Added CVarArg support to NSString and String

Author: Molanda

Sources/Foundation/NSString.swift

@@ -1644,3 +1644,23 @@ extension NSString : _StructTypeBridgeable {
         return _StructType._unconditionallyBridgeFromObjectiveC(self)
     }
 }
+
+extension NSString : CVarArg {
+    @inlinable // c-abi
+    public var _cVarArgEncoding: [Int] {
+        return _encodeBitsAsWords(unsafeBitCast(self, to: CFString.self))
+    }
+}
+
+extension String : CVarArg {
+    @inlinable // c-abi
+    public var _cVarArgEncoding: [Int] {
+        // We don't have an autorelease pool to retain the NSString until the withVaList closure is complete.
+        // So add an operation to release on the next cycle of this thread.
+        let ns = Unmanaged.passRetained(NSString(string: self))
+        OperationQueue.current?.addOperation {
+            ns.release()
+        }
+        return ns.takeUnretainedValue()._cVarArgEncoding
+    }
+}

Tests/Foundation/Tests/TestNSString.swift

@@ -813,6 +813,15 @@ class TestNSString: LoopbackServerTest {
         }
     }
 
+    func test_initializeWithFormat4() {
+        let argument: [CVarArg] = ["One", "Two", "Three"]
+        withVaList(argument) {
+            pointer in
+            let string = NSString(format: "Testing %@ %@ %@", arguments: pointer)
+            XCTAssertEqual(string, "Testing One Two Three")
+        }
+    }
+
     func test_appendingPathComponent() {
         do {
             let path: NSString = "/tmp"
@@ -1677,6 +1686,7 @@ class TestNSString: LoopbackServerTest {
             ("test_initializeWithFormat", test_initializeWithFormat),
             ("test_initializeWithFormat2", test_initializeWithFormat2),
             ("test_initializeWithFormat3", test_initializeWithFormat3),
+            ("test_initializeWithFormat4", test_initializeWithFormat4),
             ("test_appendingPathComponent", test_appendingPathComponent),
             ("test_deletingLastPathComponent", test_deletingLastPathComponent),
             ("test_getCString_simple", test_getCString_simple),