basic implementation and unit tests

Tests/ThreadSafeTests/ThreadSafeTests.swift

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+import Testing
+import Foundation
+@testable import ThreadSafe
+
+class ThreadSafetyTest: @unchecked Sendable {
+    
+    class Obj {
+        let id = UUID().uuidString
+    }
+    
+    // If @ThreadSafe is removed, the test would crash instantly
+    @ThreadSafe var obj = Obj()
+    
+    @Test func testThreadSafe() async throws {
+        let coreCount = ProcessInfo.processInfo.activeProcessorCount
+        
+        for _ in 0 ..< coreCount / 2 {
+            Thread { // write thread
+                while true {
+                    self.obj = .init()
+                }
+            }.start()
+        }
+        
+        for _ in 0 ..< coreCount / 2{
+            Thread { // read thread
+                while self.obj.id.count != 0 {}
+            }.start()
+        }
+        
+        // If it doesn't crash after 10s, consider it passed.
+        // I think it's good enough as the possibility for a false positive is microscopic.
+        // Can't think of a way to definitively prove it's thread safe
+        try? await Task.sleep(for: .seconds(10))
+    }
+}
+
+@Test func TestCopyBehaviour() async throws {
+    struct Example {
+        // valueTypeCopyOnWrite enabled by default
+        @ThreadSafe var string: String = "Hello"
+        // valueTypeCopyOnWrite disabled
+        @ThreadSafe(valueTypeCopyOnWrite: false) var anotherString: String = "Hello"
+        // won't do anything as NSMutableString is a class
+        @ThreadSafe(valueTypeCopyOnWrite: true) var nsString: NSMutableString = "Hello"
+        // same as above
+        @ThreadSafe(valueTypeCopyOnWrite: false) var anotherNSString: NSMutableString = "Hello"
+    }
+
+    let original = Example()
+    var copy = original
+    
+    copy.string.append(", World!")
+    #expect(original.string == "Hello") // copy on write triggered, original not affected
+    #expect(copy.string == "Hello, World!")
+    
+    copy.anotherString.append(", World!")
+    #expect(original.anotherString == "Hello, World!") // valueTypeCopyOnWrite == false, underlying box not copied
+    #expect(copy.anotherString == "Hello, World!")
+    
+    copy.nsString.append(", World!")
+    #expect(original.nsString == "Hello, World!")
+    #expect(copy.nsString == "Hello, World!")
+    
+    copy.anotherNSString.append(", World!")
+    #expect(original.anotherNSString == "Hello, World!")
+    #expect(copy.anotherNSString == "Hello, World!")
+}