lib/modules: Internally collect all unmatched definitions

This fundamentally changes how the module evaluation internally
handles definitions without an associated option.

Previously the values of these definitions were discarded and only
the names were propagated. This was fine because that's all that's
needed for optionally checking whether all definitions have an
associated option with _module.check.

However with the upcoming change of supporting freeform modules,
we *do* need the values of these.

With this change, the module evaluation cleanly separates definitions
that match an option, and ones that do not.

lib/modules.nix

@@ -65,38 +65,24 @@ rec {
         };
       };
 
-      options =
+      merged =
         let collected = collectModules
           (specialArgs.modulesPath or "")
           (modules ++ [ internalModule ])
           ({ inherit lib options config; } // specialArgs);
         in mergeModules prefix (reverseList collected);
 
-      config =
-        let
-          # Traverse options and extract the option values into the final
-          # config set.  At the same time, check whether all option
-          # definitions have matching declarations.
-          # !!! _module.check's value can't depend on any other config values
-          # without an infinite recursion. One way around this is to make the
-          # 'config' passed around to the modules be unconditionally unchecked,
-          # and only do the check in 'result'.
-          yieldConfig = prefix: set:
-            let res = removeAttrs (mapAttrs (n: v:
-              if isOption v then v.value
-              else yieldConfig (prefix ++ [n]) v) set) ["_definedNames"];
-            in
-            if options._module.check.value && set ? _definedNames then
-              foldl' (res: m:
-                foldl' (res: name:
-                  if set ? ${name} then res else throw "The option `${showOption (prefix ++ [name])}' defined in `${m.file}' does not exist.")
-                  res m.names)
-                res set._definedNames
-            else
-              res;
-        in yieldConfig prefix options;
+      options = merged.matchedOptions;
 
-      result = {
+      config = mapAttrsRecursiveCond (v: ! isOption v) (_: v: v.value) options;
+
+      checkUnmatched =
+        if config._module.check && merged.unmatchedDefns != [] then
+          let inherit (head merged.unmatchedDefns) file prefix;
+          in throw "The option `${showOption prefix}' defined in `${file}' does not exist."
+        else null;
+
+      result = builtins.seq checkUnmatched {
         inherit options;
         config = removeAttrs config [ "_module" ];
         inherit (config) _module;
@@ -236,7 +222,23 @@ rec {
      declarations in all modules, combining them into a single set.
      At the same time, for each option declaration, it will merge the
      corresponding option definitions in all machines, returning them
-     in the ‘value’ attribute of each option. */
+     in the ‘value’ attribute of each option.
+
+     This returns a set like
+       {
+         # A recursive set of options along with their final values
+         matchedOptions = {
+           foo = { _type = "option"; value = "option value of foo"; ... };
+           bar.baz = { _type = "option"; value = "option value of bar.baz"; ... };
+           ...
+         };
+         # A list of definitions that weren't matched by any option
+         unmatchedDefns = [
+           { file = "file.nix"; prefix = [ "qux" ]; value = "qux"; }
+           ...
+         ];
+       }
+  */
   mergeModules = prefix: modules:
     mergeModules' prefix modules
       (concatMap (m: map (config: { file = m._file; inherit config; }) (pushDownProperties m.config)) modules);
@@ -283,9 +285,9 @@ rec {
       defnsByName' = byName "config" (module: value:
           [{ inherit (module) file; inherit value; }]
         ) configs;
-    in
-    (flip mapAttrs declsByName (name: decls:
-      # We're descending into attribute ‘name’.
+
+      resultsByName = flip mapAttrs declsByName (name: decls:
+        # We're descending into attribute ‘name’.
         let
           loc = prefix ++ [name];
           defns = defnsByName.${name} or [];
@@ -294,7 +296,10 @@ rec {
         in
           if nrOptions == length decls then
             let opt = fixupOptionType loc (mergeOptionDecls loc decls);
-            in evalOptionValue loc opt defns'
+            in {
+              matchedOptions = evalOptionValue loc opt defns';
+              unmatchedDefns = [];
+            }
           else if nrOptions != 0 then
             let
               firstOption = findFirst (m: isOption m.options) "" decls;
@@ -302,9 +307,27 @@ rec {
             in
               throw "The option `${showOption loc}' in `${firstOption._file}' is a prefix of options in `${firstNonOption._file}'."
           else
-            mergeModules' loc decls defns
-      ))
-    // { _definedNames = map (m: { inherit (m) file; names = attrNames m.config; }) configs; };
+            mergeModules' loc decls defns);
+
+      matchedOptions = mapAttrs (n: v: v.matchedOptions) resultsByName;
+
+      # an attrset 'name' => list of unmatched definitions for 'name'
+      unmatchedDefnsByName =
+        # Propagate all unmatched definitions from nested option sets
+        mapAttrs (n: v: v.unmatchedDefns) resultsByName
+        # Plus the definitions for the current prefix that don't have a matching option
+        // removeAttrs defnsByName' (attrNames matchedOptions);
+    in {
+      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);
+    };
 
   /* Merge multiple option declarations into a single declaration.  In
      general, there should be only one declaration of each option.