diff --git a/gio/gaction.c b/gio/gaction.c
index c3748a902..645eb26ed 100644
--- a/gio/gaction.c
+++ b/gio/gaction.c
@@ -224,7 +224,7 @@ g_action_change_state (GAction  *action,
  * The return value (if non-%NULL) should be freed with
  * g_variant_unref() when it is no longer required.
  *
- * Returns: (transfer full): the current state of the action
+ * Returns: (nullable) (transfer full): the current state of the action
  *
  * Since: 2.28
  **/
diff --git a/gio/gactionmap.c b/gio/gactionmap.c
index bfcda8df3..077e3cfa6 100644
--- a/gio/gactionmap.c
+++ b/gio/gactionmap.c
@@ -76,7 +76,7 @@ g_action_map_default_init (GActionMapInterface *iface)
  *
  * If no such action exists, returns %NULL.
  *
- * Returns: (transfer none): a #GAction, or %NULL
+ * Returns: (nullable) (transfer none): a #GAction, or %NULL
  *
  * Since: 2.32
  */
diff --git a/gio/gappinfo.c b/gio/gappinfo.c
index bc3e5d701..0303c6188 100644
--- a/gio/gappinfo.c
+++ b/gio/gappinfo.c
@@ -173,7 +173,7 @@ g_app_info_equal (GAppInfo *appinfo1,
  * Note that the returned ID may be %NULL, depending on how
  * the @appinfo has been constructed.
  *
- * Returns: a string containing the application's ID.
+ * Returns: (nullable): a string containing the application's ID.
  **/
 const char *
 g_app_info_get_id (GAppInfo *appinfo)
@@ -240,7 +240,7 @@ g_app_info_get_display_name (GAppInfo *appinfo)
  * 
  * Gets a human-readable description of an installed application.
  *
- * Returns: a string containing a description of the 
+ * Returns: (nullable): a string containing a description of the 
  * application @appinfo, or %NULL if none. 
  **/
 const char *
@@ -284,7 +284,7 @@ g_app_info_get_executable (GAppInfo *appinfo)
  * Gets the commandline with which the application will be
  * started.  
  *
- * Returns: (type filename): a string containing the @appinfo's commandline,
+ * Returns: (nullable) (type filename): a string containing the @appinfo's commandline,
  *     or %NULL if this information is not available
  *
  * Since: 2.20
@@ -518,7 +518,7 @@ g_app_info_get_supported_types (GAppInfo *appinfo)
  * 
  * Gets the icon for the application.
  *
- * Returns: (transfer none): the default #GIcon for @appinfo or %NULL
+ * Returns: (nullable) (transfer none): the default #GIcon for @appinfo or %NULL
  * if there is no default icon.
  **/
 GIcon *
@@ -1279,7 +1279,7 @@ g_app_launch_context_get_environment (GAppLaunchContext *context)
  * applications are started on the same display as the launching
  * application, by setting the `DISPLAY` environment variable.
  *
- * Returns: a display string for the display.
+ * Returns: (nullable): a display string for the display.
  */
 char *
 g_app_launch_context_get_display (GAppLaunchContext *context,
@@ -1311,7 +1311,7 @@ g_app_launch_context_get_display (GAppLaunchContext *context,
  * Startup notification IDs are defined in the 
  * [FreeDesktop.Org Startup Notifications standard](http://standards.freedesktop.org/startup-notification-spec/startup-notification-latest.txt).
  *
- * Returns: a startup notification ID for the application, or %NULL if
+ * Returns: (nullable): a startup notification ID for the application, or %NULL if
  *     not supported.
  **/
 char *
diff --git a/gio/gapplication.c b/gio/gapplication.c
index b78fd9ab2..8a473e63f 100644
--- a/gio/gapplication.c
+++ b/gio/gapplication.c
@@ -1793,7 +1793,7 @@ g_application_new (const gchar       *application_id,
  *
  * Gets the unique identifier for @application.
  *
- * Returns: the identifier for @application, owned by @application
+ * Returns: (nullable): the identifier for @application, owned by @application
  *
  * Since: 2.28
  **/
@@ -2084,7 +2084,7 @@ g_application_get_is_remote (GApplication *application)
  * This function must not be called before the application has been
  * registered.  See g_application_get_is_registered().
  *
- * Returns: (transfer none): a #GDBusConnection, or %NULL
+ * Returns: (nullable) (transfer none): a #GDBusConnection, or %NULL
  *
  * Since: 2.34
  **/
@@ -2116,7 +2116,7 @@ g_application_get_dbus_connection (GApplication *application)
  * This function must not be called before the application has been
  * registered.  See g_application_get_is_registered().
  *
- * Returns: the object path, or %NULL
+ * Returns: (nullable): the object path, or %NULL
  *
  * Since: 2.34
  **/
@@ -2743,7 +2743,7 @@ static GApplication *default_app;
  *
  * If there is no default application then %NULL is returned.
  *
- * Returns: (transfer none): the default application for this process, or %NULL
+ * Returns: (nullable) (transfer none): the default application for this process, or %NULL
  *
  * Since: 2.32
  **/
diff --git a/gio/gapplicationcommandline.c b/gio/gapplicationcommandline.c
index d5dd55c19..7d3f0e665 100644
--- a/gio/gapplicationcommandline.c
+++ b/gio/gapplicationcommandline.c
@@ -529,7 +529,7 @@ g_application_command_line_get_options_dict (GApplicationCommandLine *cmdline)
  *
  * You must only call this function once per commandline invocation.
