MutexGuard and others should be Send

[theemathas]

MutexGuard, RwLockWriteGuard and RwLockReadGuard are currently !Send. They should be Send.

This also currently causes StdinLock and StdoutLock to be !Send.

[alexcrichton]

These are actually intentionally not sendable. To quote the standard:

If a thread attempts to unlock a mutex that it has not locked or a mutex which is unlocked, undefined behavior results.

As a result these structures are not sendable as we must guarantee that the lock is only released on the same thread that acquired it.

[smessmer]

Since Rust 1.62 has a new Mutex implementation without pthread, should this be reconsidered?

[masklinn]

Since Rust 1.62 has a new Mutex implementation without pthread, should this be reconsidered?

Unix-like OS without futexes still use pthread locks, and for windows

SRW lock must be released by the same thread that acquired it.

I also looked around and didn't find any actually stated guarantee that futexes can be released by a non-owning thread.

[haraldh]

Since Rust 1.62 has a new Mutex implementation without pthread, should this be reconsidered?

Unix-like OS without futexes still use pthread locks, and for windows

SRW lock must be released by the same thread that acquired it.

I also looked around and didn't find any actually stated guarantee that futexes can be released by a non-owning thread.

@masklinn a futex is just an AtomicU32. As long as the thread has access to the shared memory it can "own" it.