Add option to configure Zeitwerk in new gems

Add a question to configure Zeitwerk when creating new gems (false by default). It
also supports a --zeitwerk flag to do it:

    bundle gem mygem --zeitwerk

Author: jorgemanrubia

bundler/lib/bundler/cli.rb

@@ -545,6 +545,7 @@ def viz
                          desc: "Open generated gemspec in the specified editor (defaults to $EDITOR or $BUNDLER_EDITOR)"
     method_option :ext, type: :string, desc: "Generate the boilerplate for C extension code.", enum: EXTENSIONS
     method_option :git, type: :boolean, default: true, desc: "Initialize a git repo inside your library."
+    method_option :zeitwerk, type: :boolean, desc: "Configure Zeitwerk as the class loader. Set a default with `bundle config set --global gem.zeitwerk true`."
     method_option :mit, type: :boolean, desc: "Generate an MIT license file. Set a default with `bundle config set --global gem.mit true`."
     method_option :rubocop, type: :boolean, desc: "Add rubocop to the generated Rakefile and gemspec. Set a default with `bundle config set --global gem.rubocop true`."
     method_option :changelog, type: :boolean, desc: "Generate changelog file. Set a default with `bundle config set --global gem.changelog true`."

bundler/lib/bundler/cli/gem.rb

@@ -145,6 +145,12 @@ def run
         config[:ci_config_path] = ".circleci "
       end
 
+      if ask_and_set(:zeitwerk, "Do you want to use Zeitwerk to load classes?",
+        "With Zeitwerk (https://github.com/fxn/zeitwerk), Ruby can load classes automatically " \
+          "based on name conventions so that you don't have to require files manually.")
+        config[:zeitwerk] = true
+      end
+
       if ask_and_set(:mit, "Do you want to license your code permissively under the MIT license?",
         "This means that any other developer or company will be legally allowed to use your code " \
         "for free as long as they admit you created it. You can read more about the MIT license " \

bundler/lib/bundler/templates/newgem/README.md.tt

@@ -19,6 +19,12 @@ If bundler is not being used to manage dependencies, install the gem by executin
 ```bash
 gem install UPDATE_WITH_YOUR_GEM_NAME_IMMEDIATELY_AFTER_RELEASE_TO_RUBYGEMS_ORG
 ```
+<%- if config[:zeitwerk] -%>
+
+### Zeitwerk and class loading
+
+This gem is configured to use [Zeitwerk](https://github.com/fxn/zeitwerk), which, by default, loads classes lazily as they are referenced in the code. In production environments, it's common to load code eagerly for performance reasons. If you're using this gem in an execution environment that supports Zeitwerk—such as Rails or Hanami—no additional configuration is necessary. Otherwise, you may want to [eager load this gem](https://github.com/fxn/zeitwerk?tab=readme-ov-file#eager-loading).
+<%- end -%>
 
 ## Usage
 

bundler/lib/bundler/templates/newgem/lib/newgem.rb.tt

@@ -1,9 +1,25 @@
 # frozen_string_literal: true
 
+<%- unless config[:zeitwerk] -%>
 require_relative "<%= File.basename(config[:namespaced_path]) %>/version"
+<%- end -%>
 <%- if config[:ext] -%>
 require_relative "<%= File.basename(config[:namespaced_path]) %>/<%= config[:underscored_name] %>"
 <%- end -%>
+<%- if config[:zeitwerk] -%>
+require "zeitwerk"
+<%- if config[:name].include?("-") -%>
+loader = Zeitwerk::Loader.for_gem_extension(<%= config[:constant_array][0..-2].join("::") %>)
+<%- else -%>
+loader = Zeitwerk::Loader.for_gem
+<%- end -%>
+loader.setup
+
+# Client code may eager load the gem, make sure that works.
+# If some files or directories should never be eager loaded,
+# please configure eager load exceptions in the loader.
+loader.eager_load if ENV.key?('CI')
+<%- end -%>
 
 <%- config[:constant_array].each_with_index do |c, i| -%>
 <%= "  " * i %>module <%= c %>

bundler/lib/bundler/templates/newgem/newgem.gemspec.tt

@@ -46,6 +46,9 @@ Gem::Specification.new do |spec|
 <%- if config[:ext] == 'rust' -%>
   spec.add_dependency "rb_sys", "~> 0.9.91"
 <%- end -%>
+<%- if config[:zeitwerk] -%>
+  spec.add_dependency "zeitwerk"
+<%- end -%>
 
