RUSTSEC-2020-0059: futures-util: MutexGuard::map can cause a data race in safe code

Description

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 Send and Sync implementations, and by adding a PhantomData<&'a mut U> marker to the MappedMutexGuard type to tell the compiler that the guard is over U too.

More Info

https://github.com/rust-lang/futures-rs/issues/2239

Patched Versions

Unaffected Versions