Detect properties declared in interface

Makefile

@@ -40,6 +40,8 @@ lint:
 		--exclude tests/PHPStan/Rules/Functions/data/arrow-function-nullsafe-by-ref.php \
 		--exclude tests/PHPStan/Levels/data/namedArguments.php \
 		--exclude tests/PHPStan/Rules/Keywords/data/continue-break.php \
+		--exclude tests/PHPStan/Rules/Properties/data/bug-8915.php \
+		--exclude tests/PHPStan/Rules/Properties/data/properties-in-interface.php \
 		--exclude tests/PHPStan/Rules/Properties/data/read-only-property.php \
 		--exclude tests/PHPStan/Rules/Properties/data/read-only-property-phpdoc-and-native.php   \
 		--exclude tests/PHPStan/Rules/Properties/data/read-only-property-readonly-class.php \

conf/config.level0.neon

@@ -88,6 +88,7 @@ rules:
 	- PHPStan\Rules\Properties\AccessPropertiesInAssignRule
 	- PHPStan\Rules\Properties\AccessStaticPropertiesInAssignRule
 	- PHPStan\Rules\Properties\MissingReadOnlyPropertyAssignRule
+	- PHPStan\Rules\Properties\PropertiesInInterfaceRule
 	- PHPStan\Rules\Properties\PropertyAttributesRule
 	- PHPStan\Rules\Properties\ReadOnlyPropertyRule
 	- PHPStan\Rules\Traits\ConstantsInTraitsRule

src/Rules/Properties/PropertiesInInterfaceRule.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Properties;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Node\ClassPropertyNode;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+
+/**
+ * @implements Rule<ClassPropertyNode>
+ */
+class PropertiesInInterfaceRule implements Rule
+{
+
+	public function getNodeType(): string
+	{
+		return ClassPropertyNode::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		if (!$node->getClassReflection()->isInterface()) {
+			return [];
+		}
+
+		return [
+			RuleErrorBuilder::message('Interfaces may not include properties.')->nonIgnorable()->build(),
+		];
+	}
+
+}

tests/PHPStan/Rules/Properties/PropertiesInInterfaceRuleTest.php

@@ -0,0 +1,33 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Properties;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<PropertiesInInterfaceRule>
+ */
+class PropertiesInInterfaceRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new PropertiesInInterfaceRule();
+	}
+
+	public function testRule(): void
+	{
+		$this->analyse([__DIR__ . '/data/properties-in-interface.php'], [
+			[
+				'Interfaces may not include properties.',
+				7,
+			],
+			[
+				'Interfaces may not include properties.',
+				9,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Properties/data/properties-in-interface.php

@@ -0,0 +1,10 @@
+<?php declare(strict_types=1);
+
+namespace PropertiesInInterface;
+
+interface HelloWorld
+{
+    public \DateTimeInterface $dateTime;
+
+    public static \Closure $callable;
+}