In Drupal 8, how do you add attributes to a link generated by the core Link field? (or, generally a render array with #type set to "link")

Question

So I have a situation where I have a core Link field on a node. I wanted to style that link with custom attributes, so I created a field.html.twig override to do that, only to find that the link itself is printed elsewhere. There doesn't appear to be a template or theme hook for the link; given that the core Twig debug functionality doesn't report a child template being invoked.

I tried pulling the URL out directly and rendering my own link, but that class (Drupal\Core\Url) isn't on the Twig whitelist, so getting the URL out will just cause a WSOD. There doesn't appear to be an Attributes object that I could add things into.

I could use theme_preprocess_field to call Drupal\Core\Url->toString() directly and store it in another variable, but I'd be throwing out whatever attributes are generated by default. I'd rather participate in however Drupal is rendering this link directly; or, if it's not extensible, at least know how to replicate what core does.

(For the record, the solution recommended in another question of using theme_preprocess_field to write attributes into #options does not appear to work.)

EDIT: As requested, here is the code I tried for a preprocess hook to add attributes:

function MYTHEME_preprocess_field(&$variables) {
    $element = $variables['element'];
    if ($element['#name'] == 'field_extra_links') {
        $variables['items'][0]['content']['#options']['attributes']['class'][] = 'Button';
    }
}

Answer 1

Answering my own question: the original suggestion was actually a valid solution; the way I had my theme configured meant that the hook wasn't loading. Stupid mistakes!