   # For more information and examples about making a new gem, check out our
   # guide at: https://bundler.io/guides/creating_gem.html

bundler/spec/bundler/gem_helper_spec.rb

@@ -10,7 +10,7 @@
 
   before(:each) do
     global_config "BUNDLE_GEM__MIT" => "false", "BUNDLE_GEM__TEST" => "false", "BUNDLE_GEM__COC" => "false", "BUNDLE_GEM__LINTER" => "false",
-                  "BUNDLE_GEM__CI" => "false", "BUNDLE_GEM__CHANGELOG" => "false"
+                  "BUNDLE_GEM__CI" => "false", "BUNDLE_GEM__ZEITWERK" => "false", "BUNDLE_GEM__CHANGELOG" => "false"
     git("config --global init.defaultBranch main")
     bundle "gem #{app_name}"
     prepare_gemspec(app_gemspec_path)

bundler/spec/commands/newgem_spec.rb

@@ -38,7 +38,7 @@ def bundle_exec_standardrb
     git("config --global github.user bundleuser")
 
     global_config "BUNDLE_GEM__MIT" => "false", "BUNDLE_GEM__TEST" => "false", "BUNDLE_GEM__COC" => "false", "BUNDLE_GEM__LINTER" => "false",
-                  "BUNDLE_GEM__CI" => "false", "BUNDLE_GEM__CHANGELOG" => "false"
+                  "BUNDLE_GEM__CI" => "false", "BUNDLE_GEM__ZEITWERK" => "false", "BUNDLE_GEM__CHANGELOG" => "false"
   end
 
   describe "git repo initialization" do
@@ -74,6 +74,36 @@ def bundle_exec_standardrb
     end
   end
 
+  shared_examples_for "--zeitwerk flag" do
+    let(:gem_name) { "my_gem" }
+
+    before do
+      bundle "gem #{gem_name} --zeitwerk"
+    end
+    it "configures zeitwerk" do
+      gem_skeleton_assertions
+      expect(bundled_app("#{gem_name}/#{gem_name}.gemspec").read).to include('spec.add_dependency "zeitwerk"')
+      expect(bundled_app("#{gem_name}/README.md").read).to include("## Zeitwerk")
+      expect(bundled_app("#{gem_name}/lib/#{require_path}.rb").read).to include <<~RUBY
+        require "zeitwerk"
+        loader = Zeitwerk::Loader.for_gem
+        loader.setup
+      RUBY
+    end
+  end
+
+  shared_examples_for "--no-zeitwerk flag" do
+    before do
+      bundle "gem #{gem_name} --no-zeitwerk"
+    end
+    it "does not configure zeitwerk" do
+      gem_skeleton_assertions
+      expect(bundled_app("#{gem_name}/#{gem_name}.gemspec").read).to_not include('spec.add_dependency "zeitwerk"')
+      expect(bundled_app("#{gem_name}/README.md").read).to_not include("## Zeitwerk")
+      expect(bundled_app("#{gem_name}/lib/#{require_path}.rb").read).to_not include('require "zeitwerk"')
+    end
+  end
+
   shared_examples_for "--mit flag" do
     before do
       bundle "gem #{gem_name} --mit"
@@ -1408,6 +1438,28 @@ def create_temporary_dir(dir)
     end
   end
 
+  context "testing --zeitwerk option against bundle config settings" do
+    let(:gem_name) { "my_gem" }
+
+    let(:require_path) { "my_gem" }
+
+    context "with zeitwerk option in bundle config settings set to true" do
+      before do
+        global_config "BUNDLE_GEM__ZEITWERK" => "true"
+      end
+      it_behaves_like "--zeitwerk flag"
+      it_behaves_like "--no-zeitwerk flag"
+    end
+
+    context "with zeitwerk option in bundle config settings set to false" do
+      before do
+        global_config "BUNDLE_GEM__ZEITWERK" => "false"
+      end
+      it_behaves_like "--zeitwerk flag"
+      it_behaves_like "--no-zeitwerk flag"
+    end
+  end
+
   context "testing --github-username option against git and bundle config settings" do
     context "without git config set" do
       before do
@@ -1716,6 +1768,31 @@ def create_temporary_dir(dir)
       expect(bundled_app("foobar/.github/workflows/main.yml")).to exist
     end
 
+    it "asks about Zeitwerk" do
+      global_config "BUNDLE_GEM__ZEITWERK" => nil
+
+      bundle "gem foobar" do |input, _, _|
+        input.puts "yes"
+      end
+
+      expect(bundled_app("foobar/foobar.gemspec").read).to include('spec.add_dependency "zeitwerk"')
+    end
+
+    context("gem extensions") do
+      let(:gem_name) { "my-gem" }
+
+      it "configures zeitwerk detecting the gem extension" do
+        bundle "gem my-gem --zeitwerk"
+
+        expect(bundled_app("#{gem_name}/#{gem_name}.gemspec").read).to include('spec.add_dependency "zeitwerk"')
+        expect(bundled_app("#{gem_name}/lib/my/gem.rb").read).to include <<~RUBY
+          require "zeitwerk"
+          loader = Zeitwerk::Loader.for_gem_extension(My)
+          loader.setup
+        RUBY
+      end
+    end
+
     it "asks about MIT license" do
       global_config "BUNDLE_GEM__MIT" => nil
 

bundler/spec/other/major_deprecation_spec.rb

@@ -608,7 +608,7 @@
   describe "deprecating rubocop" do
     before do
       global_config "BUNDLE_GEM__MIT" => "false", "BUNDLE_GEM__TEST" => "false", "BUNDLE_GEM__COC" => "false",
-                    "BUNDLE_GEM__CI" => "false", "BUNDLE_GEM__CHANGELOG" => "false"
+                    "BUNDLE_GEM__ZEITWERK" => "false", "BUNDLE_GEM__CI" => "false", "BUNDLE_GEM__CHANGELOG" => "false"
     end
 
     context "bundle gem --rubocop" do