ok, glad I did the sanity test. hardcoded the following:

pub fn compress() -> Result<(), std::io::Error> {
  let tar_gz = File::create("archive.tar.gz")?;
  let enc = GzEncoder::new(tar_gz, Compression::default());
  let mut tar = tar::Builder::new(enc);
  let mut f = File::open("/tmp/good/gen/scrooge/297e53822ddf/wilyns.thrift.src.main.thrift.thrift-scala/c8959dfdc97a/com/twitter/wilyns/thriftscala/LookupRequest.scala").unwrap();

  tar.append_file("gen/scrooge/297e53822ddf/wilyns.thrift.src.main.thrift.thrift-scala/c8959dfdc97a/com/twitter/wilyns/thriftscala/LookupRequest.scala",
                  &mut f)?;
  Ok(())
}

generates

archive.tar.gz"

. Then

tar -xvf archive.tar.gz

produces the correct

LookupRequest.scala

, whereas using rust to untar it giving

LookupRequest.scala

as a directory.