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Custom Rules

PHPStan allows writing custom rules to check for specific situations in your own codebase. The implementation is all about applying the core concepts so check out that guide first and then continue here.

Motivation #

We want to avoid dangerous and unwanted situations in our code.

class Person
{

/**
* !!! DO NOT INSTANTIATE Person DIRECTLY! Use PersonFactory instead !!!
*/

public function __construct(string $email)
{

}

}

If a piece of code is fine from the point of the programming language and thus not detected by core PHPStan rules, but does something that we want to avoid because of our experience with the project’s codebase, we should write a custom PHPStan rule so that the situation is detected automatically and we don’t have to rely on human code review - it’s unreliable and we shouldn’t keep our brains busy with something that computers do better.

In the example above we’d like to disallow new Person().

The Rule interface #

Custom rules are classes implementing the PHPStan\Rules\Rule interface. The interface has two methods:

  • public function getNodeType(): string
  • public function processNode(PhpParser\Node $node, PHPStan\Analyser\Scope $scope): array

The getNodeType() method returns the AST node type. Every time the analyser encounters the node of this type, the second method processNode() is called with that AST node as the first argument, and the current Scope as the second argument. The goal of the processNode() method is to return an array of errors to report to the user.

Choosing the right AST node #

People unfamiliar with the AST don’t know which node type to return from the getNodeType() method to get started. Fortunately we can follow a simple guide to find the right type.

Write a small piece of code that contains the situation we want the rule to report:

class Foo
{

public function doFoo(): void
{
$p = new Person();
}

}

Create the rule class and make sure the namespace, the class name, and the rule’s location match your autoloading rules:

namespace App;

use PhpParser\Node;
use PHPStan\Analyser\Scope;

class MyRule implements \PHPStan\Rules\Rule
{

public function getNodeType(): string
{
// todo
}

public function processNode(Node $node, Scope $scope): array
{
// todo
}

}

Register it in your configuration file:

rules:
- App\MyRule

Return \PhpParser\Node::class from the getNodeType() method. This is the common interface to all AST nodes:

public function getNodeType(): string
{
return \PhpParser\Node::class;
}

And dump all the node types the rule is called with:

public function processNode(Node $node, Scope $scope): array
{
var_dump(get_class($node));
return [];
}

When you run PHPStan with this rule registered on the small sample file we wrote above:

vendor/bin/phpstan analyse -l 8 test.php

You’ll see PHP printing all the nodes the rule is called with:

string(21) "PHPStan\Node\FileNode"
string(26) "PhpParser\Node\Stmt\Class_"
string(24) "PHPStan\Node\InClassNode"
string(31) "PhpParser\Node\Stmt\ClassMethod"
string(25) "PhpParser\Node\Identifier"
string(30) "PHPStan\Node\InClassMethodNode"
string(30) "PhpParser\Node\Stmt\Expression"
string(26) "PhpParser\Node\Expr\Assign"
string(28) "PhpParser\Node\Expr\Variable"
string(24) "PhpParser\Node\Expr\New_"
string(29) "PHPStan\Node\ExecutionEndNode"
string(39) "PHPStan\Node\MethodReturnStatementsNode"
string(32) "PHPStan\Node\ClassPropertiesNode"
string(29) "PHPStan\Node\ClassMethodsNode"
string(31) "PHPStan\Node\ClassConstantsNode"

The method body ($p = new Person();) is represented by these four nodes:

string(30) "PhpParser\Node\Stmt\Expression"
string(26) "PhpParser\Node\Expr\Assign"
string(28) "PhpParser\Node\Expr\Variable"
string(24) "PhpParser\Node\Expr\New_"

The new keyword is represented by the PhpParser\Node\Expr\New_ node, and that’s the one we’re interested in. Let’s update our getNodeType(): string method:

public function getNodeType(): string
{
return Node\Expr\New_::class;
}

To have zero errors when analysing the rule itself with PHPStan, we also need to specify the TNodeType template type of the implemented Rule interface:

/**
* @implements \PHPStan\Rules\Rule<Node\Expr\New_>
*/

class MyRule implements \PHPStan\Rules\Rule
{
...

The New_ node class looks like this:

class New_ extends Expr
{
/** @var Node\Name|Expr|Node\Stmt\Class_ Class name */
public $class;

/** @var Node\Arg[] Arguments */
public $args;

...
}

The $class property can be three different types:

  • A Name instance describing the usual new Foo()
  • An Expr instance describing the more dynamic new $foo()
  • A Class_ instance describing an anonymous class: new class () {}

For the purpose of our rule we’re interested only in the first case.

Implementing the rule logic #

Our processNode() method is now called only for the New_ nodes. We need filter out the cases instantiating other node types than Name, and also the cases that don’t instantiate the Person class. We also want to allow new Person() in PersonFactory itself:

public function processNode(Node $node, Scope $scope): array
{
if (!$node->class instanceof Node\Name) {
return [];
}

if ($node->class->toString() !== Person::class) {
return [];
}

if (
$scope->isInClass()
&& $scope->getClassReflection()->getName() === PersonFactory::class
) {
return [];
}

...
}

What’s left to do is to return an array of errors to report. The processNode() method can return either an array of strings, or an array of RuleError instances. These can’t be instantiated directly, but only through PHPStan\Rules\RuleErrorBuilder. It allows to attach more information to errors like a different error line, or making the error non-ignorable. The simplest way to use it is to create an instance using the static message(string $message): RuleErrorBuilder method and return RuleError instance using the build(): RuleError method:

return [
RuleErrorBuilder::message(
'New Person instance can be created only in PersonFactory.'
)->build(),
];

