Add sendable RwLock guards

[djc]

I'm using parking_lot's RwLock in some graph-like code where I use rayon to parallelize initialization of the graph. To enforce some ordering, I'm taking a write lock when I initialize a node inside the rayon loop. Unfortunately, this means the locking happens with non-deterministic ordering, and some of the early nodes start writing edges to the neighboring nodes before those get locked.

Unfortunately, the RwLockWriteGuard is not Send, so I cannot easily bring it along inside the rayon loop. However, I noticed that there is an unsafe fn force_unlock_write(&self) method. Would it make sense to add a safe wrapper that adds a Send wrapper type which uses the forget()/force_unlock_write() pattern internally?

[Amanieu]

If you enable the send_guard feature in cargo then the guards will implement Send (but it prevents the use of the deadlock detector).

[djc]

Thanks! Might be useful to mention that somewhere in the documentation.

[Amanieu]

It is mentioned in the README.

[djc]

I find that many crates mention a list of feature flags in the top-level page of the reference documentation, for example tokio or opentelemetry.