Stop passing E_USER_ERROR to trigger_error() on PHP 8.4

ToStringTrait might be tricky, because exceptions and __toString() methods don't like each other, unless that has changed somehow?

And the suggestion in RFC is to use proper exceptions

Using exceptions instead solves all the above problems, and allows catching the error outside the problematic code path.

If the desired outcome is to terminate the program with no possible way to recover one should use the exit() function with a string argument.

Therefore we propose to deprecate passing E_USER_ERROR to trigger_error()

Looks like exception handling in __toString() methods got fixed in PHP 7.4: https://3v4l.org/5ZJ2i. That means we can drop the try/catch and even consider to deprecate that trait? It's only used in two classes in core.

Yes, just 2 implementations to add, and deprecated, now needs to clean-up _drupal_error_handler_real(), maybe better to split out fixes for the remaining usage for scope

@Berdir thank you! it works and the only test needs to be adjusted

Drupal\Tests\Core\StringTranslation\TranslatableMarkupTest::testToString
Failed asserting that null matches expected 256.

core/tests/Drupal/Tests/Core/StringTranslation/TranslatableMarkupTest.php:77

There's just 4 usages in contrib, so needs to improve message and decide with test, polish CR

blocker for PHP 8.4 adoption

Probably deprecation of ToStringTrait better to move to separate issue or it needs second change record

Added to summary why most of time it needs to decide if code needs exception or logging instead of throwing user error as everyone thinks it should be fatal but there's exit() or exception re-thrown

Drupal does not stop execution for user-level errors, allowing the application to log and continue

I think for 10.4/11.1 we should change all these to E_USER_WARNING and then open follow-ups for each case to consider adding:

1. an assert() so it fails hard during development
2. An exception from 12.x onwards if that's appropriate

Related required for BigPipe fix

kim.pepper → made their first commit to this issue’s fork.

Created an MR for #13

Some failures looks fixable

MR!9924 is green now, so it needs follow-up to deprecate ToStringTrait