Filter placeholders without arguments are not replaced when HTML corrector filter applied afterwards

Small super nitpicky change

But MR should be pointed to 11.x vs 11.0.x as 11.x is still the "main" branch.

I see the test-only feature passes when it should fail so tests seem to need some work.

As far as your comment on the MR I also could not find a spot and we don't have a filter sub-maintainer so think it's fine for now.

Thanks for reporting. I was entirely unaware of this in the filter system when I worked on the HTML5 parser upgrade, and so it only works as far as we have test coverage, and we've obviously missing test coverage for this case.

If there are no arguments, would it be better/more correct to not output the arguments attribute at all? ie. we would just have <drupal-filter-placeholder callback="callback" token="token" />? Then, the HTML5 parser/emitter does not need to special case this at all. But not sure if the placeholder system will need wider work to handle this.

In PlaceholderGenerator we did update it to optionally output the arguments for HTML5, so I do think we should try the same here:

    $placeholder_markup = '<drupal-render-placeholder callback="' . Html::escape($callback) . '"';
    if ($arguments !== '') {
      $placeholder_markup .= ' arguments="' . Html::escape($arguments) . '"';
    }
    $placeholder_markup .= ' token="' . Html::escape($token) . '"></drupal-render-placeholder>';

Pushed a change for #11.

I think in a followup we could even refactor that code into a static helper method on PlaceholderGenerator, so all placeholders are constructed in a consistent way.

Mistakenly removed the original tag when working on this at Dev Days.

I read the IS, comments and MR. There are no outstanding questions and I had no recommendations to make on the MR. Leaving at RTBC

Added tag for a followup for #15.

Committed to 11.x and backported to 11.0.x, 10.4.x and 10.3.x

Thanks folks

Opened #3463038: Add helper method to generate HTML placeholders to simplify the code here.

Wow, quite the edge case! Introduced by yours truly all the way back in #2478483: Introduce placeholders (#lazy_builder) to replace #post_render_cache.