Update LinkWidget to accommodate the attribute.

@jibran maybe we just need to add it to the UI settings? I've changed the issue summary accordingly.

It would be really fantastic to have these in the UI (class, rel and target).

Is this still possible in the RC phase? If not I guess it should be postponed to 8.1.x, but maybe in the meantime we could document the custom module route -- my guess is a lot of themers are going to want to add classes to their links.

Thinking about this more, is there a reason this couldn't be moved to a template like the other old field preprocess functions were? There's already a link-formatter-link-separate.html.twig template; if the functionality to render a link field could be moved to a template too, then themers could add attributes like class, title, rel, target etc. pretty easily.

I read more from #1898426: link.module - Convert theme_ functions to Twig and I'm really confused why there's the link-formatter-link-separate.html.twig template but nothing for a (non-separated) link. If the theming functions for theme_link_formatter_link_separate were converted, why not for regular links? In fact link_theme only even mentions link_formatter_link_separate.

Bizarre. I'm changing the title to try to get some feedback on my suggestion that we have a template for themed link fields, or at least some explanation of why that doesn't exist.

Updated issue summary with clearer explanation; fixed a couple typos.

url options can be added using $item->options but there is no UI for that in field settings page.

Can you give a specific example of how / where to add this? Can this be done in hook_preprocess_field()?

I take it that's a question for jibran.

Using the opportunity to move this to 8.1.

There seems to be some progress in contrib modules, but neither of them are working for me...
https://www.drupal.org/project/link_class
https://www.drupal.org/project/link_class_widget

The only working solution I found to this problem was to add a preprocess function in my theme!

function MYTHEME_preprocess_field(&$variables) {
  if ($variables['field_name'] == 'field_MYFIELD') {
    foreach ($variables['items'] as $delta => $item) {
      $variables['items'][$delta]['content']['#options']['attributes']['class'][] = 'MYCLASS';
    }
  }
}

I also tried in a field twig, but I could not figure out how to access the attributes array from there.

Is anything happening to include basic things like this in the Drupal core Link Formatter? Is there any reason this was not included?

@weseze Leah Wagner just wrote this up. You're right that currently the only way is to use a preprocess function, but her suggestion is a little more flexible:

In your THEME.theme file:

function THEME_preprocess_field(&$variables, $hook) {
  // Make additional variables available to the template.
  $variables['bundle'] = $variables['element']['#bundle'];
}

And then in your theme's field.twig.html:

{% extends "@stable/field/field.html.twig" %}

{# Create classes array #}
{% set classes = [
  bundle ~ '__' ~ field_name|replace({'field_' : ''})|clean_class
] %}

{% set attributes = attributes.addClass(classes) %}

This is extending the Stable theme's field template, but you could also extend the Classy theme instead. This is also applying it to all fields, but you could apply it just to link fields with a field-specific template. And this is programmatically adding classes based on the field name; if you want to just apply a specific class, you'd prepend (or replace) the bundle... line with the quoted class, i.e. 'MYCLASS'.

That said, the original suggestion in this issue -- to make adding all attributes (including not just classes but things like title and rel) possible with a core link-field Twig template -- would be more flexible (and more understandable) still. So that's what I'm pulling for.

what about title attribute?