The rationale behind `from_raw_file()` and the special treatment of non-buffered IO is that there is no `read_buffer()` API or other clean way to say "I want to read just what's currently in the buffer so that from now on I could read directly from the file descriptor without harm".

If you want to read from a buffered file object, sure, just call `from_file()`. If you want to ensure you'll get full performance benefits, call `from_raw_file()`. If you pass an eligible file object to `from_file()` you'll get the benefits anyway, because why not.