Improve SWC compiler detection and default dependencies (#2135)

## Summary
- Add `using_swc?` helper method to detect SWC configuration in
shakapacker.yml
- Automatically install SWC dependencies (@swc/core, swc-loader) when
SWC is configured
- Update spec/dummy to include SWC packages

## Key Changes

### 1. Enhanced `using_swc?` Helper
- Parses `shakapacker.yml` for `javascript_transpiler` setting
- Returns `true` when SWC is explicitly configured
- Returns `true` for Shakapacker 9.3.0+ when not specified (SWC is the
default)
- Returns `true` for fresh installations (SWC is recommended)

### 2. Generator Updates
- Add `SWC_DEPENDENCIES` constant with @swc/core and swc-loader
- Install SWC deps automatically when `using_swc?` returns true
- Add to devDependencies since they're build-time tools

### 3. spec/dummy Updates
- Add @swc/core and swc-loader to devDependencies
- Matches the `javascript_transpiler: swc` setting already in
shakapacker.yml

## Testing
- Added comprehensive RSpec tests for `using_swc?` helper
- All existing tests pass
- RuboCop checks pass

## Test Plan
- [ ] Test generator with Shakapacker 9.3.0+
- [ ] Verify SWC detection logic
- [ ] Test fallback to Babel when explicitly configured

Closes #1956

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Automatic SWC transpiler detection using Shakapacker config and
version heuristics
* Conditional installation of SWC dependencies during project setup when
applicable
  * Resilient config parsing with clear precedence and fallback behavior

* **Tests**
* Expanded test coverage for SWC detection, dependency handling, and
edge/error scenarios

<sub>✏️ Tip: You can customize this high-level summary in your review
settings.</sub>
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude <[email protected]>

Author: justin808

react_on_rails/lib/generators/react_on_rails/generator_helper.rb

@@ -124,4 +124,75 @@ def shakapacker_version_9_or_higher?
       true
     end
   end
+
+  # Check if SWC is configured as the JavaScript transpiler in shakapacker.yml
+  #
+  # @return [Boolean] true if SWC is configured or should be used by default
+  #
+  # Detection logic:
+  # 1. If shakapacker.yml exists and specifies javascript_transpiler: parse it
+  # 2. For Shakapacker 9.3.0+, SWC is the default if not specified
+  # 3. Returns true for fresh installations (SWC is recommended default)
+  #
+  # @note This method is used to determine whether to install SWC dependencies
+  #   (@swc/core, swc-loader) instead of Babel dependencies during generation.
+  #
+  # @note Caching: The result is memoized for the lifetime of the generator instance.
+  #   If shakapacker.yml changes during generator execution (unlikely), the cached
+  #   value will not update. This is acceptable since generators run quickly.
+  def using_swc?
+    return @using_swc if defined?(@using_swc)
+
+    @using_swc = detect_swc_configuration
+  end
+
+  private
+
+  def detect_swc_configuration
+    shakapacker_yml_path = File.join(destination_root, "config/shakapacker.yml")
+
+    if File.exist?(shakapacker_yml_path)
+      config = parse_shakapacker_yml(shakapacker_yml_path)
+      transpiler = config.dig("default", "javascript_transpiler")
+
+      # Explicit configuration takes precedence
+      return transpiler == "swc" if transpiler
+
+      # For Shakapacker 9.3.0+, SWC is the default
+      return shakapacker_version_9_3_or_higher?
+    end
+
+    # Fresh install: SWC is recommended default for Shakapacker 9.3.0+
+    shakapacker_version_9_3_or_higher?
+  end
+
+  def parse_shakapacker_yml(path)
+    require "yaml"
+    # Use safe_load_file for security (defense-in-depth, even though this is user's own config)
+    # permitted_classes: [Symbol] allows symbol keys which shakapacker.yml may use
+    # aliases: true allows YAML anchors (&default, *default) commonly used in Rails configs
+    YAML.safe_load_file(path, permitted_classes: [Symbol], aliases: true)
+  rescue ArgumentError
+    # Older Psych versions don't support all parameters - try without aliases
+    begin
+      YAML.safe_load_file(path, permitted_classes: [Symbol])
+    rescue ArgumentError
+      # Very old Psych - fall back to safe_load with File.read
+      YAML.safe_load(File.read(path), permitted_classes: [Symbol]) # rubocop:disable Style/YAMLFileRead
+    end
+  rescue StandardError
+    # If we can't parse the file, return empty config
+    {}
+  end
+
+  # Check if Shakapacker 9.3.0 or higher is available
+  # This version made SWC the default JavaScript transpiler
+  def shakapacker_version_9_3_or_higher?
+    return true unless defined?(ReactOnRails::PackerUtils)
+
+    ReactOnRails::PackerUtils.shakapacker_version_requirement_met?("9.3.0")
+  rescue StandardError
+    # If we can't determine version, assume latest (which uses SWC)
+    true
+  end
 end

react_on_rails/lib/generators/react_on_rails/js_dependency_manager.rb

@@ -99,6 +99,13 @@ module JsDependencyManager
         @types/react-dom
       ].freeze
 