  *
- * Returns: (transfer full): a #GInputStream for stdin
+ * Returns: (nullable) (transfer full): a #GInputStream for stdin
  *
  * Since: 2.34
  **/
@@ -610,7 +610,7 @@ g_application_command_line_get_environ (GApplicationCommandLine *cmdline)
  * The return value should not be modified or freed and is valid for as
  * long as @cmdline exists.
  *
- * Returns: the value of the variable, or %NULL if unset or unsent
+ * Returns: (nullable): the value of the variable, or %NULL if unset or unsent
  *
  * Since: 2.28
  **/
diff --git a/gio/gbytesicon.c b/gio/gbytesicon.c
index ca78be1be..b2323d905 100644
--- a/gio/gbytesicon.c
+++ b/gio/gbytesicon.c
@@ -165,7 +165,7 @@ g_bytes_icon_new (GBytes *bytes)
  *
  * Gets the #GBytes associated with the given @icon.
  *
- * Returns: (transfer none): a #GBytes, or %NULL.
+ * Returns: (transfer none): a #GBytes.
  *
  * Since: 2.38
  **/
diff --git a/gio/gdbusinterface.c b/gio/gdbusinterface.c
index fa03115d2..35fa8e460 100644
--- a/gio/gdbusinterface.c
+++ b/gio/gdbusinterface.c
@@ -73,7 +73,7 @@ g_dbus_interface_get_info (GDBusInterface *interface_)
  * the returned object is being used from other threads. See
  * g_dbus_interface_dup_object() for a thread-safe alternative.
  *
- * Returns: (transfer none): A #GDBusObject or %NULL. The returned
+ * Returns: (nullable) (transfer none): A #GDBusObject or %NULL. The returned
  *     reference belongs to @interface_ and should not be freed.
  *
  * Since: 2.30
@@ -91,7 +91,7 @@ g_dbus_interface_get_object (GDBusInterface *interface_)
  *
  * Gets the #GDBusObject that @interface_ belongs to, if any.
  *
- * Returns: (transfer full): A #GDBusObject or %NULL. The returned
+ * Returns: (nullable) (transfer full): A #GDBusObject or %NULL. The returned
  * reference should be freed with g_object_unref().
  *
  * Since: 2.32
diff --git a/gio/gdbusinterfaceskeleton.c b/gio/gdbusinterfaceskeleton.c
index a0125c541..243b4a0a4 100644
--- a/gio/gdbusinterfaceskeleton.c
+++ b/gio/gdbusinterfaceskeleton.c
@@ -767,7 +767,7 @@ set_object_path_locked (GDBusInterfaceSkeleton *interface_,
  *
  * Gets the first connection that @interface_ is exported on, if any.
  *
- * Returns: (transfer none): A #GDBusConnection or %NULL if @interface_ is
+ * Returns: (nullable) (transfer none): A #GDBusConnection or %NULL if @interface_ is
  * not exported anywhere. Do not free, the object belongs to @interface_.
  *
  * Since: 2.30
@@ -876,7 +876,7 @@ g_dbus_interface_skeleton_has_connection (GDBusInterfaceSkeleton     *interface_
  *
  * Gets the object path that @interface_ is exported on, if any.
  *
- * Returns: A string owned by @interface_ or %NULL if @interface_ is not exported
+ * Returns: (nullable): A string owned by @interface_ or %NULL if @interface_ is not exported
  * anywhere. Do not free, the string belongs to @interface_.
  *
  * Since: 2.30
diff --git a/gio/gdbusintrospection.c b/gio/gdbusintrospection.c
index dde06422e..f35b2fb44 100644
--- a/gio/gdbusintrospection.c
+++ b/gio/gdbusintrospection.c
@@ -1847,7 +1847,7 @@ g_dbus_node_info_new_for_xml (const gchar  *xml_data,
  *
  * The cost of this function is O(n) in number of annotations.
  *
- * Returns: The value or %NULL if not found. Do not free, it is owned by @annotations.
+ * Returns: (nullable): The value or %NULL if not found. Do not free, it is owned by @annotations.
  *
  * Since: 2.26
  */
@@ -1915,7 +1915,7 @@ static GHashTable *info_cache = NULL;
  * The cost of this function is O(n) in number of methods unless
  * g_dbus_interface_info_cache_build() has been used on @info.
  *
- * Returns: (transfer none): A #GDBusMethodInfo or %NULL if not found. Do not free, it is owned by @info.
+ * Returns: (nullable) (transfer none): A #GDBusMethodInfo or %NULL if not found. Do not free, it is owned by @info.
  *
  * Since: 2.26
  */
@@ -1969,7 +1969,7 @@ g_dbus_interface_info_lookup_method (GDBusInterfaceInfo *info,
  * The cost of this function is O(n) in number of signals unless
  * g_dbus_interface_info_cache_build() has been used on @info.
  *
- * Returns: (transfer none): A #GDBusSignalInfo or %NULL if not found. Do not free, it is owned by @info.
