[2.0.0-beta5] Empty field values when not renderable

Problem/Motivation

There is a huge issue in Drupal about evaluating a render array as true or false from a Twig template: #953034: [meta] Themes improperly check renderable arrays when determining visibility
Very annoying but, after 13 years, still no fix.

UI Patterns users are sometimes confronted to this issue: #3381590: Drupal entities containing optional or multiple fields do not render patterns correctly
It looks very hard to fix at a Drupal level, but we are lucky it may be doable at UI Patterns level.

Steps to reproduce

Let's do some tests on Drupal 10.1.1 with this slot:

{% if description %}<p class="fr-tile__desc">{{ description }}</p>{% endif %}

From https://git.drupalcode.org/project/ui_suite_dsfr/-/blob/1.0.x/templates/...

We expect description to be false.

Test 1: empty array

$variables['description'] = [];

OK: description is false, because in PHP an empty array is false

Test 2: non renderable properties

$variables['description'] = [
  "#cache" => [
    "contexts" => [
      "user.permissions"
    ],
    "tags" => [],
    "max-age" => -1
  ]
]

KO: description is true

Test 3: non renderable properties wrapped in a sequence

$variables['description'] = [
 [
  "#cache" => [
    "contexts" => [
      "user.permissions"
    ],
    "tags" => [],
    "max-age" => -1
  ]
]
]

KO: description is true

Test 4: non renderable properties wrapped in a mapping

Layout Builder is sometimes wrapping the render array in a mapping where the key is the block UUID:

$variables['description'] = [
"cf124739-3274-41cc-829d-c3465ee4eaa5" => [
  "#cache" => [
    "contexts" => [
      "user.permissions"
    ],
    "tags" => [],
    "max-age" => -1
  ]
]
]

KO: description is true.

Test 5: empty mapping

$variables['description'] = [
    "whatever" => []
];

KO: description is true.

Proposed resolution

So, it may be fixable at UI Patterns level because:

• we know which Twig variables are expecting a render array : fields ("slots") with an array
• we know where to put the bubbled properties: the component render array

So, in Pattern::processFields() for each component slot:

1. if the array is not a render array, do a recursive loop on items
2. if the array is a render array where all the children are empty AND where all properties are non-printable or empty:
   • move the non-printable properties (#cache, #attached... what else?) to the component render element if they are bubbable
   • remove the non-printable properties (#weight... what else?) if they are not bubbable
3. unset the keys of empty children & empty properties

Other tasks

Once fixed, share the solution with #953034: [meta] Themes improperly check renderable arrays when determining visibility

Later, we will need to port this solution to UI Patterns 2. Hopefully, SDC allows ComponentElement to be overridden.