[12.x] Ensures casts objects can be transformed into strings (#56687)

* [12.x] Adds Cast as class instances

* Modifies exception message

* Add tests

* Style changes [skip ci]

* Removes CastAttributes magic as being unreliable

* Fixes test exception message.

* Update HasAttributes.php

* Update DatabaseEloquentModelTest.php

---------

Co-authored-by: Taylor Otwell <[email protected]>

Author: DarkGhostHunter

src/Illuminate/Database/Eloquent/Concerns/HasAttributes.php

@@ -11,6 +11,7 @@
 use DateTimeImmutable;
 use DateTimeInterface;
 use Illuminate\Contracts\Database\Eloquent\Castable;
+use Illuminate\Contracts\Database\Eloquent\CastsAttributes;
 use Illuminate\Contracts\Database\Eloquent\CastsInboundAttributes;
 use Illuminate\Contracts\Support\Arrayable;
 use Illuminate\Database\Eloquent\Casts\AsArrayObject;
@@ -41,6 +42,7 @@
 use ReflectionMethod;
 use ReflectionNamedType;
 use RuntimeException;
+use Stringable;
 use ValueError;
 
 use function Illuminate\Support\enum_value;
@@ -790,6 +792,13 @@ protected function ensureCastsAreStringValues($casts)
     {
         foreach ($casts as $attribute => $cast) {
             $casts[$attribute] = match (true) {
+                is_object($cast) => value(function () use ($cast, $attribute) {
+                    return $cast instanceof Stringable
+                        ? (string) $cast
+                        : throw new InvalidArgumentException(
+                            "The cast object for the {$attribute} attribute must implement Stringable."
+                        );
+                }),
                 is_array($cast) => value(function () use ($cast) {
                     if (count($cast) === 1) {
                         return $cast[0];

tests/Database/DatabaseEloquentModelTest.php

@@ -2,6 +2,7 @@
 
 namespace Illuminate\Tests\Database;
 
+use Closure;
 use DateTime;
 use DateTimeImmutable;
 use DateTimeInterface;
@@ -59,6 +60,7 @@
 use PHPUnit\Framework\TestCase;
 use ReflectionClass;
 use stdClass;
+use Stringable as NativeStringable;
 
 include_once 'Enums.php';
 
@@ -3350,6 +3352,37 @@ public function testCastOnArrayFormatWithOneElement()
         $this->assertEquals(['bar' => 'foo'], $model->getAttribute('singleElementInArrayAttribute')->toArray());
     }
 
+    public function testUsingStringableObjectCastUsesStringRepresentation()
+    {
+        $model = new EloquentModelCastingStub;
+
+        $this->assertEquals('int', $model->getCasts()['castStringableObject']);
+    }
+
+    public function testMergeingStringableObjectCastUSesStringRepresentation()
+    {
+        $stringable = new StringableCastBuilder();
+        $stringable->cast = 'test';
+
+        $model = (new EloquentModelCastingStub)->mergeCasts([
+            'something' => $stringable
+        ]);
+
+        $this->assertEquals('test', $model->getCasts()['something']);
+    }
+
+    public function testUsingPlainObjectAsCastThrowsException()
+    {
+        $model = new EloquentModelCastingStub;
+
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The cast object for the something attribute must implement Stringable.');
+
+        $model->mergeCasts([
+            'something' => (object) [],
+        ]);
+    }
+
     public function testUnsavedModel()
     {
         $user = new UnsavedModel;
@@ -3918,6 +3951,7 @@ protected function casts(): array
             'singleElementInArrayAttribute' => [AsCollection::class],
             'duplicatedAttribute' => 'int',
             'asToObjectCast' => TestCast::class,
+            'castStringableObject' => new StringableCastBuilder(),
         ];
     }
 
@@ -4384,3 +4418,13 @@ class EloquentChildModelBootingCallbackTestStub extends EloquentModelBootingCall
 {
     public static bool $bootHasFinished = false;
 }
+
+class StringableCastBuilder implements NativeStringable
+{
+    public $cast = 'int';
+
+    public function __toString()
+    {
+        return $this->cast;
+    }
+}