lib/modules: Improve errors involving pushDownProperties (#502117)

lib/modules.nix

@@ -1322,13 +1322,24 @@ let
     : 1\. Function argument
   */
   pushDownProperties =
+    let
+      mapAttrsIfAttrs =
+        f: val:
+        if isAttrs val then
+          mapAttrs f val
+        else
+          # This does not actually work, since arriving here means we have e.g.
+          # (lib.mkIf cond nonAttrs), while an attrset is expected. However,
+          # avoiding the mapAttrs call here gives better errors later.
+          val;
+    in
     cfg:
     if cfg._type or "" == "merge" then
       concatMap pushDownProperties cfg.contents
     else if cfg._type or "" == "if" then
-      map (mapAttrs (n: v: mkIf cfg.condition v)) (pushDownProperties cfg.content)
+      map (mapAttrsIfAttrs (n: v: mkIf cfg.condition v)) (pushDownProperties cfg.content)
     else if cfg._type or "" == "override" then
-      map (mapAttrs (n: v: mkOverride cfg.priority v)) (pushDownProperties cfg.content)
+      map (mapAttrsIfAttrs (n: v: mkOverride cfg.priority v)) (pushDownProperties cfg.content)
     # FIXME: handle mkOrder?
     else
       [ cfg ];

lib/tests/modules.sh

@@ -171,6 +171,8 @@ checkConfigError() {
 # Shorthand meta attribute does not duplicate the config
 checkConfigOutput '^"one two"$' config.result ./shorthand-meta.nix
 
+checkConfigError "In module .*test-push-down-non-attrs.nix., you're trying to define a value of type \`bool'\n\s*rather than an attribute set for the option" config ./test-push-down-non-attrs.nix
+
 checkConfigOutput '^true$' config.result ./test-mergeAttrDefinitionsWithPrio.nix
 
 # Check that a module argument is passed, also when a default is available

lib/tests/modules/test-push-down-non-attrs.nix

@@ -0,0 +1,5 @@
+{ lib, ... }:
+
+{
+  config = lib.mkIf true true;
+}