class RbSys::ExtensionTask

def source_files

I'm not sure why this is necessary, can it be removed? 
def source_files
  list = FileList[
    "#{ext_dir}/**/*.{rs,rb,c,h,toml}",
    "**/Cargo.{toml,lock}",
    "**/.cargo/**/*",
    "#{ext_dir}/lib/**/*"
  ]
  list.include("#{ext_dir}/#{@source_pattern}") if @source_pattern
  list.exclude(File.join(target_directory, "**/*"))
  list
end