+      # SWC transpiler dependencies (for Shakapacker 9.3.0+ default transpiler)
+      # SWC is ~20x faster than Babel and is the default for new Shakapacker installations
+      SWC_DEPENDENCIES = %w[
+        @swc/core
+        swc-loader
+      ].freeze
+
       private
 
       def setup_js_dependencies
@@ -118,6 +125,8 @@ def add_js_dependencies
         add_css_dependencies
         # Rspack dependencies are only added when --rspack flag is used
         add_rspack_dependencies if respond_to?(:options) && options&.rspack?
+        # SWC dependencies are only added when SWC is the configured transpiler
+        add_swc_dependencies if using_swc?
         # Dev dependencies vary based on bundler choice
         add_dev_dependencies
       end
@@ -232,6 +241,26 @@ def add_rspack_dependencies
         MSG
       end
 
+      def add_swc_dependencies
+        puts "Installing SWC transpiler dependencies (20x faster than Babel)..."
+        return if add_packages(SWC_DEPENDENCIES, dev: true)
+
+        GeneratorMessages.add_warning(<<~MSG.strip)
+          ⚠️  Failed to add SWC dependencies.
+
+          SWC is the default JavaScript transpiler for Shakapacker 9.3.0+.
+          You can install them manually by running:
+            npm install --save-dev #{SWC_DEPENDENCIES.join(' ')}
+        MSG
+      rescue StandardError => e
+        GeneratorMessages.add_warning(<<~MSG.strip)
+          ⚠️  Error adding SWC dependencies: #{e.message}
+
+          You can install them manually by running:
+            npm install --save-dev #{SWC_DEPENDENCIES.join(' ')}
+        MSG
+      end
+
       def add_typescript_dependencies
         puts "Installing TypeScript dependencies..."
         return if add_packages(TYPESCRIPT_DEPENDENCIES, dev: true)

react_on_rails/spec/dummy/package.json

@@ -36,6 +36,7 @@
     "@playwright/test": "^1.55.1",
     "@pmmmwh/react-refresh-webpack-plugin": "^0.5.1",
     "@rescript/react": "^0.13.0",
+    "@swc/core": "^1.7.0",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
     "@types/react-helmet": "^6.1.5",
@@ -55,6 +56,7 @@
     "sass-resources-loader": "^2.1.0",
     "shakapacker": "9.4.0",
     "style-loader": "^3.3.1",
+    "swc-loader": "^0.2.6",
     "terser-webpack-plugin": "5.3.1",
     "url-loader": "^4.0.0",
     "webpack": "5.72.0",

