Gem::Commands::FetchCommand#execute

# File tmp/rubies/ruby-3.0.5/lib/rubygems/commands/fetch_command.rb, line 45
def execute
  version = options[:version] || Gem::Requirement.default

  platform  = Gem.platforms.last
  gem_names = get_all_gem_names

  gem_names.each do |gem_name|
    dep = Gem::Dependency.new gem_name, version
    dep.prerelease = options[:prerelease]

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

    if platform
      filtered = specs_and_sources.select {|s,| s.platform == platform }
      specs_and_sources = filtered unless filtered.empty?
    end

    spec, source = specs_and_sources.max_by {|s,| s }

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

    source.download spec

    say "Downloaded #{spec.full_name}"
  end
end