+ * Returns: (nullable) (transfer none): A #GDBusSignalInfo or %NULL if not found. Do not free, it is owned by @info.
  *
  * Since: 2.26
  */
@@ -2023,7 +2023,7 @@ g_dbus_interface_info_lookup_signal (GDBusInterfaceInfo *info,
  * The cost of this function is O(n) in number of properties unless
  * g_dbus_interface_info_cache_build() has been used on @info.
  *
- * Returns: (transfer none): A #GDBusPropertyInfo or %NULL if not found. Do not free, it is owned by @info.
+ * Returns: (nullable) (transfer none): A #GDBusPropertyInfo or %NULL if not found. Do not free, it is owned by @info.
  *
  * Since: 2.26
  */
@@ -2165,7 +2165,7 @@ g_dbus_interface_info_cache_release (GDBusInterfaceInfo *info)
  *
  * The cost of this function is O(n) in number of interfaces.
  *
- * Returns: (transfer none): A #GDBusInterfaceInfo or %NULL if not found. Do not free, it is owned by @info.
+ * Returns: (nullable) (transfer none): A #GDBusInterfaceInfo or %NULL if not found. Do not free, it is owned by @info.
  *
  * Since: 2.26
  */
diff --git a/gio/gdbusmessage.c b/gio/gdbusmessage.c
index cbcea844e..bc9386ee7 100644
--- a/gio/gdbusmessage.c
+++ b/gio/gdbusmessage.c
@@ -1085,7 +1085,7 @@ g_dbus_message_get_header_fields (GDBusMessage  *message)
  *
  * Gets the body of a message.
  *
- * Returns: (transfer none): A #GVariant or %NULL if the body is
+ * Returns: (nullable) (transfer none): A #GVariant or %NULL if the body is
  * empty. Do not free, it is owned by @message.
  *
  * Since: 2.26
@@ -1164,7 +1164,7 @@ g_dbus_message_set_body (GDBusMessage  *message,
  * to the file descriptor that can be accessed by
  * `g_unix_fd_list_get (list, 5, ...)`.
  *
- * Returns: (transfer none):A #GUnixFDList or %NULL if no file descriptors are
+ * Returns: (nullable) (transfer none): A #GUnixFDList or %NULL if no file descriptors are
  * associated. Do not free, this object is owned by @message.
  *
  * Since: 2.26
@@ -2994,7 +2994,7 @@ g_dbus_message_set_reply_serial (GDBusMessage  *message,
  *
  * Convenience getter for the %G_DBUS_MESSAGE_HEADER_FIELD_INTERFACE header field.
  *
- * Returns: The value.
+ * Returns: (nullable): The value.
  *
  * Since: 2.26
  */
@@ -3008,7 +3008,7 @@ g_dbus_message_get_interface (GDBusMessage  *message)
 /**
  * g_dbus_message_set_interface:
  * @message: A #GDBusMessage.
- * @value: The value to set.
+ * @value: (nullable): The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_INTERFACE header field.
  *
@@ -3031,7 +3031,7 @@ g_dbus_message_set_interface (GDBusMessage  *message,
  *
  * Convenience getter for the %G_DBUS_MESSAGE_HEADER_FIELD_MEMBER header field.
  *
- * Returns: The value.
+ * Returns: (nullable): The value.
  *
  * Since: 2.26
  */
@@ -3045,7 +3045,7 @@ g_dbus_message_get_member (GDBusMessage  *message)
 /**
  * g_dbus_message_set_member:
  * @message: A #GDBusMessage.
- * @value: The value to set.
+ * @value: (nullable): The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_MEMBER header field.
  *
@@ -3068,7 +3068,7 @@ g_dbus_message_set_member (GDBusMessage  *message,
  *
  * Convenience getter for the %G_DBUS_MESSAGE_HEADER_FIELD_PATH header field.
  *
- * Returns: The value.
+ * Returns: (nullable): The value.
  *
  * Since: 2.26
  */
@@ -3082,7 +3082,7 @@ g_dbus_message_get_path (GDBusMessage  *message)
 /**
  * g_dbus_message_set_path:
  * @message: A #GDBusMessage.
- * @value: The value to set.
+ * @value: (nullable): The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_PATH header field.
  *
@@ -3105,7 +3105,7 @@ g_dbus_message_set_path (GDBusMessage  *message,
  *
  * Convenience getter for the %G_DBUS_MESSAGE_HEADER_FIELD_SENDER header field.
  *
- * Returns: The value.
+ * Returns: (nullable): The value.
  *
  * Since: 2.26
  */
@@ -3119,7 +3119,7 @@ g_dbus_message_get_sender (GDBusMessage *message)
 /**
  * g_dbus_message_set_sender:
  * @message: A #GDBusMessage.
- * @value: The value to set.
+ * @value: (nullable): The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_SENDER header field.
  *
@@ -3142,7 +3142,7 @@ g_dbus_message_set_sender (GDBusMessage  *message,
  *
  * Convenience getter for the %G_DBUS_MESSAGE_HEADER_FIELD_DESTINATION header field.
  *
- * Returns: The value.
+ * Returns: (nullable): The value.
