minor symfony#28057 [DI] Improve class named servics error message (ro0NL)

This PR was squashed before being merged into the 3.4 branch (closes symfony#28057).

Discussion
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[DI] Improve class named servics error message

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets | symfony#28006
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

Commits
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61de060 [DI] Improve class named servics error message

Author: nicolas-grekas

src/Symfony/Component/DependencyInjection/Compiler/CheckDefinitionValidityPass.php

@@ -48,6 +48,15 @@ public function process(ContainerBuilder $container)
                     throw new RuntimeException(sprintf('Please add the class to service "%s" even if it is constructed by a factory since we might need to add method calls based on compile-time checks.', $id));
                 }
                 if (class_exists($id) || interface_exists($id, false)) {
+                    if (0 === strpos($id, '\\') && 1 < substr_count($id, '\\')) {
+                        throw new RuntimeException(sprintf(
+                            'The definition for "%s" has no class attribute, and appears to reference a class or interface. '
+                            .'Please specify the class attribute explicitly or remove the leading backslash by renaming '
+                            .'the service to "%s" to get rid of this error.',
+                            $id, substr($id, 1)
+                        ));
+                    }
+
                     throw new RuntimeException(sprintf(
                          'The definition for "%s" has no class attribute, and appears to reference a '
                         .'class or interface in the global namespace. Leaving out the "class" attribute '

src/Symfony/Component/DependencyInjection/Tests/ContainerBuilderTest.php

@@ -1264,6 +1264,30 @@ public function testNoClassFromGlobalNamespaceClassId()
         $container->compile();
     }
 
+    /**
+     * @expectedException \Symfony\Component\DependencyInjection\Exception\RuntimeException
+     * @expectedExceptionMessage The definition for "\DateTime" has no class attribute, and appears to reference a class or interface in the global namespace.
+     */
+    public function testNoClassFromGlobalNamespaceClassIdWithLeadingSlash()
+    {
+        $container = new ContainerBuilder();
+
+        $container->register('\\'.\DateTime::class);
+        $container->compile();
+    }
+
+    /**
+     * @expectedException \Symfony\Component\DependencyInjection\Exception\RuntimeException
+     * @expectedExceptionMessage The definition for "\Symfony\Component\DependencyInjection\Tests\FooClass" has no class attribute, and appears to reference a class or interface. Please specify the class attribute explicitly or remove the leading backslash by renaming the service to "Symfony\Component\DependencyInjection\Tests\FooClass" to get rid of this error.
+     */
+    public function testNoClassFromNamespaceClassIdWithLeadingSlash()
+    {
+        $container = new ContainerBuilder();
+
+        $container->register('\\'.FooClass::class);
+        $container->compile();
+    }
+
     /**
      * @expectedException \Symfony\Component\DependencyInjection\Exception\RuntimeException
      * @expectedExceptionMessage The definition for "123_abc" has no class.