CVE-2020-36442: Data races in beef

An issue was discovered in the beef crate before 0.5.0 for Rust. Affected versions of this crate did not have a T: Sync bound in the Send impl for Cow<'_, T, U>. This allows users to create data races by making Cow contain types that are (Send && !Sync) like Cell<_> or RefCell<_>.

Such data races can lead to memory corruption.

The flaw was corrected in commit d1c7658 by adding trait bounds T: Sync and T::Owned: Send to the Send impl for Cow<'_, T, U>.