An issue was discovered in the v9 crate through 2020-12-18 for Rust. There is an unconditional implementation of Sync for SyncRef<T>.
https://rustsec.org/advisories/RUSTSEC-2020-0127.html
https://raw.githubusercontent.com/rustsec/advisory-db/main/crates/v9/RUSTSEC-2020-0127.md