Why `MutexGuard` impls `Sync` but not `Send`?

[quinedot]

I recommend reading all of this:

Example of a type that is not `Send`?

For completeness's sake, I think it may be interesting to know of types which are Sync (safe to share across threads) but which are nevertheless not Send (safe to own across threads): Rc is neither, so the difference is harder to grasp. Some reminders By sharing I mean &-access, be it directly –&T references–, or indirectly (capable of ultimatetly yielding & access): Arc<T>, &Arc<T>, &&T, &&mut T, etc. The main cause of "unsafety to share across threads", or, more generally, the main reaso…

The part about your specific question:

For a more realistic example, it turns out that MutexGuard is not Send, since for the definition of MutexGuard to be maximally portable, it needs to support the POSIX API of pthreads, which states that a lock can only be released from within the thread where it was created.