Add @readonly rule that disallows default values

conf/config.level0.neon

@@ -6,6 +6,8 @@ conditionalTags:
 		phpstan.rules.rule: %featureToggles.nodeConnectingVisitorRule%
 	PHPStan\Rules\Properties\MissingReadOnlyByPhpDocPropertyAssignRule:
 		phpstan.rules.rule: %featureToggles.readOnlyByPhpDoc%
+	PHPStan\Rules\Properties\ReadOnlyByPhpDocPropertyRule:
+		phpstan.rules.rule: %featureToggles.readOnlyByPhpDoc%
 	PHPStan\Rules\Properties\UninitializedPropertyRule:
 		phpstan.rules.rule: %checkUninitializedProperties%
 	PHPStan\Rules\Methods\ConsistentConstructorRule:
@@ -188,6 +190,9 @@ services:
 		tags:
 			- phpstan.rules.rule
 
+	-
+		class: PHPStan\Rules\Properties\ReadOnlyByPhpDocPropertyRule
+
 	-
 		class: PHPStan\Rules\Properties\UninitializedPropertyRule
 

src/Rules/Properties/ReadOnlyByPhpDocPropertyRule.php

@@ -0,0 +1,36 @@
+<?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 ReadOnlyByPhpDocPropertyRule implements Rule
+{
+
+	public function getNodeType(): string
+	{
+		return ClassPropertyNode::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		if (!$node->isReadOnlyByPhpDoc()) {
+			return [];
+		}
+
+		$errors = [];
+		if ($node->getDefault() !== null) {
+			 $errors[] = RuleErrorBuilder::message('@readonly property cannot have a default value.')->nonIgnorable()->build();
+		}
+
+		return $errors;
+	}
+
+}

tests/PHPStan/Rules/Properties/ReadOnlyByPhpDocPropertyRuleTest.php

@@ -0,0 +1,29 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Properties;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<ReadOnlyByPhpDocPropertyRule>
+ */
+class ReadOnlyByPhpDocPropertyRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new ReadOnlyByPhpDocPropertyRule();
+	}
+
+	public function testRule(): void
+	{
+		$this->analyse([__DIR__ . '/data/read-only-property-phpdoc.php'], [
+			[
+				'@readonly property cannot have a default value.',
+				21,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Properties/data/read-only-property-phpdoc.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace ReadOnlyPropertyPhpDoc;
+
+class Foo
+{
+
+	/**
+	 * @readonly
+	 * @var int
+	 */
+	private $foo;
+
+	/** @readonly */
+	private $bar;
+
+	/**
+	 * @readonly
+	 * @var int
+	 */
+	private $baz = 0;
+
+}