Manual Synchronization should show modern locking patterns with Sendable

[ktoso]

We have developed some patterns to limit the "just sprinkle @unchecked Sendable on it" on codebases.

We should showcase some of our recent locks:

---

Maybe we should take the chance to show good lock patterns here.

Like for example https://github.com/apple/swift-nio/blob/main/Sources/NIOConcurrencyHelpers/NIOLockedValueBox.swift
which is just:

public struct NIOLockedValueBox<Value> {
    
    public init(_ value: Value)

    public func withLockedValue<T>(_ mutate: (inout Value) throws -> T) rethrows -> T
}

extension NIOLockedValueBox: Sendable where Value: Sendable {}

Or also the new Mutex type -- we can ask https://github.com/swiftlang/swift-evolution/blob/main/proposals/0433-mutex.md @Azoy to provide a small snippet

[FranzBusch]

I agree. If using new new Mutex most types don't even have to use @unchecked Sendable since Mutex is unconditionally Sendable. @unchecked Sendable is required with atomics though.

[mattmassicotte]

I think this would be excellent content, and might have a nice home in some kind of "Design Guidelines" article. That's come up many times.

[ktoso]

And we can mention I think we mention OSAllocatedUnfairLock as well