  *
  * Since: 2.26
  */
@@ -3156,7 +3156,7 @@ g_dbus_message_get_destination (GDBusMessage  *message)
 /**
  * g_dbus_message_set_destination:
  * @message: A #GDBusMessage.
- * @value: The value to set.
+ * @value: (nullable): The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_DESTINATION header field.
  *
@@ -3179,7 +3179,7 @@ g_dbus_message_set_destination (GDBusMessage  *message,
  *
  * Convenience getter for the %G_DBUS_MESSAGE_HEADER_FIELD_ERROR_NAME header field.
  *
- * Returns: The value.
+ * Returns: (nullable): The value.
  *
  * Since: 2.26
  */
@@ -3192,7 +3192,7 @@ g_dbus_message_get_error_name (GDBusMessage  *message)
 
 /**
  * g_dbus_message_set_error_name:
- * @message: A #GDBusMessage.
+ * @message: (nullable): A #GDBusMessage.
  * @value: The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_ERROR_NAME header field.
@@ -3234,7 +3234,7 @@ g_dbus_message_get_signature (GDBusMessage  *message)
 /**
  * g_dbus_message_set_signature:
  * @message: A #GDBusMessage.
- * @value: The value to set.
+ * @value: (nullable): The value to set.
  *
  * Convenience setter for the %G_DBUS_MESSAGE_HEADER_FIELD_SIGNATURE header field.
  *
@@ -3257,7 +3257,7 @@ g_dbus_message_set_signature (GDBusMessage  *message,
  *
  * Convenience to get the first item in the body of @message.
  *
- * Returns: The string item or %NULL if the first item in the body of
+ * Returns: (nullable): The string item or %NULL if the first item in the body of
  * @message is not a string.
  *
  * Since: 2.26
diff --git a/gio/gdbusmethodinvocation.c b/gio/gdbusmethodinvocation.c
index 18ae9d1be..c22e19ef0 100644
--- a/gio/gdbusmethodinvocation.c
+++ b/gio/gdbusmethodinvocation.c
@@ -194,7 +194,7 @@ g_dbus_method_invocation_get_interface_name (GDBusMethodInvocation *invocation)
  * returned.  See g_dbus_method_invocation_get_property_info() and
  * #GDBusInterfaceVTable for more information.
  *
- * Returns: A #GDBusMethodInfo or %NULL. Do not free, it is owned by @invocation.
+ * Returns: (nullable): A #GDBusMethodInfo or %NULL. Do not free, it is owned by @invocation.
  *
  * Since: 2.26
  */
@@ -221,7 +221,7 @@ g_dbus_method_invocation_get_method_info (GDBusMethodInvocation *invocation)
  *
  * If the call was GetAll, %NULL will be returned.
  *
- * Returns: (transfer none): a #GDBusPropertyInfo or %NULL
+ * Returns: (nullable) (transfer none): a #GDBusPropertyInfo or %NULL
  *
  * Since: 2.38
  */
diff --git a/gio/gdbusobject.c b/gio/gdbusobject.c
index 32393edf6..c332720c0 100644
--- a/gio/gdbusobject.c
+++ b/gio/gdbusobject.c
@@ -137,7 +137,7 @@ g_dbus_object_get_interfaces (GDBusObject *object)
  * Gets the D-Bus interface with name @interface_name associated with
  * @object, if any.
  *
- * Returns: (transfer full): %NULL if not found, otherwise a
+ * Returns: (nullable) (transfer full): %NULL if not found, otherwise a
  *   #GDBusInterface that must be freed with g_object_unref().
  *
  * Since: 2.30
diff --git a/gio/gdbusutils.c b/gio/gdbusutils.c
index ff9b44117..eb7eee9c8 100644
--- a/gio/gdbusutils.c
+++ b/gio/gdbusutils.c
@@ -533,9 +533,9 @@ g_dbus_gvariant_to_gvalue (GVariant  *value,
  * See the g_dbus_gvariant_to_gvalue() function for how to convert a
  * #GVariant to a #GValue.
  *
- * Returns: A #GVariant (never floating) of #GVariantType @type holding
- *     the data from @gvalue or %NULL in case of failure. Free with
- *     g_variant_unref().
+ * Returns: (transfer full): A #GVariant (never floating) of
+ *     #GVariantType @type holding the data from @gvalue or an empty #GVariant
+ *     in case of failure. Free with g_variant_unref().
  *
  * Since: 2.30
  */
diff --git a/gio/gdesktopappinfo.c b/gio/gdesktopappinfo.c
index b779b309b..6b9185447 100644
--- a/gio/gdesktopappinfo.c
+++ b/gio/gdesktopappinfo.c
@@ -2107,7 +2107,7 @@ g_desktop_app_info_get_is_hidden (GDesktopAppInfo *info)
  * situations such as the #GDesktopAppInfo returned from
  * g_desktop_app_info_new_from_keyfile(), this function will return %NULL.
  *
- * Returns: (type filename): The full path to the file for @info,
+ * Returns: (nullable) (type filename): The full path to the file for @info,
  *     or %NULL if not known.
  * Since: 2.24
  */
@@ -2155,7 +2155,7 @@ g_desktop_app_info_get_icon (GAppInfo *appinfo)
  *
  * Gets the categories from the desktop file.
