farm-2.x-dev/modules/core/login/farm_login.module

<?php

/**
 * @file
 * Contains farm_login.module.
 */

use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Render\Element\Email;

/**
 * Implements hook_form_FORM_ID_alter().
 */
function farm_login_form_user_login_form_alter(&$form, FormStateInterface $form_state) {
  $config = \Drupal::config('system.site');

  // Update title and description to include email as an option.
  $form['name']['#title'] = t('Email or username');
  $form['name']['#description'] = t('Enter your @s email address or username.', ['@s' => $config->get('name')]);

  // Update the maxlength to account for emails.
  $form['name']['#maxlength'] = Email::EMAIL_MAX_LENGTH;

  // Add a validation handler for the name field.
  $form['name']['#element_validate'][] = 'farm_login_user_login_validate';

  // Update password description to be more generic.
  $form['pass']['#description'] = t('Enter the password that accompanies your account.');
}

/**
 * Form element validation handler for the username in the user login form.
 *
 * Allows users to authenticate by email.
 */
function farm_login_user_login_validate($form, FormStateInterface $form_state) {

  // Check if a username was provided.
  $mail = $form_state->getValue('name');
  if (!empty($mail)) {

    // If the email address is associated with a user, use their account name
    // for later validation.
    if ($user = user_load_by_mail($mail)) {
      $form_state->setValue('name', $user->getAccountName());
    }
  }
}