Font Awesome icons in link field
You can define your own template.

For example as inline:

$link = array(
    '#type' =&gt; 'inline_template',
    '#template' =&gt; '&lt;a href="{{ url }}"&gt;&lt;i class="fa {{ fa_icon }}"&gt;&lt;/i&gt;{{ title }}&lt;/a&gt;',
    '#context' =&gt; array(
        'url' =&gt; $url,
        'title' =&gt; $title,
        'fa_icon =&gt; 'fa-pencil',
    ),
  ); 


If you are working on a theme or module you should define a template with its own name like link_fa, so that it can be overridden by other modules or themes.