  *
- * Returns: The unparsed Categories key from the desktop file;
+ * Returns: (nullable): The unparsed Categories key from the desktop file;
  *     i.e. no attempt is made to split it by ';' or validate it.
  */
 const char *
@@ -2184,9 +2184,9 @@ g_desktop_app_info_get_keywords (GDesktopAppInfo *info)
  * g_desktop_app_info_get_generic_name:
  * @info: a #GDesktopAppInfo
  *
- * Gets the generic name from the destkop file.
+ * Gets the generic name from the desktop file.
  *
- * Returns: The value of the GenericName key
+ * Returns: (nullable): The value of the GenericName key
  */
 const char *
 g_desktop_app_info_get_generic_name (GDesktopAppInfo *info)
@@ -4679,7 +4679,7 @@ G_GNUC_END_IGNORE_DEPRECATIONS
  * WM_CLASS property of the main window of the application, if launched
  * through @info.
  *
- * Returns: (transfer none): the startup WM class, or %NULL if none is set
+ * Returns: (nullable) (transfer none): the startup WM class, or %NULL if none is set
  * in the desktop file.
  *
  * Since: 2.34
@@ -4701,7 +4701,7 @@ g_desktop_app_info_get_startup_wm_class (GDesktopAppInfo *info)
  *
  * The @key is looked up in the "Desktop Entry" group.
  *
- * Returns: a newly allocated string, or %NULL if the key
+ * Returns: (nullable): a newly allocated string, or %NULL if the key
  *     is not found
  *
  * Since: 2.36
diff --git a/gio/gfileicon.c b/gio/gfileicon.c
index 48d4c8289..c77622933 100644
--- a/gio/gfileicon.c
+++ b/gio/gfileicon.c
@@ -174,7 +174,7 @@ g_file_icon_new (GFile *file)
  * 
  * Gets the #GFile associated with the given @icon.
  * 
- * Returns: (transfer none): a #GFile, or %NULL.
+ * Returns: (transfer none): a #GFile.
  **/
 GFile *
 g_file_icon_get_file (GFileIcon *icon)
diff --git a/gio/gfileinfo.c b/gio/gfileinfo.c
index be465c8ca..1b008f79a 100644
--- a/gio/gfileinfo.c
+++ b/gio/gfileinfo.c
@@ -1652,7 +1652,7 @@ g_file_info_get_edit_name (GFileInfo *info)
  *
  * Gets the icon for a file.
  *
- * Returns: (transfer none): #GIcon for the given @info.
+ * Returns: (nullable) (transfer none): #GIcon for the given @info.
  **/
 GIcon *
 g_file_info_get_icon (GFileInfo *info)
@@ -1679,7 +1679,7 @@ g_file_info_get_icon (GFileInfo *info)
  *
  * Gets the symbolic icon for a file.
  *
- * Returns: (transfer none): #GIcon for the given @info.
+ * Returns: (nullable) (transfer none): #GIcon for the given @info.
  *
  * Since: 2.34
  **/
@@ -1835,7 +1835,7 @@ g_file_info_get_modification_date_time (GFileInfo *info)
  *
  * Gets the symlink target for a given #GFileInfo.
  *
- * Returns: a string containing the symlink target.
+ * Returns: (nullable): a string containing the symlink target.
  **/
 const char *
 g_file_info_get_symlink_target (GFileInfo *info)
@@ -1859,7 +1859,7 @@ g_file_info_get_symlink_target (GFileInfo *info)
  * Gets the [entity tag][gfile-etag] for a given
  * #GFileInfo. See %G_FILE_ATTRIBUTE_ETAG_VALUE.
  *
- * Returns: a string containing the value of the "etag:value" attribute.
+ * Returns: (nullable): a string containing the value of the "etag:value" attribute.
  **/
 const char *
 g_file_info_get_etag (GFileInfo *info)
@@ -2464,8 +2464,8 @@ g_file_attribute_matcher_new (const char *attributes)
 
 /**
  * g_file_attribute_matcher_subtract:
- * @matcher: Matcher to subtract from 
- * @subtract: The matcher to subtract
+ * @matcher: (nullable): Matcher to subtract from 
+ * @subtract: (nullable): The matcher to subtract
  *
  * Subtracts all attributes of @subtract from @matcher and returns
  * a matcher that supports those attributes.
@@ -2476,7 +2476,7 @@ g_file_attribute_matcher_new (const char *attributes)
  * is a limitation of the current implementation, but may be fixed
  * in the future.
  *
- * Returns: A file attribute matcher matching all attributes of
+ * Returns: (nullable): A file attribute matcher matching all attributes of
  *     @matcher that are not matched by @subtract
  **/
 GFileAttributeMatcher *
diff --git a/gio/gfileiostream.c b/gio/gfileiostream.c
index 5919c28c0..f599bcdce 100644
--- a/gio/gfileiostream.c
+++ b/gio/gfileiostream.c
@@ -277,7 +277,7 @@ g_file_io_stream_query_info_finish (GFileIOStream     *stream,
  * This must be called after the stream has been written
  * and closed, as the etag can change while writing.
  *
- * Returns: the entity tag for the stream.
