Affected versions of the crate had a Send/Sync implementation for MappedMutexGuard that only considered variance on T, while MappedMutexGuard dereferenced to U.
This could of led to data races in safe Rust code when a closure used in MutexGuard::map() returns U that is unrelated to T.
The issue was fixed by fixing
Sync implementations, and by adding a
PhantomData<&'a mut U> marker to the
MappedMutexGuard type to tell the compiler that the guard is over