Gem::Ext::CargoBuilder#extension_nesting

# File tmp/rubies/ruby-4.0.0/lib/rubygems/ext/cargo_builder.rb, line 104
def extension_nesting(extension)
  parts = extension.to_s.split(Regexp.union([File::SEPARATOR, File::ALT_SEPARATOR].compact))

  parts = parts.each_with_object([]) do |segment, final|
    next if segment == "."
    if segment == ".."
      raise Gem::InstallError, "extension outside of gem root" if final.empty?
      next final.pop
    end
    final << segment
  end

  File.join(parts[1...-1])
end