://git.lix.systems/api/v1/repos/lix-project/lix/archive/f9a3bf6ccccf8ac6b1604c1a2980e3a565ae4f44.tar.gz?rev=f9a3bf6ccccf8ac6b1604c1a2980e3a565ae4f44"
+        "url": "https://git.lix.systems/api/v1/repos/lix-project/lix/archive/8ab5743904a06c78153281bf61b3aa8aa451a489.tar.gz?rev=8ab5743904a06c78153281bf61b3aa8aa451a489"
       },
       "original": {
         "type": "tarball",
@@ -49,11 +49,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1723175592,
+        "lastModified": 1726463316,
-        "narHash": "sha256-M0xJ3FbDUc4fRZ84dPGx5VvgFsOzds77KiBMW/mMTnI=",
+        "narHash": "sha256-gI9kkaH0ZjakJOKrdjaI/VbaMEo9qBbSUl93DnU7f4c=",
         "owner": "nixos",
         "repo": "nixpkgs",
-        "rev": "5e0ca22929f3342b19569b21b2f3462f053e497b",
+        "rev": "99dc8785f6a0adac95f5e2ab05cc2e1bf666d172",
         "type": "github"
       },
       "original": {

overlay.nix

@@ -11,7 +11,7 @@ let
   });
 
   # These packages depend on Nix features that Lix does not support
-  overridelist_2_18 = [
+  overridelist_upstream = [
     "attic-client"
     "devenv"
     "nix-du"
@@ -60,11 +60,25 @@ let
   # errors. This is a simple safeguard to put in at least something that might be seen.
   maybeWarnWrongMajor = x: if !(lib.hasPrefix supportedLixMajor lixPackageToUse.version) then builtins.trace wrongMajorWarning x else x;
 
-  overlay = 
+  overlay =
-    lib.genAttrs overridelist_2_18 (
+    lib.genAttrs overridelist_upstream (
-      name: if (lib.functionArgs prev.${name}.override ? "nix") then prev.${name}.override {
+      name:
-        nix = final.nixVersions.nix_2_18_upstream;
+      if (lib.functionArgs prev.${name}.override ? "nix") then
-      } else prev.${name}
+        let
+          # Get the two common inputs of a derivation/package.
+          inputs = prev.${name}.buildInputs ++ prev.${name}.nativeBuildInputs;
+          nixDependency = lib.findFirst
+            (drv: drv.pname == "nix")
+            prev.nixVersions.stable_upstream # default to stable nix if nix is not an input
+            inputs;
+          nixMajor = lib.versions.major nixDependency.version;
+          nixMinor = lib.versions.minor nixDependency.version;
+          nixAttr = "nix_${nixMajor}_${nixMinor}";
+          finalNix = final.nixVersions.${nixAttr};
+        in
+        prev.${name}.override {
+          nix = finalNix;
+        } else prev.${name}
     ) // {
     lix-overlay-present = 1;
     # used for things that one wouldn't necessarily want to update, but we
@@ -74,15 +88,14 @@ let
 
     lix = maybeWarnWrongMajor (maybeWarnDuplicate lixPackageToUse);
 
-    nixVersions = prev.nixVersions // rec {
+    nixVersions = prev.nixVersions // {
-      nix_2_18 = final.lix;
+      stable = final.lix;
-      stable = nix_2_18;
+      stable_upstream = prev.nixVersions.stable;
-      nix_2_18_upstream = prev.nixVersions.nix_2_18;
     };
 
     nix-eval-jobs = (prev.nix-eval-jobs.override {
       # lix
-      nix = final.nixVersions.nix_2_18;
+      nix = final.lix;
     }).overrideAttrs (old:
       let src = final.lix-sources.nix-eval-jobs;
       in {