Arrow function - check nullsafe returned by ref

Author: ondrejmirtes

build.xml

@@ -133,6 +133,8 @@
 			<arg path="tests/PHPStan/Rules/Properties/data/default-value-for-promoted-property.php"/>
 			<arg value="--exclude"/>
 			<arg path="tests/PHPStan/Rules/Operators/data/invalid-assign-var.php"/>
+			<arg value="--exclude"/>
+			<arg path="tests/PHPStan/Rules/Functions/data/arrow-function-nullsafe-by-ref.php"/>
 			<arg path="src" />
 			<arg path="tests" />
 			<arg path="compiler/src" />

conf/config.level0.neon

@@ -30,6 +30,7 @@ rules:
 	- PHPStan\Rules\Classes\InvalidPromotedPropertiesRule
 	- PHPStan\Rules\Classes\NewStaticRule
 	- PHPStan\Rules\Exceptions\ThrowExpressionRule
+	- PHPStan\Rules\Functions\ArrowFunctionReturnNullsafeByRefRule
 	- PHPStan\Rules\Functions\CallToFunctionParametersRule
 	- PHPStan\Rules\Functions\ExistingClassesInArrowFunctionTypehintsRule
 	- PHPStan\Rules\Functions\ExistingClassesInClosureTypehintsRule

src/Rules/Functions/ArrowFunctionReturnNullsafeByRefRule.php

@@ -0,0 +1,44 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Functions;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Rules\NullsafeCheck;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+
+/**
+ * @implements Rule<Node\Expr\ArrowFunction>
+ */
+class ArrowFunctionReturnNullsafeByRefRule implements Rule
+{
+
+	private NullsafeCheck $nullsafeCheck;
+
+	public function __construct(NullsafeCheck $nullsafeCheck)
+	{
+		$this->nullsafeCheck = $nullsafeCheck;
+	}
+
+	public function getNodeType(): string
+	{
+		return Node\Expr\ArrowFunction::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		if (!$node->byRef) {
+			return [];
+		}
+
+		if (!$this->nullsafeCheck->containsNullSafe($node->expr)) {
+			return [];
+		}
+
+		return [
+			RuleErrorBuilder::message('Nullsafe cannot be returned by reference.')->nonIgnorable()->build(),
+		];
+	}
+
+}

tests/PHPStan/Rules/Functions/ArrowFunctionReturnNullsafeByRefRuleTest.php

@@ -0,0 +1,34 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Functions;
+
+use PHPStan\Rules\NullsafeCheck;
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<ArrowFunctionReturnNullsafeByRefRule>
+ */
+class ArrowFunctionReturnNullsafeByRefRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new ArrowFunctionReturnNullsafeByRefRule(new NullsafeCheck());
+	}
+
+	public function testRule(): void
+	{
+		if (!self::$useStaticReflectionProvider) {
+			$this->markTestSkipped('Test requires static reflection.');
+		}
+
+		$this->analyse([__DIR__ . '/data/arrow-function-nullsafe-by-ref.php'], [
+			[
+				'Nullsafe cannot be returned by reference.',
+				6,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Functions/data/arrow-function-nullsafe-by-ref.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace ArrowFunctionNullsafeReturnByRef;
+
+function (\stdClass $foo): void {
+	fn &() => $foo?->bar;
+};
+
+function (\stdClass $foo): void {
+	fn &() => $foo->bar;
+};
+
+function (\stdClass $foo): void {
+	fn () => $foo?->bar;
+};