Support signature generation for ActionMailer

ActionMailer automatically generates class methods from the instance methods
user defined via method_missing.  For example, `AccountMailer.welcome_mail`
will be generated from `AccountMail#welcome_mail` that is defined by users.

This starts to support signature generation for auto-generated class methods
for mailer classes dynamically.

lib/rbs_rails.rb

@@ -4,6 +4,7 @@
 
 require_relative "rbs_rails/version"
 require_relative "rbs_rails/util"
+require_relative 'rbs_rails/action_mailer'
 require_relative 'rbs_rails/active_record'
 require_relative 'rbs_rails/path_helpers'
 require_relative 'rbs_rails/dependency_builder'

lib/rbs_rails/action_mailer.rb

@@ -0,0 +1,63 @@
+module RbsRails
+  module ActionMailer
+    def self.class_to_rbs(klass)
+      Generator.new(klass).generate
+    end
+
+    class Generator
+      def initialize(klass)
+        @klass = klass
+        @klass_name = Util.module_name(klass, abs: false)
+      end
+
+      def generate
+        Util.format_rbs klass_decl
+      end
+
+      private def klass_decl
+        <<~RBS
+          #{header}
+          #{methods}
+          #{footer}
+        RBS
+      end
+
+      private def header
+        namespace = +''
+        klass_name(abs: false).split('::').map do |mod_name|
+          namespace += "::#{mod_name}"
+          mod_object = Object.const_get(namespace)
+          case mod_object
+          when Class
+            # @type var superclass: Class
+            superclass = _ = mod_object.superclass
+            superclass_name = Util.module_name(superclass, abs: false)
+
+            "class #{mod_name} < ::#{superclass_name}"
+          when Module
+            "module #{mod_name}"
+          else
+            raise 'unreachable'
+          end
+        end.join("\n")
+      end
+
+      private def methods
+        klass.action_methods.map do |method_name|
+          "def self.#{method_name}: (*untyped) -> ActionMailer::MessageDelivery"
+        end.join("\n")
+      end
+
+      private def footer
+        "end\n" * klass_name(abs: false).split('::').size
+      end
+
+      private def klass_name(abs: true)
+        abs ? "::#{@klass_name}" : @klass_name
+      end
+
+      private
+      attr_reader :klass
+    end
+  end
+end

lib/rbs_rails/rake_task.rb

@@ -15,14 +15,15 @@ def initialize(name = :rbs_rails, &block)
       block.call(self) if block
 
       def_generate_rbs_for_models
+      def_generate_rbs_for_mailers
       def_generate_rbs_for_path_helpers
       def_all
     end
 
     def def_all
       desc 'Run all tasks of rbs_rails'
 
-      deps = [:"#{name}:generate_rbs_for_models", :"#{name}:generate_rbs_for_path_helpers"]
+      deps = [:"#{name}:generate_rbs_for_models", :"#{name}:generate_rbs_for_mailers", :"#{name}:generate_rbs_for_path_helpers"]
       task("#{name}:all": deps)
     end
 
@@ -34,7 +35,7 @@ def def_generate_rbs_for_models
         Rails.application.eager_load!
 
         dep_builder = DependencyBuilder.new
-        
+
         ::ActiveRecord::Base.descendants.each do |klass|
           next unless RbsRails::ActiveRecord.generatable?(klass)
           next if ignore_model_if&.call(klass)
@@ -53,6 +54,23 @@ def def_generate_rbs_for_models
       end
     end
 
+    def def_generate_rbs_for_mailers
+      desc 'Generate RBS files for Active Mailer mailers'
+      task("#{name}:generate_rbs_for_mailers": :environment) do
+        require 'rbs_rails'
+
+        Rails.application.eager_load!
+
+        ::ActionMailer::Base.descendants.each do |klass|
+          path = signature_root_dir / "app/mailers/#{klass.name.underscore}.rbs"
+          path.dirname.mkpath
+
+          sig = RbsRails::ActionMailer.class_to_rbs(klass)
+          path.write sig
+        end
+      end
+    end
+
     def def_generate_rbs_for_path_helpers
       desc 'Generate RBS files for path helpers'
       task("#{name}:generate_rbs_for_path_helpers": :environment) do

sig/rbs_rails/action_mailer.rbs

@@ -0,0 +1,27 @@
+module RbsRails
+  module ActionMailer
+    def self.class_to_rbs: (untyped klass) -> String
+
+    class Generator
+      @klass_name: String
+
+      def initialize: (singleton(ActionMailer::Base) klass) -> void
+
+      def generate: () -> String
+
+      def klass_decl: () -> String
+
+      def header: () -> String
+
+      def methods: () -> String
+
+      def footer: () -> String
+
+      def klass_name: (?abs: boolish) -> String
+
+      private
+
+      attr_reader klass: singleton(ActionMailer::Base)
+    end
+  end
+end

sig/rbs_rails/rake_task.rbs

@@ -17,6 +17,8 @@ module RbsRails
 
     def def_generate_rbs_for_models: () -> void
 
+    def def_generate_rbs_for_mailers: () -> void
+
     def def_generate_rbs_for_path_helpers: () -> void
 
     private

test/app/Gemfile

@@ -10,6 +10,9 @@ gem 'bcrypt'
 
 gem 'rbs_rails', path: '../../'
 
