Add support for PHP 8.1 Enums as allowed values for list_* field types

10.0 requires 8.1

Form field/widget side @ #3249599: Add support for PHP 8.1 Backed Enums in Select, Checkboxes, Radio elements

There's no patch or MR to review

Love this. But curious how it would work in the Field UI. Is the idea that if a field uses this to manage allowed values, that those parts of the UI where you define the values and labels would disappear or something?

PHP 8.3 added Backed Enums, which opens up the possibility of key/value enums.

So, if we supported Backed Enums, then we essentially have the equivalent of a function which returns an array for the keys/values.

Backed Enums can have methods, like classes, the Enum class could define a label() method which returns the enum value using TranslatableMarkup.

This opens up another possibility where the Enum could have methods that the field could make use of for other things (not sure what yet), but there's potential!

Here's an example of a Status enum that I built to use as the allowed values of a list_string base field.

declare(strict_types=1);

namespace Drupal\my_module;

use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * The available states for an edition.
 */
enum State: string {

  case Draft = 'draft';
  case Published = 'published';
  case Archived = 'archived';

  /**
   * Gets the human-readable label for this state.
   */
  public function label(): TranslatableMarkup {
    return match ($this) {
      self::Draft => new TranslatableMarkup('Draft'),
      self::Published => new TranslatableMarkup('Published'),
      self::Archived => new TranslatableMarkup('Archived'),
    };
  }

  /**
   * Gets the states as options suitable for a list field.
   *
   * @return array<string, \Drupal\Core\StringTranslation\TranslatableMarkup>
   *   The state values keyed by their stored value.
   */
  public static function options(): array {
    $options = [];
    foreach (self::cases() as $state) {
      $options[$state->value] = $state->label();
    }
    return $options;
  }

}

If list fields supported Backed Enums, I wouldn't need the options() method.

But you can see the potential here!