react_on_rails/spec/react_on_rails/generators/generator_helper_spec.rb

@@ -109,4 +109,147 @@ def self.read
       end
     end
   end
+
+  describe "#using_swc?" do
+    let(:shakapacker_yml_path) { File.join(destination_root, "config/shakapacker.yml") }
+
+    before do
+      # Clear memoized value before each test
+      remove_instance_variable(:@using_swc) if instance_variable_defined?(:@using_swc)
+      FileUtils.mkdir_p(File.join(destination_root, "config"))
+    end
+
+    after do
+      FileUtils.rm_rf(File.join(destination_root, "config"))
+    end
+
+    context "when shakapacker.yml exists with javascript_transpiler: swc" do
+      before do
+        File.write(shakapacker_yml_path, <<~YAML)
+          default: &default
+            javascript_transpiler: swc
+        YAML
+      end
+
+      it "returns true" do
+        expect(using_swc?).to be true
+      end
+    end
+
+    context "when shakapacker.yml exists with javascript_transpiler: babel" do
+      before do
+        File.write(shakapacker_yml_path, <<~YAML)
+          default: &default
+            javascript_transpiler: babel
+        YAML
+      end
+
+      it "returns false" do
+        expect(using_swc?).to be false
+      end
+    end
+
+    context "when shakapacker.yml exists without javascript_transpiler setting" do
+      before do
+        File.write(shakapacker_yml_path, <<~YAML)
+          default: &default
+            source_path: app/javascript
+        YAML
+        # Stub to simulate Shakapacker 9.3.0+ where SWC is default
+        stub_const("ReactOnRails::PackerUtils", Class.new do
+          def self.shakapacker_version_requirement_met?(version)
+            version == "9.3.0"
+          end
+        end)
+      end
+
+      it "returns true for Shakapacker 9.3.0+ (SWC is default)" do
+        expect(using_swc?).to be true
+      end
+    end
+
+    context "when shakapacker.yml does not exist" do
+      before do
+        FileUtils.rm_f(shakapacker_yml_path)
+        # Stub to simulate Shakapacker 9.3.0+ where SWC is default
+        stub_const("ReactOnRails::PackerUtils", Class.new do
+          def self.shakapacker_version_requirement_met?(version)
+            version == "9.3.0"
+          end
+        end)
+      end
+
+      it "returns true for fresh installations with Shakapacker 9.3.0+" do
+        expect(using_swc?).to be true
+      end
+    end
+
+    context "when shakapacker.yml has parse errors" do
+      before do
+        File.write(shakapacker_yml_path, "invalid: yaml: [}")
+        # Stub to simulate Shakapacker 9.3.0+ where SWC is default
+        stub_const("ReactOnRails::PackerUtils", Class.new do
+          def self.shakapacker_version_requirement_met?(version)
+            version == "9.3.0"
+          end
+        end)
+      end
+
+      it "returns true (assumes latest Shakapacker with SWC default)" do
+        expect(using_swc?).to be true
+      end
+    end
+
+    context "with version boundary scenarios" do
+      before do
+        File.write(shakapacker_yml_path, <<~YAML)
+          default: &default
+            source_path: app/javascript
+        YAML
+      end
+
+      context "when Shakapacker version is 9.3.0+ (SWC default)" do
+        before do
+          stub_const("ReactOnRails::PackerUtils", Class.new do
+            def self.shakapacker_version_requirement_met?(version)
+              version == "9.3.0"
+            end
+          end)
+        end
+
+        it "returns true when no transpiler is specified" do
+          expect(using_swc?).to be true
+        end
+      end
+
+      context "when Shakapacker version is below 9.3.0 (Babel default)" do
+        before do
+          stub_const("ReactOnRails::PackerUtils", Class.new do
+            def self.shakapacker_version_requirement_met?(version)
+              # Only meets requirements for versions below 9.3.0
+              version != "9.3.0"
+            end
+          end)
+        end
+
+        it "returns false when no transpiler is specified" do
+          expect(using_swc?).to be false
+        end
+      end
+
+      context "when PackerUtils raises an error during version check" do
+        before do
+          stub_const("ReactOnRails::PackerUtils", Class.new do
+            def self.shakapacker_version_requirement_met?(_version)
+              raise StandardError, "Cannot determine version"
+            end
+          end)
+        end
+
+        it "defaults to true (assumes latest Shakapacker)" do
+          expect(using_swc?).to be true
+        end
+      end
+    end
+  end
 end

react_on_rails/spec/react_on_rails/generators/install_generator_spec.rb

@@ -12,6 +12,26 @@
 
     include_examples "base_generator", application_js: true
     include_examples "no_redux_generator"
+
+    it "installs appropriate transpiler dependencies based on Shakapacker version" do
+      assert_file "package.json" do |content|
+        package_json = JSON.parse(content)
+        # This test verifies the generator adapts to the Shakapacker version in the current environment.
+        # CI runs with both minimum (Shakapacker 8.x) and latest (Shakapacker 9.x) configurations,
+        # so this test validates correct behavior for whichever version is installed.
+        # SWC is the default transpiler for Shakapacker 9.3.0+; Babel is the default for older versions.
+        swc_is_default = ReactOnRails::PackerUtils.shakapacker_version_requirement_met?("9.3.0")
+
+        if swc_is_default
+          expect(package_json["devDependencies"]).to include("@swc/core")
+          expect(package_json["devDependencies"]).to include("swc-loader")
+        else
+          # For older Shakapacker versions, SWC is NOT installed by default
+          # (Babel is the default, but we don't install Babel deps since Shakapacker handles it)
+          expect(package_json["devDependencies"]).not_to include("@swc/core")
+        end
+      end
+    end
   end
 
   context "with --redux" do

react_on_rails/spec/react_on_rails/generators/js_dependency_manager_spec.rb

@@ -26,6 +26,13 @@ def destination_root
         "/test/path"
       end
 
+      # Mock using_swc? from GeneratorHelper (defaults to true for SWC testing)
+      def using_swc?
+        @using_swc.nil? ? true : @using_swc
+      end
+
+      attr_writer :using_swc
+
       # Test helpers
       attr_writer :add_npm_dependencies_result
 
@@ -109,6 +116,12 @@ def errors
                                                                                            ])
     end
 
+    it "defines SWC_DEPENDENCIES" do
+      expect(ReactOnRails::Generators::JsDependencyManager::SWC_DEPENDENCIES).to(
+        eq(%w[@swc/core swc-loader])
+      )
+    end
+
     it "does not include Babel presets in REACT_DEPENDENCIES" do
       expect(ReactOnRails::Generators::JsDependencyManager::REACT_DEPENDENCIES).not_to include(
         "@babel/preset-react"