RecipeInputFormTrait lets you collect recipe inputs in a form

The new trait, Drupal\Core\Recipe\RecipeInputFormTrait, can be added to form classes if you want to create a UI that collects input values for a recipe which needs them.

For recipe authors

To work with forms, your recipe needs to define how it wants its inputs to look when rendered in a form. For example, from core's feedback_contact_form recipe:

input:
  recipient:
    data_type: email
    description: 'The email address that should receive submissions from the feedback form.'
    constraints:
      NotBlank: []
    prompt:
      method: ask
      arguments:
        question: 'What email address should receive website feedback?'
    form:
      '#type': email
      '#title': 'Feedback form email address'
    default:
      source: config
      config: ['system.site', 'mail']

Anything under the form key will be used to build the form element for this input. If form is not present, the input will never be shown in a form at all. If it is present, it must be an array containing only form element properties (i.e., every key must begin with #) -- it cannot contain any child elements.

For developers building a recipe UI

In your form class, load the recipe as an object using \Drupal\Core\Recipe\Recipe::createFromDirectory(), and then generate a set of elements for the recipe's inputs and all of its dependencies, validate them, and apply the recipe programmatically with the submitted values:

class MyRecipeForm extends FormBase {

  use RecipeInputFormTrait;

  public function buildForm(array $form, FormStateInterface $form_state): array {
    $recipe = Recipe::createFromDirectory('core/recipes/feedback_contact_form');
    $form += $this->buildRecipeInputForm($recipe);

    // $form['feedback_contact_form']['recipient'] will now exist, and have '#type' => 'email'.

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state): void {
    $recipe = Recipe::createFromDirectory('core/recipes/feedback_contact_form');
    $this->validateRecipeInput($recipe, $form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state): void {
    $recipe = Recipe::createFromDirectory('core/recipes/feedback_contact_form');
    $this->setRecipeInput($recipe, $form_state);
    RecipeRunner::processRecipe($recipe);
  }

}

See core/modules/system/tests/modules/form_test/src/Form/FormTestRecipeInputForm.php for an example of how this works.