add validator for typoed emails

It happens quite often that people typo their email address (I blame
phone keyboards for that).  Catch common cases in a validator and
tell the user that they have a typo in their email.

Why yes, I did write the tests for this first, thanks for asking!

app/models/user.rb

@@ -61,7 +61,7 @@ class User < ApplicationRecord
     screen_name.strip!
   end
 
-  validates :email, fake_email: true
+  validates :email, fake_email: true, typoed_email: true
   validates :screen_name, presence: true, format: { with: SCREEN_NAME_REGEX }, uniqueness: { case_sensitive: false }, screen_name: true
 
   mount_uploader :profile_picture, ProfilePictureUploader, mount_on: :profile_picture_file_name

app/validators/typoed_email_validator.rb

@@ -0,0 +1,44 @@
+# frozen_string_literal: true
+
+class TypoedEmailValidator < ActiveModel::EachValidator
+  # this array contains "forbidden" email address endings
+  INVALID_ENDINGS = [
+    # without @:
+    *%w[
+      .con
+      .coom
+    ],
+
+    # with @:
+    *%w[
+      fmail.com
+      gemail.com
+      gmail.co
+      gmaile.com
+      gmaill.com
+      icluod.com
+      proton.mail
+    ].map { "@#{_1}" }
+  ].freeze
+
+  def validate_each(record, attribute, value)
+    return if valid?(value)
+
+    record.errors[attribute] << "contains a typo"
+  end
+
+  private
+
+  def valid?(value)
+    # needs an @
+    return false unless value.include?('@')
+
+    # part after the @ needs to have at least one period
+    return false if value.split('@', 2).last.count('.') == 0
+
+    # finally, common typos
+    return false if INVALID_ENDINGS.any? { value.end_with?(_1) }
+
+    true
+  end
+end

spec/models/user_spec.rb

@@ -33,6 +33,61 @@ RSpec.describe User, type: :model do
     end
   end
 
+  describe 'email validation' do
+    subject do
+      FactoryBot.build(:user, email: email).tap(&:validate).errors[:email]
+    end
+
+    shared_examples_for 'valid email' do |example_email|
+      context "when email is #{example_email}" do
+        let(:email) { example_email }
+
+        it "does not have validation errors" do
+          expect(subject).to be_empty
+        end
+      end
+    end
+
+    shared_examples_for 'invalid email' do |example_email|
+      context "when email is #{example_email}" do
+        let(:email) { example_email }
+
+        it "has validation errors" do
+          expect(subject).not_to be_empty
+        end
+      end
+    end
+
+    include_examples 'valid email', 'ifyouusethismailyouarebanned@nilsding.org'
+    include_examples 'valid email', 'fritz.fantom@gmail.com'
+    include_examples 'valid email', 'fritz.fantom@columbiamail.co'
+    include_examples 'valid email', 'fritz.fantom@protonmail.com'
+    include_examples 'valid email', 'fritz.fantom@enterprise.k8s.420stripes.k8s.needs.more.k8s.jira.atlassian.k8s.eu-central-1.s3.amazonaws.com'
+    include_examples 'invalid email', '@jack'
+
+    # examples from the real world:
+
+    # .con is not a valid TLD
+    include_examples 'invalid email', 'fritz.fantom@gmail.con'
+    include_examples 'invalid email', 'fritz.fantom@protonmail.con'
+    # neither is .coom
+    include_examples 'invalid email', 'fritz.fantom@gmail.coom'
+    # common typos:
+    include_examples 'invalid email', 'fritz.fantom@fmail.com'
+    include_examples 'invalid email', 'fritz.fantom@gemail.com'
+    include_examples 'invalid email', 'fritz.fantom@gmail.co'
+    include_examples 'invalid email', 'fritz.fantom@gmailcom'
+    include_examples 'invalid email', 'fritz.fantom@gmaile.com'
+    include_examples 'invalid email', 'fritz.fantom@gmaill.com'
+    include_examples 'invalid email', 'fritz.fantom@hotmailcom'
+    include_examples 'invalid email', 'fritz.fantom@icluod.com'
+    # no TLD
+    include_examples 'invalid email', 'fritz.fantom@gmail'
+    include_examples 'invalid email', 'fritz.fantom@protonmail'
+    # not registered as of 2022-01-11
+    include_examples 'invalid email', 'fritz.fantom@proton.mail'
+  end
+
   # -- User::TimelineMethods --
 
   shared_examples_for 'result is blank' do