lib/modules: Short-circuit unmatchedDefns when configs is empty

lib/modules.nix

@@ -13,8 +13,6 @@ let
     elem
     filter
     findFirst
-    flip
-    foldl
     foldl'
     getAttrFromPath
     head
@@ -403,7 +401,7 @@ rec {
           [{ inherit (module) file; inherit value; }]
         ) configs;
 
-      resultsByName = flip mapAttrs declsByName (name: decls:
+      resultsByName = mapAttrs (name: decls:
         # We're descending into attribute ‘name’.
         let
           loc = prefix ++ [name];
@@ -424,7 +422,7 @@ rec {
             in
               throw "The option `${showOption loc}' in `${firstOption._file}' is a prefix of options in `${firstNonOption._file}'."
           else
-            mergeModules' loc decls defns);
+            mergeModules' loc decls defns) declsByName;
 
       matchedOptions = mapAttrs (n: v: v.matchedOptions) resultsByName;
 
@@ -438,12 +436,19 @@ rec {
       inherit matchedOptions;
 
       # Transforms unmatchedDefnsByName into a list of definitions
-      unmatchedDefns = concatLists (mapAttrsToList (name: defs:
-        map (def: def // {
-          # Set this so we know when the definition first left unmatched territory
-          prefix = [name] ++ (def.prefix or []);
-        }) defs
-      ) unmatchedDefnsByName);
+      unmatchedDefns =
+        if configs == []
+        then
+          # When no config values exist, there can be no unmatched config, so
+          # we short circuit and avoid evaluating more _options_ than necessary.
+          []
+        else
+          concatLists (mapAttrsToList (name: defs:
+            map (def: def // {
+              # Set this so we know when the definition first left unmatched territory
+              prefix = [name] ++ (def.prefix or []);
+            }) defs
+          ) unmatchedDefnsByName);
     };
 
   /* Merge multiple option declarations into a single declaration.  In
@@ -857,7 +862,7 @@ rec {
   mkMergedOptionModule = from: to: mergeFn:
     { config, options, ... }:
     {
-      options = foldl recursiveUpdate {} (map (path: setAttrByPath path (mkOption {
+      options = foldl' recursiveUpdate {} (map (path: setAttrByPath path (mkOption {
         visible = false;
         # To use the value in mergeFn without triggering errors
         default = "_mkMergedOptionModule";
@@ -941,7 +946,7 @@ rec {
 
   /* Use this function to import a JSON file as NixOS configuration.
 
-     importJSON -> path -> attrs
+     modules.importJSON :: path -> attrs
   */
   importJSON = file: {
     _file = file;
@@ -950,7 +955,7 @@ rec {
 
   /* Use this function to import a TOML file as NixOS configuration.
 
-     importTOML -> path -> attrs
+     modules.importTOML :: path -> attrs
   */
   importTOML = file: {
     _file = file;

lib/tests/modules/declare-attrsOf.nix

@@ -1,6 +1,13 @@
-{ lib, ... }: {
+{ lib, ... }:
+let
+  deathtrapArgs = lib.mapAttrs
+    (k: _: throw "The module system is too strict, accessing an unused option's ${k} mkOption-attribute.")
+    (lib.functionArgs lib.mkOption);
+in
+{
   options.value = lib.mkOption {
     type = lib.types.attrsOf lib.types.str;
     default = {};
   };
+  options.testing-laziness-so-don't-read-me = lib.mkOption deathtrapArgs;
 }

lib/tests/modules/freeform-nested.nix

@@ -1,7 +1,14 @@
-{ lib, ... }: {
+{ lib, ... }:
+let
+  deathtrapArgs = lib.mapAttrs
+    (k: _: throw "The module system is too strict, accessing an unused option's ${k} mkOption-attribute.")
+    (lib.functionArgs lib.mkOption);
+in
+{
   options.nest.foo = lib.mkOption {
     type = lib.types.bool;
     default = false;
   };
+  options.nest.unused = lib.mkOption deathtrapArgs;
   config.nest.bar = "bar";
 }