Gem::Installer#install

# File tmp/rubies/ruby-4.1.0-preview1/lib/rubygems/installer.rb, line 274
def install
  assign_content_address
  pre_install_checks

  run_pre_install_hooks

  # Set loaded_from to ensure extension_dir is correct
  spec.loaded_from = spec_file

  # Completely remove any previous gem files
  FileUtils.rm_rf gem_dir
  FileUtils.rm_rf spec.extension_dir

  dir_mode = options[:dir_mode]
  FileUtils.mkdir_p gem_dir, mode: dir_mode && 0o755

  extract_files

  build_extensions
  write_build_info_file
  run_post_build_hooks

  generate_bin
  if options[:install_plugin] == false
    remove_stale_plugins
    warn_skipped_plugins
  else
    generate_plugins
  end

  write_spec
  write_cache_file

  File.chmod(dir_mode, gem_dir) if dir_mode

  say clean_text(spec.post_install_message.to_s) if options[:post_install_message] && !spec.post_install_message.nil?

  if incompatible_abi_install?
    say "#{spec.full_name} is scoped to Ruby ABI #{spec.ruby_abi} and will not be visible to the running Ruby (ABI #{Gem.ruby_abi})"
  else
    Gem::Specification.add_spec(spec) unless @install_dir
  end

  load_plugin unless options[:install_plugin] == false

  run_post_install_hooks

  spec
rescue Errno::EACCES => e
  # Permission denied - /path/to/foo
  raise Gem::FilePermissionError, e.message.split(" - ").last
end