nixpkgs: pass lib to config function (#457120)

doc/release-notes/rl-2605.section.md

@@ -9,6 +9,32 @@
 - Node.js default version has been updated from 22 LTS to 24 LTS.
   This introduces some breaking changes; Refer to the [upstream migration article](https://nodejs.org/en/blog/migrations/v22-to-v24) for details.
 
+- Nixpkgs configuration, specified for NixOS using `nixpkgs.config`, or using the `config` argument when importing nixpkgs, has learned to accept a `lib` argument as well as `pkgs`, which allows the configuration to be computed without depending on the `pkgs` fixed point.  If you are referencing licenses in `lib.licenses` by having this configuration be a function taking a `pkgs` arg, you may wish to change to using `lib` for faster computation and to avoid infinite recursion errors if pkgs depends on parts of the computed configuration in future.
+
+    For example, if you currently have configuration that looks like this:
+
+        { pkgs, ... }:
+        {
+          allowlistedLicenses = [ pkgs.lib.licenses.nasa13 ];
+          blocklistedLicenses = with pkgs.lib.licenses; [ gpl3Only gpl3Plus ];
+        }
+
+    You may wish to update it to something like this:
+
+        { lib, ... }:
+        {
+          allowlistedLicenses = [ lib.licenses.nasa13 ];
+          blocklistedLicenses = with lib.licenses; [ gpl3Only gpl3Plus ];
+        }
+
+    Or, if you need configuration that works with both 26.05 and 25.11:
+
+        { pkgs, lib ? pkgs.lib, ... }:
+        {
+          allowlistedLicenses = [ lib.licenses.nasa13 ];
+          blocklistedLicenses = with lib.licenses; [ gpl3Only gpl3Plus ];
+        }
+
 ## Backward Incompatibilities {#sec-nixpkgs-release-26.05-incompatibilities}
 
 <!-- To avoid merge conflicts, consider adding your item at an arbitrary place in the list instead. -->

nixos/modules/misc/nixpkgs.nix

@@ -16,8 +16,8 @@ let
   mergeConfig =
     lhs_: rhs_:
     let
-      lhs = optCall lhs_ { inherit pkgs; };
-      rhs = optCall rhs_ { inherit pkgs; };
+      lhs = optCall lhs_ { inherit lib pkgs; };
+      rhs = optCall rhs_ { inherit lib pkgs; };
     in
     lib.recursiveUpdate lhs rhs
     // lib.optionalAttrs (lhs ? packageOverrides) {

pkgs/top-level/default.nix

@@ -109,7 +109,7 @@ let
   # Allow both:
   # { /* the config */ } and
   # { pkgs, ... } : { /* the config */ }
-  config1 = if lib.isFunction config0 then config0 { inherit pkgs; } else config0;
+  config1 = if lib.isFunction config0 then config0 { inherit lib pkgs; } else config0;
 
   configEval = lib.evalModules {
     modules = [