BC promise rule for trait use

conf/config.level0.neon

@@ -15,6 +15,8 @@ conditionalTags:
 		phpstan.rules.rule: %featureToggles.apiRules%
 	PHPStan\Rules\Api\ApiStaticCallRule:
 		phpstan.rules.rule: %featureToggles.apiRules%
+	PHPStan\Rules\Api\ApiTraitUseRule:
+		phpstan.rules.rule: %featureToggles.apiRules%
 	PHPStan\Rules\Api\PhpStanNamespaceIn3rdPartyPackageRule:
 		phpstan.rules.rule: %featureToggles.apiRules%
 	PHPStan\Rules\Functions\ClosureUsesThisRule:
@@ -90,6 +92,9 @@ services:
 	-
 		class: PHPStan\Rules\Api\ApiStaticCallRule
 
+	-
+		class: PHPStan\Rules\Api\ApiTraitUseRule
+
 	-
 		class: PHPStan\Rules\Api\PhpStanNamespaceIn3rdPartyPackageRule
 

src/Rules/Api/ApiTraitUseRule.php

@@ -0,0 +1,66 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Api;
+
+use PhpParser\Node;
+use PHPStan\Analyser\Scope;
+use PHPStan\Reflection\ReflectionProvider;
+use PHPStan\Rules\Rule;
+use PHPStan\Rules\RuleErrorBuilder;
+
+/**
+ * @implements Rule<Node\Stmt\TraitUse>
+ */
+class ApiTraitUseRule implements Rule
+{
+
+	private ApiRuleHelper $apiRuleHelper;
+
+	private ReflectionProvider $reflectionProvider;
+
+	public function __construct(
+		ApiRuleHelper $apiRuleHelper,
+		ReflectionProvider $reflectionProvider
+	)
+	{
+		$this->apiRuleHelper = $apiRuleHelper;
+		$this->reflectionProvider = $reflectionProvider;
+	}
+
+	public function getNodeType(): string
+	{
+		return Node\Stmt\TraitUse::class;
+	}
+
+	public function processNode(Node $node, Scope $scope): array
+	{
+		if ($this->apiRuleHelper->isInPhpStanNamespace($scope->getNamespace())) {
+			return [];
+		}
+
+		$errors = [];
+		$tip = sprintf(
+			"If you think it should be covered by backward compatibility promise, open a discussion:\n   %s\n\n   See also:\n   https://phpstan.org/developing-extensions/backward-compatibility-promise",
+			'https://github.com/phpstan/phpstan/discussions'
+		);
+		foreach ($node->traits as $traitName) {
+			$traitName = $traitName->toString();
+			if (!$this->reflectionProvider->hasClass($traitName)) {
+				continue;
+			}
+
+			$traitReflection = $this->reflectionProvider->getClass($traitName);
+			if (!$this->apiRuleHelper->isInPhpStanNamespace($traitReflection->getName())) {
+				continue;
+			}
+
+			$errors[] = RuleErrorBuilder::message(sprintf(
+				'Using %s is not covered by backward compatibility promise. The trait might change in a minor PHPStan version.',
+				$traitReflection->getDisplayName()
+			))->tip($tip)->build();
+		}
+
+		return $errors;
+	}
+
+}

tests/PHPStan/Rules/Api/ApiTraitUseRuleTest.php

@@ -0,0 +1,39 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\Rules\Api;
+
+use PHPStan\Testing\RuleTestCase;
+
+/**
+ * @extends RuleTestCase<ApiTraitUseRule>
+ */
+class ApiTraitUseRuleTest extends RuleTestCase
+{
+
+	protected function getRule(): \PHPStan\Rules\Rule
+	{
+		return new ApiTraitUseRule(new ApiRuleHelper(), $this->createReflectionProvider());
+	}
+
+	public function testRuleInPhpStan(): void
+	{
+		$this->analyse([__DIR__ . '/data/trait-use-in-phpstan.php'], []);
+	}
+
+	public function testRuleOutOfPhpStan(): void
+	{
+		$tip = sprintf(
+			"If you think it should be covered by backward compatibility promise, open a discussion:\n   %s\n\n   See also:\n   https://phpstan.org/developing-extensions/backward-compatibility-promise",
+			'https://github.com/phpstan/phpstan/discussions'
+		);
+
+		$this->analyse([__DIR__ . '/data/trait-use-out-of-phpstan.php'], [
+			[
+				'Using PHPStan\Type\JustNullableTypeTrait is not covered by backward compatibility promise. The trait might change in a minor PHPStan version.',
+				15,
+				$tip,
+			],
+		]);
+	}
+
+}

tests/PHPStan/Rules/Api/data/trait-use-in-phpstan.php

@@ -0,0 +1,12 @@
+<?php declare(strict_types = 1);
+
+namespace PHPStan\TraitUse;
+
+use PHPStan\Type\JustNullableTypeTrait;
+
+class Foo
+{
+
+	use JustNullableTypeTrait;
+
+}

tests/PHPStan/Rules/Api/data/trait-use-out-of-phpstan.php

@@ -0,0 +1,18 @@
+<?php declare(strict_types = 1);
+
+namespace App\TraitUse;
+
+use PHPStan\Type\JustNullableTypeTrait;
+
+trait FooTrait
+{
+
+}
+
+class Foo
+{
+
+	use JustNullableTypeTrait;
+	use FooTrait;
+
+}