lib.types.submodule: Disable check during docs generation

Put simply docs generation is a weird eval, and we should avoid
generating any unnecessary errors.

lib/tests/misc.nix

@@ -2830,6 +2830,42 @@ runTests {
     expected = "unknown";
   };
 
+  # https://github.com/NixOS/nixpkgs/issues/396849
+  "test: submodule definitions aren't unchecked when evaluating submodule documentation" = {
+    expr =
+      let
+        module =
+          { lib, ... }:
+          {
+            options.foo = lib.mkOption { type = lib.types.submodule submodule; };
+          };
+
+        submodule = {
+          options.bar = lib.mkOption { type = lib.types.int; };
+          config.submoduleWrong = throw "yikes";
+        };
+
+        options = (evalModules { modules = [ module ]; }).options;
+
+        renderableOpts = filter (o: !o.internal) (optionAttrSetToDocList options);
+        # Evaluate the whole docs
+      in
+      builtins.deepSeq renderableOpts
+        # Return the locations
+        (map (o: o.loc) renderableOpts);
+    expected = [
+      [
+        "_module"
+        "args"
+      ]
+      [ "foo" ]
+      [
+        "foo"
+        "bar"
+      ]
+    ];
+  };
+
   testFreeformOptions = {
     expr =
       let

lib/types.nix

@@ -290,6 +290,15 @@ let
       unparenthesize: t:
       if unparenthesize (t.descriptionClass or null) then t.description else "(${t.description})";
 
+    noCheckForDocsModule = {
+      # When generating documentation, our goal isn't to check anything.
+      # Quite the opposite in fact. Generating docs is somewhat of a
+      # challenge, evaluating modules in a *lacking* context. Anything
+      # that makes the docs avoid an error is a win.
+      config._module.check = lib.mkForce false;
+      _file = "<built-in module that disables checks for the purpose of documentation generation>";
+    };
+
     # When adding new types don't forget to document them in
     # nixos/doc/manual/development/option-types.section.md!
     types = rec {
@@ -1209,7 +1218,14 @@ let
         in
         mkOptionType {
           inherit name;
-          description = if description != null then description else freeformType.description or name;
+          description =
+            if description != null then
+              description
+            else
+              let
+                docsEval = base.extendModules { modules = [ noCheckForDocsModule ]; };
+              in
+              docsEval._module.freeformType.description or name;
           check = x: isAttrs x || isFunction x || path.check x;
           merge =
             loc: defs:
@@ -1222,7 +1238,18 @@ let
           };
           getSubOptions =
             prefix:
-            (base.extendModules { inherit prefix; }).options
+            let
+              docsEval = (
+                base.extendModules {
+                  inherit prefix;
+                  modules = [ noCheckForDocsModule ];
+                }
+              );
+              # Intentionally shadow the freeformType from the possibly *checked*
+              # configuration. See `noCheckForDocsModule` comment.
+              inherit (docsEval._module) freeformType;
+            in
+            docsEval.options
             // optionalAttrs (freeformType != null) {
               # Expose the sub options of the freeform type. Note that the option
               # discovery doesn't care about the attribute name used here, so this