Summary

When running rails generate react_on_rails:install --rspack, the generator creates all config files under config/webpack/. However, Shakapacker 9.x expects rspack configs in config/rspack/ and prints a deprecation warning on every build when it falls back to config/webpack/.

Steps to Reproduce

rails new test_app --skip-javascript
cd test_app
# Add react_on_rails and shakapacker to Gemfile
bundle install
git init && git add -A && git commit -m "initial"
rails generate react_on_rails:install --rspack
bin/shakapacker  # trigger a build

Expected Behavior

• Generator creates rspack config files in config/rspack/ (Shakapacker 9.x standard location)
• Builds run cleanly without deprecation warnings

Actual Behavior

Generator creates all configs in config/webpack/. Every build prints:

[Shakapacker] Looking for Rspack config in: config/rspack/rspack.config.ts, config/rspack/rspack.config.js
[Shakapacker] Rspack config not found, checking for webpack config fallback...
[Shakapacker] Checking config/webpack/ for backward compatibility...
⚠️  DEPRECATION WARNING: Found webpack config in config/webpack/ but assets_bundler is set to 'rspack'.
   For rspack, configs should be in config/rspack/ directory.

The build succeeds via backward-compat fallback, but the warning appears on every single build.

Root Cause

React on Rails PR #1852 introduced --rspack with a "unified config" approach — all config files go in config/webpack/ regardless of bundler. Meanwhile, Shakapacker moved rspack to a separate directory:

• Shakapacker PR #590 (Sep 2025): Implemented dual bundler architecture with config/rspack/ as the rspack config location
• Shakapacker PR #594 (Sep 2025): Standardized rspack config to config/rspack/rspack.config.js
• Shakapacker PR #734 (Oct 2025): Added config/webpack/ as a deprecated fallback for rspack, with deprecation warning

Shakapacker's configuration.rb explicitly returns different paths per bundler:

# lib/shakapacker/configuration.rb:373-378
def assets_bundler_config_path
  custom_path = fetch(:assets_bundler_config_path)
  return custom_path if custom_path
  rspack? ? "config/rspack" : "config/webpack"
end

Additional Impact

This also affects shakapacker_configured? in install_generator.rb (line 251-255), which hardcodes config/webpack/webpack.config.js:

def shakapacker_configured?
  shakapacker_binaries_exist? &&
    File.exist?("config/shakapacker.yml") &&
    File.exist?("config/webpack/webpack.config.js")
end

If a user installs Shakapacker with rspack before running the React on Rails generator, shakapacker_configured? returns false (because rspack creates config/rspack/rspack.config.js, not config/webpack/webpack.config.js), triggering an unnecessary full Shakapacker reinstall.

Suggested Fix

1. When --rspack is passed, create config files in config/rspack/ instead of config/webpack/
2. Update shakapacker_configured? to check the correct config path based on the bundler setting in config/shakapacker.yml

Environment

• React on Rails: master (16.4.0.rc.1)
• Shakapacker: 9.4.0
• Rails: 8.1.2

Found while investigating #2289.