drupal_module__contact_field/contact_field.module

<?php
/**
 * @file
 * A contact field for use with contact.module
 *
 * - can be used as just a formatter for the user object
 */

function contact_field_field_formatter_info() {
  $ret = array(
    'user_contact_form_link' => array(
      'label' => t('Contact form'),
      'field types' => array('user_contact'),
    ),
    'user_contact_mailto_link' => array(
      'label' => t('mailto: link'),
      'field types' => array('user_contact'),
    ),
  );

  // dsm(get_defined_vars(), __FUNCTION__);
  return $ret;
}

function contact_field_field_info() {
  $ret = array(
    'user_contact' => array(
      'label' => t('User contact'),
      'description' => t('This field builds a contact link from the entity to which it is attached, as long as that entity has a uid field linking to a user account'),
      'settings' => array(),
      'instance settings' => array(),
      'default_widget' => NULL,
      'default_formatter' => 'user_contact_link',
      'cardinality' => 1,
    ),
  );

  // dsm(get_defined_vars(), __FUNCTION__);
  return $ret;
}

/**
 * Implements hook_field_is_empty().
 */
function contact_field_field_is_empty($item, $field) {
  return empty($item);
}

function contact_field_field_schema($field) {
  if ($field['type'] != 'user_contact') {
    return;
  }

  $ret = array(
    'columns' => array(),
    'indexes' => array(),

  );

  // dsm(get_defined_vars(), __FUNCTION__);
  return $ret;
}

function contact_field_field_widget_info() {
  $ret = array(
    'contact_field_simple' => array(
      'label' => t('No input necessary'),
      'description' => t('Take the information from the underlying account'),
      'field types' => array('user_contact'),
      'behaviors' => array(
        'multiple values' => FIELD_BEHAVIOR_CUSTOM, // No values input anyway
        'default value' => FIELD_BEHAVIOR_DEFAULT,
      ),
    ),
  );

  // dsm(get_defined_vars(), __FUNCTION__);
  return $ret;
}

function contact_field_field_widget_form(&$form, &$form_state, $field,
  $instance, $langcode, $items, $delta, $element) {
  // dsm(get_defined_vars(), __FUNCTION__);

  $ret = array();
  if (isset($form['uid'])) {
    // This is not the widget settings form
    $uid = $form['uid']['#value'];
    $account = user_load($uid);
    $ret['mail'] = array(
      '#type' => 'value',
      '#title' => $instance['label'],
      '#disabled' => TRUE,
      '#value' => $account->mail,
    );
  }
  return $ret;

}

/**
 * Implements hook_field_formatter_view().
 */
function contact_field_field_formatter_view($obj_type, $object, $field,
  $instance, $langcode, $items, $display) {

  $uid = $object->uid;
  $name = $object->name;

  $ret = array();
  switch ($display['type']) {
    case 'user_contact_form_link':
      foreach (array_keys($items) as $delta) {
        // Cannot use theme_username() in D7.7. See http://drupal.org/node/1238094
        $ret[$delta] = array('#markup' => l($name, "user/$uid/contact"));
      }
      break;

    case 'user_contact_mailto_link':
      $account = user_load($uid);
      $mail = $account->mail;
      foreach (array_keys($items) as $delta) {
        // Cannot use theme_username() in D7.7. See http://drupal.org/node/1238094
        $ret[$delta] = array('#markup' => l($name, "mailto:$mail"));
      }
      break;

    default:
      return;
  }

  $ret = array($instance['id'] => $ret);
  // dsm(get_defined_vars(), __FUNCTION__);
  return $ret;
}

/**
 * Force cardinality: there is no point in having multiple instances of this
 * field since it is single-valued per entity instance, instead of having
 * standalone storage.
 */
function contact_field_form_field_ui_field_edit_form_alter(&$form, $form_state, $form_id) {
  dsm($form);
  if ($form['#field']['type'] == 'user_contact') {
    $override = array(
      '#access' => FALSE,
      '#value' => 1,
    );
    $form['field']['cardinality'] = $override + $form['field']['cardinality'];
  }
}