Detect unused constructor method calls

Author: ondrejmirtes

conf/config.level4.neon

@@ -8,6 +8,7 @@ rules:
 	- PHPStan\Rules\DeadCode\UnreachableStatementRule
 	- PHPStan\Rules\Exceptions\DeadCatchRule
 	- PHPStan\Rules\Functions\CallToFunctionStamentWithoutSideEffectsRule
+	- PHPStan\Rules\Methods\CallToConstructorStatementWithoutSideEffectsRule
 	- PHPStan\Rules\Methods\CallToMethodStamentWithoutSideEffectsRule
 	- PHPStan\Rules\Methods\CallToStaticMethodStamentWithoutSideEffectsRule
 	- PHPStan\Rules\TooWideTypehints\TooWideArrowFunctionReturnTypehintRule

src/Rules/Methods/CallToConstructorStatementWithoutSideEffectsRule.php

@@ -0,0 +1,64 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Methods;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+
+/**
+ * @implements \PHPStan\Rules\Rule<\PhpParser\Node\Stmt\Expression>
+ */
+class CallToConstructorStatementWithoutSideEffectsRule implements Rule
+{
+
+	private ReflectionProvider $reflectionProvider;
+
+	public function __construct(ReflectionProvider $reflectionProvider)
+	{
+		$this->reflectionProvider = $reflectionProvider;
+	}
+
+	public function getNodeType(): string
+	{
+		return Node\Stmt\Expression::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		if (!$node->expr instanceof Node\Expr\New_) {
+			return [];
+		}
+
+		$instantiation = $node->expr;
+		if (!$instantiation->class instanceof Node\Name) {
+			return [];
+		}
+
+		$className = $scope->resolveName($instantiation->class);
+		if (!$this->reflectionProvider->hasClass($className)) {
+			return [];
+		}
+
+		$classReflection = $this->reflectionProvider->getClass($className);
+		if (!$classReflection->hasConstructor()) {
+			return [];
+		}
+
+		$constructor = $classReflection->getConstructor();
+		if ($constructor->hasSideEffects()->no()) {
+			return [
+				RuleErrorBuilder::message(sprintf(
+					'Call to %s::%s() on a separate line has no effect.',
+					$classReflection->getDisplayName(),
+					$constructor->getName()
+				))->build(),
+			];
+		}
+
+		return [];
+	}
+
+}

tests/PHPStan/Rules/Methods/CallToConstructorStatementWithoutSideEffectsRuleTest.php

@@ -0,0 +1,29 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Methods;
+
+use PHPStan\Rules\Rule;
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<CallToConstructorStatementWithoutSideEffectsRule>
+ */
+class CallToConstructorStatementWithoutSideEffectsRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): Rule
+	{
+		return new CallToConstructorStatementWithoutSideEffectsRule($this->createReflectionProvider());
+	}
+
+	public function testRule(): void
+	{
+		$this->analyse([__DIR__ . '/data/constructor-statement-no-side-effects.php'], [
+			[
+				'Call to Exception::__construct() on a separate line has no effect.',
+				6,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Methods/data/constructor-statement-no-side-effects.php

@@ -0,0 +1,13 @@
+<?php
+
+namespace ConstructorStatementNoSideEffects;
+
+function () {
+	new \Exception();
+	throw new \Exception();
+};
+
+function () {
+	new \PDOStatement();
+	new \stdClass();
+};