Fail if validate hander or submit handler does not exist

[alberto56]

Feature request

Let's say I'm altering a form and I'm adding:

function MYMODULE_form_alter(array &$form, FormStateInterface $form_state) {
    $form['#validate'][] = 'this_function_does_not_exist';
}

it would be nice if I got an error if this_function_does_not_exist() is not a function. Same for the submit hander:

function MYMODULE_form_alter(array &$form, FormStateInterface $form_state) {
    foreach (array_keys($form['actions']) as $action) {
      if ($action != 'preview' && isset($form['actions'][$action]['#type']) && $form['actions'][$action]['#type'] === 'submit') {
        $form['actions'][$action]['#submit'][] = 'this_function_does_not_exist_either';
      }
    }
}

Would be nice if we throw an error if this_function_does_not_exist_either() does not exist.

[mglaman]

This should be possible and is similar to RenderCallbackRule which checks ArrayItem.

However, I don't think it's been tested with the same value assignment that usually happens with submit handlers and validate handlers in Form API:

$form['#validate'][]

So the rule would need to check on ArrayItem and then check that all assigne values are callable, which is what RenderCallbackRule does.

  public function prepareCallback($callback) {
    if (is_string($callback) && substr($callback, 0, 2) == '::') {
      $callback = [$this->getFormObject(), substr($callback, 2)];
    }
    return $callback;
  }

Looks like service name patterns are not supported. But the shorthand ::METHOD_NAME is, which is the same as Class::METHOD_NAME but invoked as an instance method not statically.