A user should be able to construct std::io::fs::Directories in user code in order to extend the filesystem APIs. Right now std::io::fs::Directories has private fields and I don't see a constructor.

For example, I want to make a version of walk_dir which never returns a error, returning an empty Directories instead, in order to be able to write

  if last_modified (fail_free_walk_dir (...) .filter (...)) > last_modified (fail_free_walk_dir (...) .filter (...)) {...}

Curently I can't do that without moving the contents of the Directories into a temporary vector or making a new Iterator implementation, because Directories doesn't have a public constructor.