Create a HaveCorrespondingUnit expression

This will allow us to ensure that certain classes always have a test,
or that every test has a matching class and their namespaces are correct.

README.md

@@ -152,6 +152,25 @@ For example: `phparkitect debug:expression ResideInOneOfTheseNamespaces App`
 
 Currently, you can check if a class:
 
+### Has a corresponding code unit in another namespace
+
+This will allow us to ensure that certain classes always have a test,
+or that every test has a matching class and their namespaces are correct.
+
+```php
+$rules = Rule::allClasses()
+    ->that(new ResideInOneOfTheseNamespaces('App\Core\Component\**\Command\*'))
+    ->should(new HaveCorrespondingUnit(
+            // This will assert that class `App\Core\Component\MyComponent\Command\MyCommand`
+            // has a test class in `Tests\App\Core\Component\MyComponent\Command\MyCommandTest`
+            function ($fqcn) {
+                return 'Tests\'.$fqcn.'Test';
+            }
+        )
+    )
+    ->because('we want all our command handlers to have a test');
+```
+
 ### Depends on a namespace
 
 ```php

src/Expression/ForClasses/HaveCorrespondingUnit.php

@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Expression\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Expression\Description;
+use Arkitect\Expression\Expression;
+use Arkitect\Rules\Violation;
+use Arkitect\Rules\Violations;
+
+final class HaveCorrespondingUnit implements Expression
+{
+    /** @var \Closure */
+    private $inferFqnFunction;
+
+    public function __construct(\Closure $inferFqnFunction)
+    {
+        $this->inferFqnFunction = $inferFqnFunction;
+    }
+
+    public function describe(ClassDescription $theClass, string $because): Description
+    {
+        $correspondingFqn = $this->inferCorrespondingFqn($theClass);
+
+        return new Description("should have a matching unit named: '$correspondingFqn'", $because);
+    }
+
+    public function evaluate(ClassDescription $theClass, Violations $violations, string $because): void
+    {
+        $correspondingFqn = $this->inferCorrespondingFqn($theClass);
+
+        if (
+            !trait_exists($correspondingFqn)
+            && !class_exists($correspondingFqn)
+            && !interface_exists($correspondingFqn)
+        ) {
+            $violations->add(
+                new Violation(
+                    $theClass->getFQCN(),
+                    $this->describe($theClass, $because)->toString()
+                )
+            );
+        }
+    }
+
+    /**
+     * @return class-string
+     */
+    public function inferCorrespondingFqn(ClassDescription $theClass): string
+    {
+        $inferFqn = $this->inferFqnFunction;
+
+        return $inferFqn($theClass->getFQCN());
+    }
+}

tests/Unit/Expressions/ForClasses/DummyClasses/Cat.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+final class Cat
+{
+}

tests/Unit/Expressions/ForClasses/DummyClasses/CatTestCase.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+final class CatTestCase
+{
+}

tests/Unit/Expressions/ForClasses/DummyClasses/Dog.php

@@ -0,0 +1,9 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses;
+
+final class Dog
+{
+}

tests/Unit/Expressions/ForClasses/HaveCorrespondingUnitTest.php

@@ -0,0 +1,77 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Arkitect\Tests\Unit\Expressions\ForClasses;
+
+use Arkitect\Analyzer\ClassDescription;
+use Arkitect\Analyzer\FullyQualifiedClassName;
+use Arkitect\Expression\ForClasses\HaveCorrespondingUnit;
+use Arkitect\Rules\Violations;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\Cat;
+use Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\Dog;
+use PHPUnit\Framework\TestCase;
+
+class HaveCorrespondingUnitTest extends TestCase
+{
+    public function test_it_should_pass_the_validation(): void
+    {
+        $class = Cat::class;
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString($class),
+            [],
+            [],
+            null,
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+        $constraint = new HaveCorrespondingUnit(
+            function ($fqn) {
+                return $fqn.'TestCase';
+            }
+        );
+
+        $because = 'we want all our command handlers to have a test';
+        $violations = new Violations();
+        $constraint->evaluate($classDescription, $violations, $because);
+
+        self::assertEquals(0, $violations->count());
+    }
+
+    public function test_it_should_return_violation_error(): void
+    {
+        $class = Dog::class;
+        $classDescription = new ClassDescription(
+            FullyQualifiedClassName::fromString($class),
+            [],
+            [],
+            null,
+            false,
+            false,
+            false,
+            false,
+            false
+        );
+        $constraint = new HaveCorrespondingUnit(
+            function ($fqn) {
+                return $fqn.'TestCase';
+            }
+        );
+
+        $because = 'we want all our command handlers to have a test';
+        $violations = new Violations();
+        $constraint->evaluate($classDescription, $violations, $because);
+
+        self::assertNotEquals(0, $violations->count());
+
+        $violationError = $constraint->describe($classDescription, $because)->toString();
+        $this->assertEquals(
+            'should have a matching unit named: '
+            ."'Arkitect\Tests\Unit\Expressions\ForClasses\DummyClasses\DogTestCase' because $because",
+            $violationError
+        );
+    }
+}