Missing `Sync` bound on `PyCFunction::new_closure` closures

PyCFunction::newclosure and the temporary newclosurebound complement in the 0.21–0.22 series required the supplied closure to be Send + 'static but not Sync. The resulting PyCFunction is a Python callable that can be invoked from any Python thread, which means the closure may be called concurrent...

CVE-2020-36442

An issue was discovered in the beef crate before 0.5.0 for Rust. beef::Cow has no Sync bound on its Send trait...