Add value accessor methods to Mutex and RwLock

[EFanZh]

• ACP: Add get, set and replace methods to Mutex and RwLock libs-team#485.
• Tracking issue: Tracking Issue for lock_value_accessors #133407.

This PR adds get, set and replace methods to the Mutex and RwLock types for quick access to their contained values.

One possible optimization would be to check for poisoning first and return an error immediately, without attempting to acquire the lock. I didn’t implement this because I consider poisoning to be relatively rare, adding this extra check could slow down common use cases.

[rustbot]

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[EFanZh]

@rustbot ready

[kennytm]

Just to avoid the discussion getting lost - as I mentioned in rust-lang/libs-team#497, the type PoisonError<T> assumes the T is the result if you simply disregard poisoning. In this PR these are used to store the original input instead.

I suggest we either reword the documentation of PoisonError

Consumes this error indicating that a lock is poisoned, returning the underlying guard to allow access regardless.

or create a different error type that is not associated with LockResult.

[EFanZh]

I have updated the documentation.

[Noratrieb]

The docs changes to the lock result are fine, moving the writing about the guard being contained to the methods explicitly probably makes it more clear even without the new use of the result.

[Noratrieb]

@bors r+

[bors]

📌 Commit 242c6c3 has been approved by Noratrieb

It is now in the queue for this repository.