Make assigning property via reference impure

src/Analyser/ImpurePoint.php

@@ -6,7 +6,7 @@
 use PHPStan\Node\VirtualNode;
 
 /**
- * @phpstan-type ImpurePointIdentifier = 'echo'|'die'|'exit'|'propertyAssign'|'propertyUnset'|'methodCall'|'new'|'functionCall'|'include'|'require'|'print'|'eval'|'superglobal'|'yield'|'yieldFrom'|'static'|'global'|'betweenPhpTags'
+ * @phpstan-type ImpurePointIdentifier = 'echo'|'die'|'exit'|'propertyAssign'|'propertyAssignByRef'|'propertyUnset'|'methodCall'|'new'|'functionCall'|'include'|'require'|'print'|'eval'|'superglobal'|'yield'|'yieldFrom'|'static'|'global'|'betweenPhpTags'
  * @api
  */
 class ImpurePoint

src/Analyser/NodeScopeResolver.php

@@ -2138,7 +2138,23 @@ public function processExprNode(Node\Stmt $stmt, Expr $expr, MutatingScope $scop
 				$nodeCallback,
 				$context,
 				function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $context): ExpressionResult {
+					$impurePoints = [];
 					if ($expr instanceof AssignRef) {
+						$referencedExpr = $expr->expr;
+						while ($referencedExpr instanceof ArrayDimFetch) {
+							$referencedExpr = $referencedExpr->var;
+						}
+
+						if ($referencedExpr instanceof PropertyFetch || $referencedExpr instanceof StaticPropertyFetch) {
+							$impurePoints[] = new ImpurePoint(
+								$scope,
+								$expr,
+								'propertyAssignByRef',
+								'property assignment by reference',
+								false,
+							);
+						}
+
 						$scope = $scope->enterExpressionAssign($expr->expr);
 					}
 
@@ -2153,7 +2169,7 @@ function (MutatingScope $scope) use ($stmt, $expr, $nodeCallback, $context): Exp
 					$result = $this->processExprNode($stmt, $expr->expr, $scope, $nodeCallback, $context->enterDeep());
 					$hasYield = $result->hasYield();
 					$throwPoints = $result->getThrowPoints();
-					$impurePoints = $result->getImpurePoints();
+					$impurePoints = array_merge($impurePoints, $result->getImpurePoints());
 					$scope = $result->getScope();
 
 					if ($expr instanceof AssignRef) {

tests/PHPStan/Rules/Pure/PureMethodRuleTest.php

@@ -157,4 +157,14 @@ public function testPureConstructor(): void
 		]);
 	}
 
+	public function testImpureAssignRef(): void
+	{
+		$this->analyse([__DIR__ . '/data/impure-assign-ref.php'], [
+			[
+				'Possibly impure property assignment by reference in pure method ImpureAssignRef\HelloWorld::bar6().',
+				49,
+			],
+		]);
+	}
+
 }

tests/PHPStan/Rules/Pure/data/impure-assign-ref.php

@@ -0,0 +1,63 @@
+<?php declare(strict_types = 1); // lint >= 7.4
+
+namespace ImpureAssignRef;
+
+class HelloWorld
+{
+	public int $value = 0;
+	/** @var array<int, int> */
+	public array $arr = [];
+	/** @var array<int, \stdClass> */
+	public array $objectArr = [];
+	public static int $staticValue = 0;
+	/** @var array<int, int> */
+	public static array $staticArr = [];
+
+	private function bar1(): void
+	{
+		$value = &$this->value;
+		$value = 1;
+	}
+
+	private function bar2(): void
+	{
+		$value = &$this->arr[0];
+		$value = 1;
+	}
+
+	private function bar3(): void
+	{
+		$value = &self::$staticValue;
+		$value = 1;
+	}
+
+	private function bar4(): void
+	{
+		$value = &self::$staticArr[0];
+		$value = 1;
+	}
+
+	private function bar5(self $other): void
+	{
+		$value = &$other->value;
+		$value = 1;
+	}
+
+	/** @phpstan-pure */
+	private function bar6(): int
+	{
+		$value = &$this->objectArr[0]->foo;
+
+		return 1;
+	}
+
+	public function foo(): void
+	{
+		$this->bar1();
+		$this->bar2();
+		$this->bar3();
+		$this->bar4();
+		$this->bar5(new self());
+		$this->bar6();
+	}
+}