Retrieves the instance definition array for a widget's helper callbacks.
Widgets helper element callbacks (such as #process, #element_validate, #value_callback, ...) should use field_widget_field() and field_widget_instance() instead of field_info_field() and field_info_instance() when they need to access field or instance properties. See hook_field_widget_form() for more details.
$element: The structured array for the widget.
$form_state: The form state.
The $instance definition array for the current widget.
hook_field_widget_form()
function field_widget_instance($element, $form_state) {
$field_state = field_form_get_state($element['#field_parents'], $element['#field_name'], $element['#language'], $form_state);
return $field_state['instance'];
}