+# to run Rails6.1 under Ruby 3.1+
+gem 'mail', '>= 2.8.0'
+
 # to run Rails6.1 under Ruby 3.4
 gem 'base64'
 gem 'bigdecimal'

test/app/Gemfile.lock

@@ -76,6 +76,7 @@ GEM
     builder (3.2.4)
     concurrent-ruby (1.1.10)
     crass (1.0.6)
+    date (3.3.4)
     drb (2.2.0)
       ruby2_keywords
     erubi (1.10.0)
@@ -86,15 +87,27 @@ GEM
     loofah (2.21.3)
       crass (~> 1.0.2)
       nokogiri (>= 1.12.0)
-    mail (2.7.1)
+    mail (2.8.1)
       mini_mime (>= 0.1.1)
+      net-imap
+      net-pop
+      net-smtp
     marcel (1.0.1)
     method_source (1.0.0)
     mini_mime (1.0.3)
     mini_portile2 (2.8.2)
     minitest (5.17.0)
     msgpack (1.4.2)
     mutex_m (0.2.0)
+    net-imap (0.4.10)
+      date
+      net-protocol
+    net-pop (0.1.2)
+      net-protocol
+    net-protocol (0.2.2)
+      timeout
+    net-smtp (0.4.0.1)
+      net-protocol
     nio4r (2.5.8)
     nokogiri (1.15.2)
       mini_portile2 (~> 2.8.2)
@@ -147,6 +160,7 @@ GEM
       sprockets (>= 3.0.0)
     sqlite3 (1.4.2)
     thor (1.1.0)
+    timeout (0.4.1)
     tzinfo (2.0.5)
       concurrent-ruby (~> 1.0)
     websocket-driver (0.7.3)
@@ -163,6 +177,7 @@ DEPENDENCIES
   bigdecimal
   bootsnap
   drb
+  mail (>= 2.8.0)
   mutex_m
   psych (< 4)
   puma

test/app/app/mailers/application_mailer.rb

@@ -0,0 +1,4 @@
+class ApplicationMailer < ActionMailer::Base
+  default from: '[email protected]'
+  layout 'mailer'
+end

test/app/app/mailers/user_mailer.rb

@@ -0,0 +1,9 @@
+class UserMailer < ApplicationMailer
+  default from: '[email protected]'
+
+  def welcome_email
+    @user = params[:user]
+    @url  = 'http://example.com/login'
+    mail(to: @user.email, subject: 'Welcome to My Awesome Site')
+  end
+end

test/app/app/views/layouts/mailer.text.erb

@@ -0,0 +1 @@
+<%= yield %>

test/app/config/application.rb

@@ -7,7 +7,7 @@
 require "active_record/railtie"
 require "active_storage/engine"
 require "action_controller/railtie"
-# require "action_mailer/railtie"
+require "action_mailer/railtie"
 # require "action_mailbox/engine"
 # require "action_text/engine"
 require "action_view/railtie"

test/expectations/user_mailer.rbs

@@ -0,0 +1,3 @@
+class UserMailer < ::ApplicationMailer
+  def self.welcome_email: (*untyped) -> ActionMailer::MessageDelivery
+end

test/rbs_rails/action_mailer_test.rb

@@ -0,0 +1,43 @@
+require 'test_helper'
+
+class ActionMailerTest < Minitest::Test
+  def test_type_check
+    clean_test_signatures
+
+    setup!
+
+    dir = app_dir
+    sh!('steep', 'check', chdir: dir)
+  end
+
+  def test_user_mailer_rbs_snapshot
+    clean_test_signatures
+
+    setup!
+
+    rbs_path = app_dir.join('sig/rbs_rails/app/mailers/user_mailer.rbs')
+    expect_path = expectations_dir / 'user_mailer.rbs'
+    # Code to re-generate the expectation files
+    # expect_path.write rbs_path.read
+
+    assert_equal expect_path.read, rbs_path.read
+  end
+
+  def app_dir
+    Pathname(__dir__).join('../app')
+  end
+
+  def expectations_dir
+    Pathname(__dir__).join('../expectations')
+  end
+
+  def setup!
+    dir = app_dir
+
+    Bundler.with_unbundled_env do
+      sh!('bundle', 'install', chdir: dir)
+      sh!('bin/rake', 'db:create', 'db:schema:load', chdir: dir)
+      sh!('bin/rake', 'rbs_rails:all', '--trace', chdir: dir)
+    end
+  end
+end