+ * Returns: (nullable) (transfer full): the entity tag for the stream.
  *
  * Since: 2.22
  **/
diff --git a/gio/gfilenamecompleter.c b/gio/gfilenamecompleter.c
index bad2c6e7a..a290c0322 100644
--- a/gio/gfilenamecompleter.c
+++ b/gio/gfilenamecompleter.c
@@ -404,9 +404,9 @@ init_completion (GFilenameCompleter *completer,
  *
  * Obtains a completion for @initial_text from @completer.
  *  
- * Returns: a completed string, or %NULL if no completion exists. 
- *     This string is not owned by GIO, so remember to g_free() it 
- *     when finished.
+ * Returns: (nullable) (transfer full): a completed string, or %NULL if no
+ *     completion exists. This string is not owned by GIO, so remember to g_free()
+ *     it when finished.
  **/
 char *
 g_filename_completer_get_completion_suffix (GFilenameCompleter *completer,
diff --git a/gio/gfileoutputstream.c b/gio/gfileoutputstream.c
index d0839c2b2..1dc536feb 100644
--- a/gio/gfileoutputstream.c
+++ b/gio/gfileoutputstream.c
@@ -271,7 +271,7 @@ g_file_output_stream_query_info_finish (GFileOutputStream     *stream,
  * This must be called after the stream has been written
  * and closed, as the etag can change while writing.
  * 
- * Returns: the entity tag for the stream.
+ * Returns: (nullable) (transfer full): the entity tag for the stream.
  **/
 char *
 g_file_output_stream_get_etag (GFileOutputStream  *stream)
diff --git a/gio/gicon.c b/gio/gicon.c
index 42c0a096a..f1ba0e2dc 100644
--- a/gio/gicon.c
+++ b/gio/gicon.c
@@ -562,7 +562,7 @@ g_icon_deserialize_emblemed (GVariant *value)
  *
  * Deserializes a #GIcon previously serialized using g_icon_serialize().
  *
- * Returns: (transfer full): a #GIcon, or %NULL when deserialization fails.
+ * Returns: (nullable) (transfer full): a #GIcon, or %NULL when deserialization fails.
  *
  * Since: 2.38
  */
@@ -653,7 +653,7 @@ g_icon_deserialize (GVariant *value)
  * makes sense to transfer the #GVariant between processes on the same machine,
  * (as opposed to over the network), and within the same file system namespace.
  *
- * Returns: (transfer full): a #GVariant, or %NULL when serialization fails. The #GVariant will not be floating.
+ * Returns: (nullable) (transfer full): a #GVariant, or %NULL when serialization fails. The #GVariant will not be floating.
  *
  * Since: 2.38
  */
diff --git a/gio/gmenu.c b/gio/gmenu.c
index 95602520c..bb5202cd3 100644
--- a/gio/gmenu.c
+++ b/gio/gmenu.c
@@ -781,7 +781,7 @@ g_menu_item_set_link (GMenuItem   *menu_item,
  * type, %NULL is returned.  %NULL is also returned if the attribute
  * simply does not exist.
  *
- * Returns: (transfer full): the attribute value, or %NULL
+ * Returns: (nullable) (transfer full): the attribute value, or %NULL
  *
  * Since: 2.34
  */
@@ -865,7 +865,7 @@ g_menu_item_get_attribute (GMenuItem   *menu_item,
  *
  * Queries the named @link on @menu_item.
  *
- * Returns: (transfer full): the link, or %NULL
+ * Returns: (nullable) (transfer full): the link, or %NULL
  *
  * Since: 2.34
  */
diff --git a/gio/gmenumodel.c b/gio/gmenumodel.c
index 9d7b76615..bd60dc5f6 100644
--- a/gio/gmenumodel.c
+++ b/gio/gmenumodel.c
@@ -541,7 +541,7 @@ g_menu_model_iterate_item_attributes (GMenuModel *model,
  * If the attribute does not exist, or does not match the expected type
  * then %NULL is returned.
  *
- * Returns: (transfer full): the value of the attribute
+ * Returns: (nullable) (transfer full): the value of the attribute
  *
  * Since: 2.32
  */
@@ -648,7 +648,7 @@ g_menu_model_iterate_item_links (GMenuModel *model,
  * If the link exists, the linked #GMenuModel is returned.  If the link
  * does not exist, %NULL is returned.
  *
- * Returns: (transfer full): the linked #GMenuModel, or %NULL
+ * Returns: (nullable) (transfer full): the linked #GMenuModel, or %NULL
  *
  * Since: 2.32
  */
diff --git a/gio/gmountoperation.c b/gio/gmountoperation.c
index 05fc50674..4bc19a88b 100644
--- a/gio/gmountoperation.c
+++ b/gio/gmountoperation.c
@@ -638,7 +638,7 @@ g_mount_operation_new (void)
  * 
  * Get the user name from the mount operation.
  *
- * Returns: a string containing the user name.
+ * Returns: (nullable): a string containing the user name.
  **/
 const char *
 g_mount_operation_get_username (GMountOperation *op)
@@ -650,7 +650,7 @@ g_mount_operation_get_username (GMountOperation *op)
 /**
  * g_mount_operation_set_username:
  * @op: a #GMountOperation.
- * @username: input username.
