lib.modules: Add _prefix module argument

lib/modules.nix

@@ -258,6 +258,7 @@ let
                     specialArgs
                     ;
                   _class = class;
+                  _prefix = prefix;
                 }
                 // specialArgs
               );

lib/tests/modules.sh

@@ -352,6 +352,9 @@ checkConfigOutput '^"submodule"$' options.submodule.type.description ./declare-s
 ## Paths should be allowed as values and work as expected
 checkConfigOutput '^true$' config.submodule.enable ./declare-submoduleWith-path.nix
 
+## _prefix module argument is available at import time and contains the prefix
+checkConfigOutput '^true$' config.foo.ok ./prefix-module-argument.nix
+
 ## deferredModule
 # default module is merged into nodes.foo
 checkConfigOutput '"beta"' config.nodes.foo.settingsDict.c ./deferred-module.nix

lib/tests/modules/prefix-module-argument.nix

@@ -0,0 +1,17 @@
+{ lib, ... }:
+{
+  options.foo = lib.mkOption {
+    type = lib.types.submodule { };
+    default = { };
+  };
+
+  config = {
+    foo =
+      { _prefix, ... }:
+      assert _prefix == [ "foo" ];
+      {
+        options.ok = lib.mkOption { };
+        config.ok = true;
+      };
+  };
+}