Weird error when passing guards to another thread

[Ved-s]

Guards implement Send for some reason?

[bjorn3]

parking_lot's RwLockWriteGuard implements Send when the protected value implements it.

[Amanieu]

The error message with std is nicer because it uses an unstable feature to directly implement !Send on mutex guards. parking_lot uses a different method (see GuardNoSend) which effectively has the same effect.

[Ved-s]

what was the reason of doing that instead of !Send ?

[bjorn3]

impl !Send for Type {} is unstable, so parking_lot can't use it. The standard library is privileged in that it can use any unstable feature even on stable rustc.

[Ved-s]

Makes sense, though I've seen parking_lot use impl !Sync (ig it's stable?)