+ * @username: (nullable): input username.
  *
  * Sets the user name within @op to @username.
  **/
@@ -670,7 +670,7 @@ g_mount_operation_set_username (GMountOperation *op,
  *
  * Gets a password from the mount operation. 
  *
- * Returns: a string containing the password within @op.
+ * Returns: (nullable): a string containing the password within @op.
  **/
 const char *
 g_mount_operation_get_password (GMountOperation *op)
@@ -682,7 +682,7 @@ g_mount_operation_get_password (GMountOperation *op)
 /**
  * g_mount_operation_set_password:
  * @op: a #GMountOperation.
- * @password: password to set.
+ * @password: (nullable): password to set.
  * 
  * Sets the mount operation's password to @password.  
  *
@@ -741,7 +741,7 @@ g_mount_operation_set_anonymous (GMountOperation *op,
  * 
  * Gets the domain of the mount operation.
  * 
- * Returns: a string set to the domain. 
+ * Returns: (nullable): a string set to the domain.
  **/
 const char *
 g_mount_operation_get_domain (GMountOperation *op)
@@ -753,7 +753,7 @@ g_mount_operation_get_domain (GMountOperation *op)
 /**
  * g_mount_operation_set_domain:
  * @op: a #GMountOperation.
- * @domain: the domain to set.
+ * @domain: (nullable): the domain to set.
  * 
  * Sets the mount operation's domain. 
  **/  
diff --git a/gio/gnetworkaddress.c b/gio/gnetworkaddress.c
index b51d9c58d..ef1940a92 100644
--- a/gio/gnetworkaddress.c
+++ b/gio/gnetworkaddress.c
@@ -611,7 +611,7 @@ g_network_address_get_port (GNetworkAddress *addr)
  *
  * Gets @addr's scheme
  *
- * Returns: @addr's scheme (%NULL if not built from URI)
+ * Returns: (nullable): @addr's scheme (%NULL if not built from URI)
  *
  * Since: 2.26
  */
diff --git a/gio/gproxy.c b/gio/gproxy.c
index 60bb5b8e5..4c80a4746 100644
--- a/gio/gproxy.c
+++ b/gio/gproxy.c
@@ -55,7 +55,7 @@ g_proxy_default_init (GProxyInterface *iface)
  * Find the `gio-proxy` extension point for a proxy implementation that supports
  * the specified protocol.
  *
- * Returns: (transfer full): return a #GProxy or NULL if protocol
+ * Returns: (nullable) (transfer full): return a #GProxy or NULL if protocol
  *               is not supported.
  *
  * Since: 2.26
diff --git a/gio/gproxyaddress.c b/gio/gproxyaddress.c
index a02b31faf..a9405eb48 100644
--- a/gio/gproxyaddress.c
+++ b/gio/gproxyaddress.c
@@ -409,7 +409,7 @@ g_proxy_address_get_destination_port (GProxyAddress *proxy)
  *
  * Gets @proxy's username.
  *
- * Returns: the @proxy's username
+ * Returns: (nullable): the @proxy's username
  *
  * Since: 2.26
  */
@@ -425,7 +425,7 @@ g_proxy_address_get_username (GProxyAddress *proxy)
  *
  * Gets @proxy's password.
  *
- * Returns: the @proxy's password
+ * Returns: (nullable): the @proxy's password
  *
  * Since: 2.26
  */
@@ -442,7 +442,7 @@ g_proxy_address_get_password (GProxyAddress *proxy)
  *
  * Gets the proxy URI that @proxy was constructed from.
  *
- * Returns: the @proxy's URI, or %NULL if unknown
+ * Returns: (nullable): the @proxy's URI, or %NULL if unknown
  *
  * Since: 2.34
  */
diff --git a/gio/gsettingsschema.c b/gio/gsettingsschema.c
index 0b94f76f6..cbc3fad27 100644
--- a/gio/gsettingsschema.c
+++ b/gio/gsettingsschema.c
@@ -1001,7 +1001,7 @@ g_settings_schema_get_value (GSettingsSchema *schema,
  * therefore describe multiple sets of keys at different locations.  For
  * relocatable schemas, this function will return %NULL.
  *
- * Returns: (transfer none): the path of the schema, or %NULL
+ * Returns: (nullable) (transfer none): the path of the schema, or %NULL
  *
  * Since: 2.32
  **/
@@ -1660,7 +1660,7 @@ g_settings_schema_key_get_name (GSettingsSchemaKey *key)
  * function has to parse all of the source XML files in the schema
  * directory.
  *
- * Returns: the summary for @key, or %NULL
+ * Returns: (nullable): the summary for @key, or %NULL
  *
  * Since: 2.34
  **/
@@ -1695,7 +1695,7 @@ g_settings_schema_key_get_summary (GSettingsSchemaKey *key)
  * function has to parse all of the source XML files in the schema
  * directory.
  *
- * Returns: the description for @key, or %NULL
+ * Returns: (nullable): the description for @key, or %NULL
  *
  * Since: 2.34
  **/
diff --git a/gio/gsocketclient.c b/gio/gsocketclient.c
index 373774682..62b1afbcd 100644
--- a/gio/gsocketclient.c
+++ b/gio/gsocketclient.c
@@ -491,7 +491,7 @@ g_socket_client_set_protocol (GSocketClient   *client,
  *
  * See g_socket_client_set_local_address() for details.
