How to write hook_field_placeholder_info() for the link module? The contributed project
Field placeholder
lets one use HTML5 placeholder attribute.  The project page says:


  By default, this module integrates text, number and mail modules,
  but any other widget type can be added through
  hook_field_placeholder_info().


I've tested this with a “Text” field, and it works as advertised.

I want to add hook_field_placeholder_info() for the
Link module.

According to field_placeholder.api.php, this hook should return:


  An array whose keys are the widget names and whose value are the
  widget item where the placeholder will be attached.


So far I've tried this:

/**
 * Implements hook_field_placeholder_info().
 */
function mymodule_field_placeholder_info() {
  return array(
    'link_field' =&gt; 'value',
  );
}


This puts the widget for the placeholder settings on the admin page
where one manage the link file, but setting a placeholder value has no
effect when I create a new node with a link field.

I believe the problem is “value”.  According to the documentation,
it should be the “widget item where the placeholder will be attached”.

I am not sure what that is.  So far I've tried “value”, “link_field”,
 “link”,  and “url” (clearing the cache before testing each in
turn), but none of these work for the link field.

So: How to write hook_field_placeholder_info() for the link module?
There is a feature request to add support for the link module. Unfortunate that patch has not been committed yet, but you can use/apply the patch in #5 and use

/**
 * Implements hook_field_placeholder_info().
 */
function mymodule_field_placeholder_info() {
  return array(
    'link_field' =&gt; 'url',
  );
}