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 uses [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 running this gem in a context 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