Drupal - get user id from username or email without logging in

You can load the user with loadByProperties():

$users = \Drupal::entityTypeManager()->getStorage('user')
  ->loadByProperties(['mail' => $mail]);
$user = reset($users);
if ($user) {
  $uid = $user->id();
  $rids = $user->getRoles();
}

Drupal 8 now also has these specific functions:

user_load_by_name()

$user = user_load_by_name('some user');
$uid = $user->id();

https://api.drupal.org/api/drupal/core!modules!user!user.module/function/user_load_by_name/8.9.x

user_load_by_mail()

$user = user_load_by_mail('[email protected]');
$uid = $user->id();

https://api.drupal.org/api/drupal/core!modules!user!user.module/function/user_load_by_mail/8.9.x