Add manual Sync impl for ReentrantLockGuard

[programmerjake]

Fixes: #125526
Tracking Issue: #121440

this impl is even shown in the summary in the tracking issue, but apparently was forgotten in the actual implementation

[rustbot]

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[workingjubilee]

@programmerjake Can you add a test for this? Something that fails to compile if ReentrantLock is not Sync, like I added in #125392

Or something that fails to compile if ReentrantLock is !Sync or ?Sync, if you're feeling silly, that's fine too.

[programmerjake]

@workingjubilee

@programmerjake Can you add a test for this? Something that fails to compile if ReentrantLock is not Sync

done, except that this checks ReentrantLockGuard, not ReentrantLock.

[workingjubilee]

ah, yes, I did... 🙃 thank you for doing what I meant instead of what I managed to say.

[workingjubilee]

@bors r+

[bors]

📌 Commit f4b9ac6 has been approved by workingjubilee

It is now in the queue for this repository.

[workingjubilee]

cc @joboet just so you're aware.

[workingjubilee]

@bors rollup