"tokio::io::ReadHalf<T>::unsplit" can violate the "Pin" contract The soundness issue is described in the "tokio/issues#5372" (https://github.com/tokio-rs/tokio/issues/5372) Specific set of conditions needed to trigger an issue (a !Unpin type in ReadHalf) is unusual, combined with the difficulty of making any arbitrary use-after-free exploitable in Rust without doing a lot of careful alignment of data types in the surrounding code. The "tokio" feature "io-util" is also required to be enabled to trigger this soundness issue. Thanks to zachs18 reporting the issue to Tokio team responsibly and taiki-e and carllerche appropriately responding and fixing the soundness bug. Tokio before 0.2.0 used "futures" 0.1 that did not have "Pin", so it is not affected by this issue.