// Alter the submitted value in $form_state.
$form_state->setValueForElement($form['name'], 'value changed by setValueForElement() in #validate');
// Output the element's value from $form_state.
- drupal_set_message(t('@label value: @value', ['@label' => $form['name']['#title'], '@value' => $form_state->getValue('name')]));
+ $this->messenger()->addStatus($this->t('@label value: @value', ['@label' => $form['name']['#title'], '@value' => $form_state->getValue('name')]));
// Trigger a form validation error to see our changes.
$form_state->setErrorByName('');