Enums cannot implement Serializable

src/Rules/Classes/EnumSanityRule.php

@@ -4,9 +4,11 @@
 
 use PhpParser\Node;
 use PHPStan\Analyser\Scope;
+use PHPStan\Reflection\ReflectionProvider;
 use PHPStan\Rules\Rule;
 use PHPStan\Rules\RuleErrorBuilder;
 use PHPStan\ShouldNotHappenException;
+use Serializable;
 use function array_key_exists;
 use function sprintf;
 
@@ -22,6 +24,12 @@ class EnumSanityRule implements Rule
 		'__invoke' => true,
 	];
 
+	public function __construct(
+		private ReflectionProvider $reflectionProvider,
+	)
+	{
+	}
+
 	public function getNodeType(): string
 	{
 		return Node\Stmt\Enum_::class;
@@ -103,6 +111,17 @@ public function processNode(Node $node, Scope $scope): array
 			))->line($node->scalarType->getLine())->nonIgnorable()->build();
 		}
 
+		if ($this->reflectionProvider->hasClass($node->namespacedName->toString())) {
+			$classReflection = $this->reflectionProvider->getClass($node->namespacedName->toString());
+
+			if ($classReflection->implementsInterface(Serializable::class)) {
+				$errors[] = RuleErrorBuilder::message(sprintf(
+					'Enum %s cannot implement the Serializable interface.',
+					$node->namespacedName->toString(),
+				))->line($node->getLine())->nonIgnorable()->build();
+			}
+		}
+
 		return $errors;
 	}
 

tests/PHPStan/Rules/Classes/EnumSanityRuleTest.php

@@ -14,13 +14,13 @@ class EnumSanityRuleTest extends RuleTestCase
 
 	protected function getRule(): Rule
 	{
-		return new EnumSanityRule();
+		return new EnumSanityRule($this->createReflectionProvider());
 	}
 
 	public function testRule(): void
 	{
-		if (PHP_VERSION_ID < 80000) {
-			$this->markTestSkipped('Test requires PHP 8.0');
+		if (PHP_VERSION_ID < 80100) {
+			$this->markTestSkipped('Test requires PHP 8.1');
 		}
 
 		$this->analyse([__DIR__ . '/data/enum-sanity.php'], [
@@ -76,6 +76,10 @@ public function testRule(): void
 				'Enum EnumSanity\EnumWithSerialize contains magic method __unserialize().',
 				81,
 			],
+			[
+				'Enum EnumSanity\EnumMayNotSerializable cannot implement the Serializable interface.',
+				86,
+			],
 		]);
 	}
 

tests/PHPStan/Rules/Classes/data/enum-sanity.php

@@ -1,4 +1,4 @@
-<?php
+<?php // lint >= 8.1
 
 namespace EnumSanity;
 
@@ -83,3 +83,10 @@ public function __unserialize(array $data) {
 	}
 }
 
+enum EnumMayNotSerializable implements \Serializable {
+
+	public function serialize() {
+	}
+	public function unserialize($data) {
+	}
+}