  *
- * Returns: (transfer none): a #GSocketAddress or %NULL. Do not free.
+ * Returns: (nullable) (transfer none): a #GSocketAddress or %NULL. Do not free.
  *
  * Since: 2.22
  */
diff --git a/gio/gsubprocesslauncher.c b/gio/gsubprocesslauncher.c
index c27960949..9e077bd20 100644
--- a/gio/gsubprocesslauncher.c
+++ b/gio/gsubprocesslauncher.c
@@ -303,7 +303,7 @@ g_subprocess_launcher_unsetenv (GSubprocessLauncher *self,
  * On UNIX, the returned string can be an arbitrary byte string.
  * On Windows, it will be UTF-8.
  *
- * Returns: (type filename): the value of the environment variable,
+ * Returns: (nullable) (type filename): the value of the environment variable,
  *     %NULL if unset
  *
  * Since: 2.40
diff --git a/gio/gtlscertificate.c b/gio/gtlscertificate.c
index 0668d4905..9d00272f8 100644
--- a/gio/gtlscertificate.c
+++ b/gio/gtlscertificate.c
@@ -784,7 +784,7 @@ g_tls_certificate_list_new_from_file (const gchar  *file,
  *
  * Gets the #GTlsCertificate representing @cert's issuer, if known
  *
- * Returns: (transfer none): The certificate of @cert's issuer,
+ * Returns: (nullable) (transfer none): The certificate of @cert's issuer,
  * or %NULL if @cert is self-signed or signed with an unknown
  * certificate.
  *
diff --git a/gio/gtlsclientconnection.c b/gio/gtlsclientconnection.c
index f2a61c425..d0a740f4f 100644
--- a/gio/gtlsclientconnection.c
+++ b/gio/gtlsclientconnection.c
@@ -224,7 +224,7 @@ g_tls_client_connection_set_validation_flags (GTlsClientConnection  *conn,
  *
  * Gets @conn's expected server identity
  *
- * Returns: (transfer none): a #GSocketConnectable describing the
+ * Returns: (nullable) (transfer none): a #GSocketConnectable describing the
  * expected server identity, or %NULL if the expected identity is not
  * known.
  *
diff --git a/gio/gunixmounts.c b/gio/gunixmounts.c
index b0fdfa8c0..2aac6419a 100644
--- a/gio/gunixmounts.c
+++ b/gio/gunixmounts.c
@@ -2491,7 +2491,7 @@ g_unix_mount_point_get_fs_type (GUnixMountPoint *mount_point)
  * 
  * Gets the options for the mount point.
  * 
- * Returns: a string containing the options.
+ * Returns: (nullable): a string containing the options.
  *
  * Since: 2.32
  */
diff --git a/gio/gvolumemonitor.c b/gio/gvolumemonitor.c
index 65ec955a9..056999f6a 100644
--- a/gio/gvolumemonitor.c
+++ b/gio/gvolumemonitor.c
@@ -353,7 +353,7 @@ g_volume_monitor_get_mounts (GVolumeMonitor *volume_monitor)
  * 
  * Finds a #GVolume object by its UUID (see g_volume_get_uuid())
  * 
- * Returns: (transfer full): a #GVolume or %NULL if no such volume is available.
+ * Returns: (nullable) (transfer full): a #GVolume or %NULL if no such volume is available.
  *     Free the returned object with g_object_unref().
  **/
 GVolume *
@@ -377,7 +377,7 @@ g_volume_monitor_get_volume_for_uuid (GVolumeMonitor *volume_monitor,
  * 
  * Finds a #GMount object by its UUID (see g_mount_get_uuid())
  * 
- * Returns: (transfer full): a #GMount or %NULL if no such mount is available.
+ * Returns: (nullable) (transfer full): a #GMount or %NULL if no such mount is available.
  *     Free the returned object with g_object_unref().
  **/
 GMount *
diff --git a/gio/gzlibcompressor.c b/gio/gzlibcompressor.c
index 53f3e4745..b68f8823a 100644
--- a/gio/gzlibcompressor.c
+++ b/gio/gzlibcompressor.c
@@ -296,7 +296,7 @@ g_zlib_compressor_new (GZlibCompressorFormat format,
  *
  * Returns the #GZlibCompressor:file-info property.
  *
- * Returns: (transfer none): a #GFileInfo, or %NULL
+ * Returns: (nullable) (transfer none): a #GFileInfo, or %NULL
  *
  * Since: 2.26
  */
diff --git a/gio/gzlibdecompressor.c b/gio/gzlibdecompressor.c
index cf2542ef8..e3f225150 100644
--- a/gio/gzlibdecompressor.c
+++ b/gio/gzlibdecompressor.c
@@ -282,7 +282,7 @@ g_zlib_decompressor_new (GZlibCompressorFormat format)
  * or the header data was not fully processed yet, or it not present in the
  * data stream at all.
  *
- * Returns: (transfer none): a #GFileInfo, or %NULL
+ * Returns: (nullable) (transfer none): a #GFileInfo, or %NULL
  *
  * Since: 2.26
  */