From 830dd996ea485f4e725b429a009fbced50caddec Mon Sep 17 00:00:00 2001
From: Eman Resu <78693624+quatquatt@users.noreply.github.com>
Date: Wed, 3 Jun 2026 13:49:00 -0400
Subject: [PATCH] lib.modules: avoid filtering option decls in most cases

---
 lib/modules.nix | 37 ++++++++++++++++++++++++++++++++-----
 1 file changed, 32 insertions(+), 5 deletions(-)

diff --git a/lib/modules.nix b/lib/modules.nix
index 8b26f2c462e8..a42690b9a0c1 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -881,13 +881,12 @@ let
           loc = prefix ++ [ name ];
           defns = pushedDownDefinitionsByName.${name} or [ ];
           defns' = rawDefinitionsByName.${name} or [ ];
-          optionDecls = filter (
+          isOptionDecl =
             m:
             m.options ? _type
-            && (m.options._type == "option" || throwDeclarationTypeError loc m.options._type m._file)
-          ) decls;
+            && (m.options._type == "option" || throwDeclarationTypeError loc m.options._type m._file);
         in
-        if length optionDecls == length decls then
+        if all isOptionDecl decls then
           let
             opt = fixupOptionType loc (mergeOptionDecls loc decls);
           in
@@ -895,7 +894,35 @@ let
             matchedOptions = evalOptionValue loc opt defns';
             unmatchedDefns = [ ];
           }
-        else if optionDecls != [ ] then
+        # this may look like duplicate computations are performed, but testing
+        # with a minimal NixOS config (defined below):
+        #
+        # 1. in 87% of cases, the above `all isOptionDecl decls` passes and we
+        # short-circuit
+        # 2. In 12% of cases, there's only one decl and we short-circuit
+        # 3. in another 1% of cases, we loop again and short-circuit when
+        # `any isOptionDecl decls` fails
+        # 4. In the final case, we pass the `any` and have to refilter. This
+        # only triggers in 1 of the 48000 calls to this function.
+        #
+        # The minimal config was defined with this expression:
+        #
+        # let nixos = import ./nixos/lib/eval-config.nix {
+        #   modules = [
+        #     ./nixos/modules/profiles/minimal.nix
+        #     {
+        #       fileSystems."/" = {
+        #         device = "/dev/sda1";
+        #         fsType = "ext4";
+        #       };
+        #       boot.loader.grub.devices = [ "/dev/sda" ];
+        #     }
+        #   ];
+        # }; in nixos.config.system.build.toplevel
+        else if length decls != 1 && any isOptionDecl decls then
+          let
+            optionDecls = filter isOptionDecl decls;
+          in
           if
             all (x: x.options.type.name or null == "submodule") optionDecls
           # Raw options can only be merged into submodules. Merging into