kernel: Make lazier (fix infinite recursion)

pkgs/os-specific/linux/kernel/generic.nix

···
   }; # end of configfile derivation
 
   kernel = (callPackage ./manual-config.nix { inherit lib stdenv buildPackages; }) (basicArgs // {
-    inherit kernelPatches randstructSeed extraMakeFlags extraMeta configfile;
+    inherit kernelPatches randstructSeed extraMakeFlags extraMeta configfile modDirVersion;
     pos = builtins.unsafeGetAttrPos "version" args;
 
-    config = { CONFIG_MODULES = "y"; CONFIG_FW_LOADER = "m"; } // lib.optionalAttrs withRust { CONFIG_RUST = "y"; };
-  } // lib.optionalAttrs (modDirVersion != null) { inherit modDirVersion; });
+    config = {
+      CONFIG_MODULES = "y";
+      CONFIG_FW_LOADER = "m";
+      CONFIG_RUST = lib.mkIf withRust "y";
+    };
+  });
 
 in
 kernel.overrideAttrs (finalAttrs: previousAttrs: {

pkgs/os-specific/linux/kernel/manual-config.nix

···
   extraMakeFlags ? [],
   # The name of the kernel module directory
   # Needs to be X.Y.Z[-extra], so pad with zeros if needed.
-  modDirVersion ? lib.versions.pad 3 version,
+  modDirVersion ? null /* derive from version */,
   # The kernel source (tarball, git checkout, etc.)
   src,
   # a list of { name=..., patch=..., extraConfig=...} patches
···
 }:
 
 let
+  # Provide defaults. Note that we support `null` so that callers don't need to use optionalAttrs,
+  # which can lead to unnecessary strictness and infinite recursions.
+  modDirVersion_ = if modDirVersion == null then lib.versions.pad 3 version else modDirVersion;
+in
+let
+  # Shadow the un-defaulted parameter; don't want null.
+  modDirVersion = modDirVersion_;
   inherit (lib)
     hasAttr getAttr optional optionals optionalString optionalAttrs maintainers platforms;