Replace user_authenticate with a UserAuth service

+1 for the idea!

1. +++ b/core/modules/user/lib/Drupal/user/Form/UserLoginForm.php
   @@ -165,7 +165,7 @@ public function validateAuthentication(array &$form, array &$form_state) {
   -      $form_state['uid'] = user_authenticate($form_state['values']['name'], $password);
   +      $form_state['uid'] = \Drupal::service('user.auth')->authenticate($form_state['values']['name'], $password);
   

   Seriously we should inject it, so other people also follow us.

2. +++ b/core/modules/user/lib/Drupal/user/UserAuth.php
   @@ -0,0 +1,75 @@
   +/**
   + * Class UserAuth.
   + */
   +class UserAuth {
   

   Shit, really?

3. +++ b/core/modules/user/lib/Drupal/user/UserAuth.php
   @@ -0,0 +1,75 @@
   +   * @param $name
   +   *   User name to authenticate.
   +   * @param $password
   +   *   A plain-text password, such as trimmed text from form values.
   +   * @return int|bool
   +   *   The user's uid on success, or FALSE on failure to authenticate.
   +   */
   +  public function authenticate($name, $password) {
   

   A bit more types would be helpful.

1. +++ b/core/modules/user/lib/Drupal/user/UserAuth.php
   @@ -0,0 +1,70 @@
   +  /**
   +   * The password service.
   +   *
   +   * @var \Drupal\Core\Password\PasswordInterface
   +   */
   +  protected $password;
   

   That's a bad name. Since we also have $password in the authenticate() method this can get confusing. Better use $passwordService or $passwordChecker or $passwordHasher.

2. +++ b/core/modules/user/lib/Drupal/user/UserAuth.php
   @@ -0,0 +1,70 @@
   +   * @param PasswordInterface $password
   

   Missing @param docs.

The PHPUnit test looks superb, great work!

You should also convert BasicAuth to use this service and then remove user_authenticate() (or mark it @deprecated and call the service?). UserLoginForm and BasicAuth are the only two instances that use it.

+++ b/core/modules/user/tests/Drupal/user/Tests/UserAuthTest.php
@@ -0,0 +1,215 @@
+class UserAuthTest extends UnitTestCase {

<3

We need a change record for this - once one exists can set back to rtbc.

Committed 8bab667 and pushed to 8.x. Thanks!