Gem::Commands::RebuildCommand#download_gem

# File tmp/rubies/ruby-3.4.1/lib/rubygems/commands/rebuild_command.rb, line 235
def download_gem(gem_name, gem_version, old_file)
  # This code was based loosely off the `gem fetch` command.
  version = "= #{gem_version}"
  dep = Gem::Dependency.new gem_name, version

  specs_and_sources, errors =
    Gem::SpecFetcher.fetcher.spec_for_dependency dep

  # There should never be more than one item in specs_and_sources,
  # since we search for an exact version.
  spec, source = specs_and_sources[0]

  if spec.nil?
    show_lookup_failure gem_name, version, errors, options[:domain]
    terminate_interaction 1
  end

  download_path = source.download spec

  FileUtils.move(download_path, old_file)

  say "Downloaded #{gem_name} version #{gem_version} as #{old_file}."
end