Drupal: Taxonomy terms as autocomplete using form API.

foo.module

<?php

function bar_form($form, &$state) {
  $form['bar'] = array(
    '#type' => 'textfield',
    '#title' => t('Bar'),
    '#autocomplete_path' => 'taxonomy/autocomplete/field_bar_term', // Field name here.
    '#element_validate' => array('foo_taxonomy_autocomplete_validate'),
  );
  
  return $form;
}

function bar_form_submit($form, &$state) {
  //--
  $entity->bar[LANGUAGE_NONE] = $values["bar"];
  //--
}

/**
 * Form element validate handler for taxonomy term autocomplete element.
 *
 * By some reasons field_widget_field() does not load field.
 *  - I guess it because $element['#field_parents'] & $element['#field_name'] are empty.
 *
 * @HACK: This validator is basically clone of taxonomy_autocomplete_validate()
 *  - with some changes to load field.
 *
 * @see: taxonomy_autocomplete_validate().
 */
function foo_taxonomy_autocomplete_validate($element, &$form_state) {
  // Autocomplete widgets do not send their tids in the form, so we must detect
  // them here and process them independently.
  $value = array();
  if ($tags = $element['#value']) {
    // Collect candidate vocabularies.
    $field = field_info_field($element['#name']); //@HACK: is here!
 
    $vocabularies = array();
    foreach ($field['settings']['allowed_values'] as $tree) {
      if ($vocabulary = taxonomy_vocabulary_machine_name_load($tree['vocabulary'])) {
        $vocabularies[$vocabulary->vid] = $vocabulary;
      }
    }
 
    // Translate term names into actual terms.
    $typed_terms = drupal_explode_tags($tags);
    foreach ($typed_terms as $typed_term) {
      // See if the term exists in the chosen vocabulary and return the tid;
      // otherwise, create a new 'autocreate' term for insert/update.
      if ($possibilities = taxonomy_term_load_multiple(array(), array('name' => trim($typed_term), 'vid' => array_keys($vocabularies)))) {
        $term = array_pop($possibilities);
      }
      else {
        $vocabulary = reset($vocabularies);
        $term = array(
          'tid' => 'autocreate',
          'vid' => $vocabulary->vid,
          'name' => $typed_term,
          'vocabulary_machine_name' => $vocabulary->machine_name,
        );
      }
      $value[] = (array)$term;
    }
  }
 
  form_set_value($element, $value, $form_state);
}

Thank you.