From 31f4f25f6a001ec18eac9658471f78cb6f624c4a Mon Sep 17 00:00:00 2001
From: Jade Lovelace <lix@jade.fyi>
Date: Mon, 30 Sep 2024 13:35:21 -0700
Subject: [PATCH] Merge pull request #37 into main

Closes https://git.lix.systems/lix-project/nixos-module/pulls/37

(cherry picked from commit ca0cc16273f903340bfbefc876b5eb87a2acc715)
---
 overlay.nix | 42 ++++++++++++++++++++++++++++++++----------
 1 file changed, 32 insertions(+), 10 deletions(-)

diff --git a/overlay.nix b/overlay.nix
index 52d3143..821cd33 100644
--- a/overlay.nix
+++ b/overlay.nix
@@ -1,15 +1,12 @@
 { lix, versionSuffix ? "" }:
 final: prev:
 let
-  # This is kind of scary to not override the nix version to pretend to be
-  # 2.18 since nixpkgs can introduce new breakage in its Nix unstable CLI
-  # usage.
-  # https://github.com/nixos/nixpkgs/blob/6afb255d976f85f3359e4929abd6f5149c323a02/nixos/modules/config/nix.nix#L121
   lixPackageFromSource = final.callPackage (lix + "/package.nix") ({
     inherit versionSuffix;
   });
 
-  # These packages depend on Nix features that Lix does not support
+  # These packages should receive CppNix since they may link to it or otherwise
+  # cause problems (or even just silly mass-rebuilds) if we give them Lix
   overridelist_upstream = [
     "attic-client"
     "devenv"
@@ -59,12 +56,37 @@ 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 = 
+  # It is not enough to *just* throw whatever the default nix version is at
+  # anything in the "don't give lix" list, we have to *also* ensure that we
+  # give whatever upstream version as specified in the callPackage invocation.
+  #
+  # Unfortunately I don't think there is any actual way to directly query that,
+  # so we instead do something extremely evil and guess which version it
+  # probably was. This code is not generalizable to arbitrary derivations, so
+  # it will hopefully not make us cry, at least.
+  useCppNixOverlay =
     lib.genAttrs overridelist_upstream (
-      name: if (lib.functionArgs prev.${name}.override ? "nix") then prev.${name}.override {
-        nix = final.nixVersions.stable_upstream;
-      } else prev.${name}
-    ) // {
+      name:
+      if (lib.functionArgs prev.${name}.override ? "nix") then
+        let
+          # Get the two common inputs of a derivation/package.
+          inputs = prev.${name}.buildInputs ++ prev.${name}.nativeBuildInputs;
+          nixDependency = lib.findFirst
+            (drv: (drv.pname or "") == "nix")
+            prev.nixVersions.stable_upstream # default to stable nix if nix is not an input
+            inputs;
+          nixMajor = lib.versions.major (nixDependency.version or "");
+          nixMinor = lib.versions.minor (nixDependency.version or "");
+          nixAttr = "nix_${nixMajor}_${nixMinor}";
+          finalNix = final.nixVersions.${nixAttr};
+        in
+        prev.${name}.override {
+          nix = finalNix;
+        }
+      else prev.${name}
+    );
+
+  overlay = useCppNixOverlay // {
     lix-overlay-present = 1;
     # used for things that one wouldn't necessarily want to update, but we
     # nevertheless shove it in the overlay and fixed-point it in case one *does*