Fix fixed NSString* parameters in variadic functions

Interop/Runtime/src/native/kotlin/kotlinx/cinterop/Varargs.kt

@@ -40,7 +40,15 @@ private tailrec fun convertArgument(
     }
 
     is String -> {
-        location.reinterpret<CPointerVar<*>>()[0] = argument.cstr.getPointer(additionalPlacement)
+        location.reinterpret<CPointerVar<*>>()[0] = if (!isVariadic) {
+            // If it is fixed argument, then it is not C string because it must have been already converted;
+            // then treat it as NSString.
+            // TODO: handle fixed NSString arguments in the stub instead.
+            interpretCPointer<COpaque>(CreateNSStringFromKString(argument))
+        } else {
+            // It is passed as variadic argument; no type information available, so treat it as C string.
+            argument.cstr.getPointer(additionalPlacement)
+        }
         FFI_TYPE_KIND_POINTER
     }
 

backend.native/tests/build.gradle

@@ -2332,6 +2332,7 @@ if (isMac()) {
         goldValue = "84\nFoo\nHello, World!\nKotlin says: Hello, everybody!\nHello from Kotlin\n2, 1\n" +
                 "true\ntrue\n" +
                 "Global string\nAnother global string\nnull\nglobal object\n" +
+                "5\n" +
                 "Deallocated\nDeallocated\n"
 
         source = "interop/objc/smoke.kt"

backend.native/tests/interop/objc/smoke.h

@@ -55,3 +55,5 @@ Class (^ _Nonnull getClassGetter(NSObject* obj))(void);
 
 extern NSString* globalString;
 extern NSObject* globalObject;
+
+int formatStringLength(NSString* format, ...);

backend.native/tests/interop/objc/smoke.kt

@@ -60,6 +60,8 @@ fun run() {
         override fun description() = "global object"
     }
     println(globalObject)
+
+    println(formatStringLength("%d %d", 42, 17))
 }
 
 fun MutablePairProtocol.swap() {

backend.native/tests/interop/objc/smoke.m

@@ -60,3 +60,11 @@ Class (^ _Nonnull getClassGetter(NSObject* obj))() {
 
 NSString* globalString = @"Global string";
 NSObject* globalObject = nil;
+
+int formatStringLength(NSString* format, ...) {
+  va_list args;
+  va_start(args, format);
+  NSString* result = [[NSString alloc] initWithFormat:format arguments:args];
+  va_end(args);
+  return result.length;
+}