Remove the 'hidden widget' plugin, it is actually never used

$form_display->getRenderer($field_name) just returns NULL

This is a bug, it did return the hidden widget plugin at some point in either the EntityFormDisplay patch or the form modes one, but it's definitely the form modes patch that lost it..

Why don't we stay that way and remove the "hidden" widget then ?

IMO, because we would have to hardcode the 'hidden' option in several places. One random example that comes to mind would be something like Rules, which could display a list of widget plugins for a condition that checks how a field is displayed in an entity form.

Mmm, I hate to chime in without understanding CMI but before this ship sails, perhaps it is because getDefaultValueWidget is still trying to find $this->instance['widget']?

#2028759: Clean up instance['widget'] in Field UI

That's being fixed in another issue (#2056405: Let field types provide their support for "default values" and associated input UI), and this discussion is not at all about CMI..

I was talking about this part: $this->instance['widget'] that's being fixed there. At least I saw it was removed in a patch from several days ago, didn't check since then.

I think the performance issues can be overcome, probably by instantiating the hidden formatter only once and keeping it "cached" in EntityDisplay. But I dunno, as you prefer..

@amateescu, indeed, I wrote without grokking...

Right, no more objections from me then.

Yep, it's not related, I was just replying to #3.

Makes sense indeed. Tested it manually by adding a default value to a textfield, moving to hidden on form display and then creating content, it still works.

One thing I've wondering though: in #2014821: Introduce form modes UI and their configuration entity we removed following line:

if ($this->instance['required'] && empty($this->instance['default_value']) && empty($this->instance['default_value_function']) && $this->instance['widget']['type'] == 'field_hidden') {
-      drupal_set_message(t('Field %label is required and uses the "hidden" widget. You might want to configure a default value.', array('%label' => $this->instance['label'])), 'warning');
-    }

But we didn't add a replacement for it. Not sure whether we need to bring that back, but probably not in this issue.

tagging

Okay, lets start with a reroll. But locally it looked like it's not working. Also some more tests are failing, confirming my manual tests. This may needs some more work.

Updated patch

Cool!

We can obsolete (or maybe delete?) https://drupal.org/node/1968224 and add it to the change notice for form modes.

Sounds good to me.

Patch no longer applies.

Rerolled

duh

Patch no longer applies.

Sad reroll :/

iirc we added this before form display modes got in. Fine with removing it if it's not used - nothing stops it being added back if there's a need for it. Committed/pushed to 8.x, thanks!

I think we should create/update change notice.

I think we just need to delete this one: https://drupal.org/node/1968224

#35 Looks like the way to go.

Updated #1968224: 'Hidden' field widget is added to Drupal 8 (then removed again in favour of form display configuration).