Report error when $this is re-assigned

- Added $this reassignment check to InvalidAssignVarRule
- Handles direct assignment ($this = ...), compound assignment ($this .= ...), and destructuring ([$this] = ...)
- Error is reported in all contexts (instance methods, static methods, functions)
- New regression test in tests/PHPStan/Rules/Operators/data/bug-3585.php

Closes phpstan/phpstan#3585

Author: VincentLanglet

src/Rules/Operators/InvalidAssignVarRule.php

@@ -66,9 +66,38 @@ public function processNode(Node $node, Scope $scope): array
 			];
 		}
 
+		if ($this->containsThisVariable($node->var)) {
+			return [
+				RuleErrorBuilder::message('Cannot re-assign $this.')
+					->identifier('assign.this')
+					->nonIgnorable()
+					->build(),
+			];
+		}
+
 		return [];
 	}
 
+	private function containsThisVariable(Expr $expr): bool
+	{
+		if ($expr instanceof Expr\Variable && $expr->name === 'this') {
+			return true;
+		}
+
+		if ($expr instanceof Expr\List_) {
+			foreach ($expr->items as $item) {
+				if ($item === null) {
+					continue;
+				}
+				if ($this->containsThisVariable($item->value)) {
+					return true;
+				}
+			}
+		}
+
+		return false;
+	}
+
 	private function containsNonAssignableExpression(Expr $expr): bool
 	{
 		if ($expr instanceof Expr\Variable) {

tests/PHPStan/Rules/Operators/InvalidAssignVarRuleTest.php

@@ -17,6 +17,36 @@ protected function getRule(): Rule
 		return new InvalidAssignVarRule(new NullsafeCheck());
 	}
 
+	public function testBug3585(): void
+	{
+		$this->analyse([__DIR__ . '/data/bug-3585.php'], [
+			[
+				'Cannot re-assign $this.',
+				9,
+			],
+			[
+				'Cannot re-assign $this.',
+				10,
+			],
+			[
+				'Cannot re-assign $this.',
+				11,
+			],
+			[
+				'Cannot re-assign $this.',
+				12,
+			],
+			[
+				'Cannot re-assign $this.',
+				17,
+			],
+			[
+				'Cannot re-assign $this.',
+				23,
+			],
+		]);
+	}
+
 	public function testRule(): void
 	{
 		$this->analyse([__DIR__ . '/data/invalid-assign-var.php'], [

tests/PHPStan/Rules/Operators/data/bug-3585.php

@@ -0,0 +1,24 @@
+<?php declare(strict_types = 1);
+
+namespace Bug3585;
+
+class Foo
+{
+	public function doFoo(): void
+	{
+		$this = 1;
+		$this = new self();
+		$this .= 'foo';
+		[$this] = [1];
+	}
+
+	public static function doBar(): void
+	{
+		$this = 1; // allowed in static context? Actually no, PHP still forbids it
+	}
+}
+
+function baz(): void
+{
+	$this = 1; // PHP forbids this too
+}