minor #42135 [DoctrineBridge] Backport workaround for closures (derrabus)

This PR was squashed before being merged into the 5.4 branch.

Discussion
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[DoctrineBridge] Backport workaround for closures

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | https://github.com/symfony/symfony/pull/42109/files#r670585475
| License       | MIT
| Doc PR        | N/A

Backported from #42109.

`DoctrineChoiceLoader` inspects the callable `$value` to check if it's an array where it can extract an `IdReader` from. For 6.0, we want to wrap that callable in a closure. If we do so, that check fails silently, breaking the optimization code here which means that all entities are being loaded (instead of just the selected one).

I "fixed" that by extracting the `IdReader` from the closure now, but I really believe `DoctrineChoiceLoader` should not make that kind of assumption.

Commits
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b474c39f78 [DoctrineBridge] Backport workaround for closures

Author: derrabus

Form/ChoiceList/DoctrineChoiceLoader.php

@@ -91,18 +91,27 @@ protected function doLoadChoicesForValues(array $values, ?callable $value): arra
             trigger_deprecation('symfony/doctrine-bridge', '5.1', 'Not defining explicitly the IdReader as value callback when query can be optimized is deprecated. Don\'t pass the IdReader to "%s" or define the "choice_value" option instead.', __CLASS__);
         }
 
+        $idReader = null;
+        if (\is_array($value) && $value[0] instanceof IdReader) {
+            $idReader = $value[0];
+        } elseif ($value instanceof \Closure && ($rThis = (new \ReflectionFunction($value))->getClosureThis()) instanceof IdReader) {
+            $idReader = $rThis;
+        } elseif ($legacy) {
+            $idReader = $this->idReader;
+        }
+
         // Optimize performance in case we have an object loader and
         // a single-field identifier
-        if (($legacy || \is_array($value) && $this->idReader === $value[0]) && $this->objectLoader) {
+        if ($idReader && $this->objectLoader) {
             $objects = [];
             $objectsById = [];
 
             // Maintain order and indices from the given $values
             // An alternative approach to the following loop is to add the
             // "INDEX BY" clause to the Doctrine query in the loader,
             // but I'm not sure whether that's doable in a generic fashion.
-            foreach ($this->objectLoader->getEntitiesByIds($this->idReader->getIdField(), $values) as $object) {
-                $objectsById[$this->idReader->getIdValue($object)] = $object;
+            foreach ($this->objectLoader->getEntitiesByIds($idReader->getIdField(), $values) as $object) {
+                $objectsById[$idReader->getIdValue($object)] = $object;
             }
 
             foreach ($values as $i => $id) {