Add Twig filters for drupal_html_class() and drupal_clean_id_identifier()

Yep for sure, I even used this as an example in the Twig playground in Austin.

+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -50,6 +50,10 @@ public function getFilters() {
+      // CSS class and ID filters

Add a period here ;)

Then I can RTBC!

I like it! Thanks @Dave Reid :)

Adding another related issue.

Just adding another test to make sure we don't accidentally break BEM syntax as we have in D7.

Let's remove the one for drupal_html_id() - that needs to go: #1305882: drupal_html_id() considered harmful; remove ajax_html_ids to use GET (not POST) AJAX requests.

@Dave Reid and @Cottser, from a sober second thought, I'd like to propose a name change as |class is very ambiguous.

Any chance 'sanitize_class' would fly as a name change?

Also, re #8 @Catch, et al would drupal_clean_id_identifier work for a 'sanitize_id' filter?

FWIW, I'm also fine with 'clean_class' and 'clean_id', though I can think of a few cases that those |class and |id filters could be used to do different things...

We opened up a sandbox for a poc for the banana https://www.drupal.org/sandbox/cottser/2297653

@Dave Reid any thoughts on #9?

I agree with #12, lets use clean_id and clean_class and use drupal_clean_id_identifier and drupal_html_class to make those work.

Here's #13. I considered taking out the second ID from the test but I think it's useful that we are differentiating between using drupal_clean_id_identifier() and drupal_html_id(), so I tweaked the text used in the test a bit to explain it for people looking at this code later.

Interdiff is from #7.

**RTBC**

+++ b/core/lib/Drupal/Core/Template/TwigExtension.php
@@ -82,6 +82,10 @@ public function getFilters() {
+      new \Twig_SimpleFilter('clean_id', 'drupal_clean_id_identifier'),

+++ b/core/modules/system/tests/modules/twig_theme_test/templates/twig_theme_test.filter.html.twig
@@ -20,3 +20,4 @@
+<div id="{{ attributes.id|clean_id }}"><span class="{{ 'Gray like a bunny!'|clean_class }} {{ 'BEM__ized--Top Feature'|clean_class }}" id="{{ attributes.id|clean_id }}">ID and class. Having the same ID twice is not valid markup but we want to make sure the filter doesn't use drupal_html_id().</span></div>

It might be nice to test that drupal_clean_id_identifier is call correctly... with the id being "quotes" it is not actaully cleaning anything. I like the fact we're testing it is not using drupal_html_id().

Also the issue summary could do with an update to accord with the latest patch.

Huge +1 here. One comment though about the name of the filter...

This will drastically improve implementation and syntax as CSS classes are moved out of preprocess functions, such as the work going on here: #2217731: Move field classes out of preprocess and into templates.

The template code in that example is already very long:

<div class="field field-{{ css_entity_type }}--{{ css_field_name }} field-name-{{ css_field_name }} field-type-{{ css_field_type }} field-label-{{ css_label_display }}{{ (attributes.class is defined) ? ' ' ~ attributes.class  : '' }}"{{ attributes }}>

If we were to switch to using this method, it would be nice to be able to shorten the code using a filter called c instead of clean_css.

<div class="field field-{{ entity_type|c }}--{{ field_name|c }} field-name-{{ field_name|c }} field-type-{{ field_type|c }} field-label-{{ label_display|c }}{{ (attributes.class is defined) ? ' ' ~ attributes.class  : '' }}"{{ attributes }}>

• We already have the t filter that themers will have to learn.
• This filter will be used more often than the t function in templates (so we have precedence).
• LEAP OF FAITH WARNING: Teaching themers (and devs!) what c means will be a no brainer.
• For something so ubiquitous for themers, just as t is for backend devs, it makes sense to use something brief.

@jwilson3 there is a historical presidence for the t filter and t() from drupal, and the |e and |escape from twig. I'm a bit leery at |c being for |clean_class. I understand it would have a wide use, but still worries me. We can maybe open up an issue for 8.1.x shorteners and punt that to a contrib twig extension so we can have some time to deliberate on that?

Single letter things are often hard to look up.

Readabilty trumps typing. We can always add aliases later if needs be, so don't let that block here. clean_class is fine.

Added id text to be filtered, and updated some text. Updated summary.

Are we agreeing on clean_class and clean_id?

clean_class and clean_id make good sense. Patch addresses concerns raised in #17. Back to RTBC.

Committed 3f3fc8d and pushed to 8.0.x. Thanks!