drupal_map_assoc() has been removed

drupal_map_assoc() existed to work around a PHP bug with array_combine().
In PHP 5.4 this bug was fixed, which resulted in this function's removal in Drupal 8 as the same behavior can be achieved with array_combine() and array_map().

In earlier versions of Drupal 8 \Drupal\Component\Utitlity\MapArray::copyValuesToKeys() existed for the same purpose. This method (and the containing class) has been removed, as well.

D7

$options = drupal_map_assoc($my_array);
// With callback
$options = drupal_map_assoc($my_array, $callback);

D8

$options = array_combine($my_array, $my_array);
// With callback
$options = array_map($callback, array_combine($my_array, $my_array));

Earlier D8 (no longer valid)

use Drupal\Component\Utility\MapArray;

$options = MapArray::copyValuesToKeys($my_array);
// With callback
$options = MapArray::copyValuesToKeys($my_array, $callback);