Registering the rule in the configuration #

PHPStan needs to know it’s supposed to execute the rule. The simplest way is to add the rule class to the rules section if your configuration file:

rules:
- App\MyRule

This only works if the rule class doesn’t have a constructor, or all the constructor parameters can be autowired. If we have non-autowirable parameters in the constructor, we need to resort the rule as a service in the services section of the configuration file, specify the arguments, and the phpstan.rules.rule tag:

	-
class: App\MyRule
arguments:
reportMaybes: %reportMaybes%
tags:
- phpstan.rules.rule

Learn more about dependency injection and configuration »

Testing the rule #

PHPStan supports testing with the industry standard PHPUnit framework. Custom rules can be tested in a test case extended from PHPStan\Testing\RuleTestCase. A typical test can look like this:

<?php declare(strict_types = 1);

namespace App;

use PHPStan\Testing\RuleTestCase;

/**
* @extends RuleTestCase<MyRule>
*/

class MyRuleTest extends RuleTestCase
{

protected function getRule(): \PHPStan\Rules\Rule
{
// getRule() method needs to return an instance of the tested rule
return new MyRule();
}

public function testRule(): void
{
// first argument: path to the example file that contains some errors that should be reported by MyRule
// second argument: an array of expected errors,
// each error consists of the asserted error message, and the asserted error file line
$this->analyse([__DIR__ . '/data/my-rule.php'], [
[
'X should not be Y', // asserted error message
15, // asserted error line
],
]);

// the test fails, if the expected error does not occur,
// or if there are other errors reported beside the expected one
}

}

Virtual nodes #

You might have noticed that some of the nodes in the Choosing the right AST node section do not come from the PHP-Parser library but from PHPStan itself.

Some situations we want to report are hard to detect with the out-of-the-box AST. For example if we want to report that’s something is missing in the AST (like a return statement), there’s no node type to register our rule for.

That’s why PHPStan introduces custom virtual nodes to use in rules. Some of the common use-cases are:

  • We need to check something is at the beginning of each file, like declare(strict_types = 1) or a required namespace. We can use FileNode for that.
  • Rules are called before the scope is updated with the information of the registered node type. That means if we’re registering our rule for the PhpParser\Node\Stmt\Class_ node, the scope will say we’re not in a class. But if we use the virtual InClassNode, $scope->getClassReflection() will contain the class reflection. Same goes for InClassMethodNode and InFunctionNode.
  • If we’re interested in class properties regardless of whether they’re traditional or promoted, we can use the ClassPropertyNode.

More custom rules examples #

Disallow comparing DateTime objects with === and !==

Comparing DateTime and DateTimeImmutable objects with === and !== is probably a mistake made by the developer, because === only ever returns true if it compares the same object instance. But in case of DateTime instances, we probably want to compare the time values they contain. Operators == and != should be used for that instead.

use PhpParser\Node;
use PhpParser\Node\Expr\BinaryOp;
use PHPStan\Analyser\Scope;
use PHPStan\Rules\Rule;
use PHPStan\Type\ObjectType;

/**
* @implements Rule<BinaryOp>
*/

class CompareDateTimeRule implements Rule
{

public function getNodeType(): string
{
return BinaryOp::class;
}

public function processNode(Node $node, Scope $scope): array
{
if (
!$node instanceof BinaryOp\Identical
&& !$node instanceof BinaryOp\NotIdentical
) {
return [];
}

$leftType = $scope->getType($node->left);
$rightType = $scope->getType($node->right);
$dateTimeType = new ObjectType(\DateTimeInterface::class);

if (
!$dateTimeType->isSuperTypeOf($leftType)->yes()
|| !$dateTimeType->isSuperTypeOf($rightType)->yes()
) {
return [];
}

return [
RuleErrorBuilder::message(sprintf(
'Cannot compare DateTime instances with %s.',
$node->getOperatorSigil()
))->build(),
];
}

}
Disallow assigning properties outside of constructor
use PhpParser\Node;
use PHPStan\Analyser\Scope;
use PHPStan\Reflection\MethodReflection;
use PHPStan\Rules\Rule;
use PHPStan\Rules\RuleErrorBuilder;

/**
* @implements Rule<Node\Expr\Assign>
*/

class ImmutableObjectRule implements Rule
{

public function getNodeType(): string
{
// we're interested only in assignments
return Node\Expr\Assign::class;
}

public function processNode(Node $node, Scope $scope): array
{
if (!$scope->isInClass()) {
// perhaps an error should be reported here too
// if we want to disallow assigning properties outside of classes
return [];
}
if (!$node->var instanceof Node\Expr\PropertyFetch) {
// we're interested only in property on the left side of assignment
// so variable assignments and similar are ignored by this rule
return [];
}
if (!$node->var->name instanceof Node\Identifier) {
// we're interested only in non-dynamic fetches like $this->foo
return [];
}
$inMethod = $scope->getFunction();
if (!$inMethod instanceof MethodReflection) {
return [];
}
if ($inMethod->getName() === '__construct') {
// nothing should be reported if we're in a constructor
return [];
}

return [
RuleErrorBuilder::message(sprintf(
'Immutability violated - assigning $%s property outside constructor.',
$node->var->name->toString()
))->build(),
];
}

}

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