Custom theme function for a Field

I want to change a theme hook for one field from a custom FieldFormatter. It's #theme callback is 'field' but I want to give the user the option to toggle a setting that changes this to 'field_raw', which then uses a Twig template with no markup. I can't figure out the theme hook.

Twig template:

{%- for item in items %}{{ item.content }}{% endfor -%}

Module hook:

function mymodule_theme($existing, $type, $theme, $path) {
  return [
    'field_raw' => [
      'items' => NULL,
    ],
  ];
}

I am unsure of what to pass here that makes it to the twig template. I can output text in the template so I know it is being hit. What is the right variables to set here so the template receives it?

I also tried:

'render element' => 'element'

'render element' => 'elements'

'render element' => 'children'

Using the field base hook

If this template with no markup should output field values {{ item.content }} you need the core code processing these values by defining a base hook.

See for example comment_theme():

/**
 * Implements hook_theme().
 */
function comment_theme() {
  return [
    ... 
    'field__comment' => [
      'base hook' => 'field',
    ],
  ];
}

In this case it's not enough to start the theme hook name with the base hook, it also needs double underscores: field__raw.

Implementing a custom theme hook

Not recommended, for demonstration purpose, you can copy the relevant core code to your own template:

mymodule.module:

function mymodule_theme($existing, $type, $theme, $path) {
  return [
    'field_raw' => [
      'render element' => 'element',
    ],
  ];
}

function template_preprocess_field_raw(&$variables, $hook) {
  $element = $variables['element'];
  $variables['items'] = [];
  $delta = 0;
  while (!empty($element[$delta])) {
    $variables['items'][$delta]['content'] = $element[$delta];
    $delta++;
  }
}

In this case you can choose a random name for the theme hook, but it must not start with field__.

Switch to this template in the custom field formatter:

  public function view(FieldItemListInterface $items, $langcode = NULL) {
    $elements = parent::view($items, $langcode);
    if (isset($elements['#theme'])) {
      $elements['#theme'] = 'field_raw';
    }
    return $elements;
  }

And finally place the minimal template in the module folder

mymodule/templates/field-raw.html.twig

{%- for item in items %}{{ item.content }}{% endfor -%}