Backports the Swift 6 type Mutex<Value> to all Apple platforms that support OSAllocatedUnfairLock

Mutex.swift

// Backports the Swift 6 type Mutex<Value> to all Darwin platforms via OSAllocatedUnfairLock.
// Lightweight version of https://github.com/swhitty/swift-mutex
// Feel free to use any part of this gist.

import struct os.OSAllocatedUnfairLock

// Backports the Swift 6.0 Mutex API
@available(iOS, introduced: 16.0, deprecated: 18.0, message: "use Mutex from Synchronization module")
@available(macOS, introduced: 13.0, deprecated: 15.0, message: "use Mutex from Synchronization module")
public struct Mutex<Value: ~Copyable>: @unchecked Sendable, ~Copyable {
    private let lock = OSAllocatedUnfairLock()
    private let storage: Storage

    public init(_ initialValue: consuming sending Value) {
        self.storage = Storage(initialValue)
    }

    public borrowing func withLock<Result, E: Error>(
        _ body: (inout sending Value) throws(E) -> sending Result
    ) throws(E) -> sending Result {
        lock.lock()
        defer { lock.unlock() }
        return try body(&storage.value)
    }

    public borrowing func withLockIfAvailable<Result, E: Error>(
        _ body: (inout sending Value) throws(E) -> sending Result
    ) throws(E) -> sending Result? {
        guard lock.lockIfAvailable() else { return nil }
        defer { lock.unlock() }
        return try body(&storage.value)
    }

    private final class Storage {
        var value: Value

        init(_ initialValue: consuming Value) {
            self.value = initialValue
        }
    }
}