message: Feature gate copy_file_range_impl on unix
  Only available there...
diff: @@ -441,6 +441,7 @@ impl Mmap {
  /// Needed because that function is not available on all platforms.
  // TODO: better implementation! (this is very quick, low effort)
  // TODO: this function needs tests!
  +#[cfg(target_family = "unix")]
  #[allow(dead_code)]
  fn copy_file_range_impl(
  source_fd: i32,
  