Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/symfony/validator/README.md

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+Validator Component
+===================
+
+This component is based on the JSR-303 Bean Validation specification and
+enables specifying validation rules for classes using XML, YAML, PHP or
+annotations, which can then be checked against instances of these classes.
+
+Usage
+-----
+
+The component provides "validation constraints", which are simple objects
+containing the rules for the validation. Let's validate a simple string
+as an example:
+
+```php
+use Symfony\Component\Validator\Validation;
+use Symfony\Component\Validator\Constraints\Length;
+
+$validator = Validation::createValidator();
+
+$violations = $validator->validateValue('Bernhard', new Length(array('min' => 10)));
+```
+
+This validation will fail because the given string is shorter than ten
+characters. The precise errors, here called "constraint violations",  are
+returned by the validator. You can analyze these or return them to the user.
+If the violation list is empty, validation succeeded.
+
+Validation of arrays is possible using the `Collection` constraint:
+
+```php
+use Symfony\Component\Validator\Validation;
+use Symfony\Component\Validator\Constraints as Assert;
+
+$validator = Validation::createValidator();
+
+$constraint = new Assert\Collection(array(
+    'name' => new Assert\Collection(array(
+        'first_name' => new Assert\Length(array('min' => 101)),
+        'last_name' => new Assert\Length(array('min' => 1)),
+    )),
+    'email' => new Assert\Email(),
+    'simple' => new Assert\Length(array('min' => 102)),
+    'gender' => new Assert\Choice(array(3, 4)),
+    'file' => new Assert\File(),
+    'password' => new Assert\Length(array('min' => 60)),
+));
+
+$violations = $validator->validateValue($input, $constraint);
+```
+
+Again, the validator returns the list of violations.
+
+Validation of objects is possible using "constraint mapping". With such
+a mapping you can put constraints onto properties and objects of classes.
+Whenever an object of this class is validated, its properties and
+method results are matched against the constraints.
+
+```php
+use Symfony\Component\Validator\Validation;
+use Symfony\Component\Validator\Constraints as Assert;
+
+class User
+{
+    /**
+     * @Assert\Length(min = 3)
+     * @Assert\NotBlank
+     */
+    private $name;
+
+    /**
+     * @Assert\Email
+     * @Assert\NotBlank
+     */
+    private $email;
+
+    public function __construct($name, $email)
+    {
+        $this->name = $name;
+        $this->email = $email;
+    }
+
+    /**
+     * @Assert\IsTrue(message = "The user should have a Google Mail account")
+     */
+    public function isGmailUser()
+    {
+        return false !== strpos($this->email, '@gmail.com');
+    }
+}
+
+$validator = Validation::createValidatorBuilder()
+    ->enableAnnotationMapping()
+    ->getValidator();
+
+$user = new User('John Doe', 'john@example.com');
+
+$violations = $validator->validate($user);
+```
+
+This example uses the annotation support of Doctrine Common to
+map constraints to properties and methods. You can also map constraints
+using XML, YAML or plain PHP, if you dislike annotations or don't want
+to include Doctrine. Check the documentation for more information about
+these drivers.
+
+Resources
+---------
+
+Silex integration:
+
+https://github.com/silexphp/Silex/blob/master/src/Silex/Provider/ValidatorServiceProvider.php
+
+Documentation:
+
+https://symfony.com/doc/2.7/book/validation.html
+
+JSR-303 Specification:
+
+http://jcp.org/en/jsr/detail?id=303
+
+You can run the unit tests with the following command:
+
+    $ cd path/to/Symfony/Component/Validator/
+    $ composer install
+    $ phpunit