Cacheable breadcrumbs block, and fix breadcrumb builders

Unfortunately, all of the breadcrumb classes don't even comply with the interface they're implementing: they're returning an array of rendered links, not a render array! Therefore, it is currently impossible to set cache tags!

Maybe we can create a sub task for this issue and discuss. I can help in fixing the issue.

1. +++ b/core/modules/system/lib/Drupal/system/Plugin/Block/SystemBreadcrumbBlock.php
   @@ -35,4 +35,23 @@ public function build() {
   +   * {@inheritdoc}
   +   */
   +  public function defaultConfiguration() {
   +    // It's the responsibility of each breadcrumb builder to set the appropriate
   +    // cache tags.
   +    return array('cache' => array('max_age' => \Drupal\Core\Cache\Cache::PERMANENT));
   +  }
   +
   +  /**
   +   * {@inheritdoc}
   +   */
   +  protected function getRequiredCacheContexts() {
   +    // The 'Breadcrumbs' block must be cached per URL, role and language: the
   +    // breadcrumbs themselves depend on the current URL, may not be accessible
   +    // to users of a certain role, and may be translated.
   +    return array('cache_context.url', 'cache_context.user.roles', 'cache_context.language');
   +  }
   +
   

   Do we really want to cache it, even we know already that its existence depends on access? These links can use any route access checkers ...

2. +++ b/core/modules/taxonomy/lib/Drupal/taxonomy/TermBreadcrumbBuilder.php
   @@ -40,6 +40,14 @@ public function build(array $attributes) {
    
   +    // @todo BreadcrumbBuilderInterface::build() says:
   +    //   * @return array
   +    //   *   A render array for the breadcrumbs. Returning an empty array will
   +    //   *   suppress all breadcrumbs."
   +    //  BUT… as you can see above, we're actually returning an array of links.
   +    //  And that's in fact the only way this can work, because what we return
   +    //  here, is being assigned to '#breadcrumb', which may only contain links…
   +
   

   This certainly will be replaced by #2314599: Use title/url instead of l() for building breadcrumb

I really hope we don't treat performance over security here. In case we do, have fun with chx :P

+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/TermBreadcrumbBuilder.php
@@ -40,6 +40,14 @@ public function build(array $attributes) {
+    //  here, is being assigned to '#breadcrumb', which may only contain links…

If we want a text, aka non-link breadcrumb, is that do-able?

@joelpittet this is worked on in #2345801: template_preprocess_breadcrumb() assumes all breadcrumbs are \Drupal\Core\Link objects.

@fgm merci bien;)

I think we can re-evaluate this once #2483183: Make breadcrumb block cacheable is in and decide whether it's a duplicate or maybe we can further improve the subsystem.