Make linkage issues more understandable in Objective-C interop

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

@@ -124,6 +124,19 @@ annotation class InteropStubs()
 @Retention(AnnotationRetention.SOURCE)
 private annotation class ObjCMethodImp(val selector: String, val encoding: String)
 
+@konan.internal.ExportForCppRuntime("Kotlin_Interop_getObjCClass")
+private fun getObjCClassByName(name: NativePtr): NativePtr {
+    val result = objc_lookUpClass(name)
+    if (result == nativeNullPtr) {
+        val className = interpretCPointer<ByteVar>(name)!!.toKString()
+        val message = """Objective-C class '$className' not found.
+            |Ensure that the containing framework or library was linked.""".trimMargin()
+
+        throw RuntimeException(message)
+    }
+    return result
+}
+
 @konan.internal.ExportForCompiler
 private fun <T : ObjCObject> allocObjCObject(clazz: NativePtr): T {
     val rawResult = objc_allocWithZone(clazz)
@@ -183,3 +196,6 @@ external fun objc_retain(ptr: NativePtr): NativePtr
 
 @SymbolName("Kotlin_objc_release")
 external fun objc_release(ptr: NativePtr)
+
+@SymbolName("Kotlin_objc_lookUpClass")
+external fun objc_lookUpClass(name: NativePtr): NativePtr

backend.native/compiler/ir/backend.native/src/org/jetbrains/kotlin/backend/konan/llvm/IrToBitcode.kt

@@ -2081,7 +2081,7 @@ internal class CodeGeneratorVisitor(val context: Context, val lifetimes: Map<IrE
         assert(!classDescriptor.isInterface)
 
         return if (classDescriptor.isExternalObjCClass()) {
-            val lookUpFunction = context.llvm.externalFunction("objc_lookUpClass",
+            val lookUpFunction = context.llvm.externalFunction("Kotlin_Interop_getObjCClass",
                     functionType(int8TypePtr, false, int8TypePtr))
 
             call(lookUpFunction,

runtime/src/main/cpp/ObjCInterop.cpp

@@ -27,6 +27,8 @@
 
 extern "C" {
 
+Class Kotlin_Interop_getObjCClass(const char* name);
+
 struct KotlinClassData {
   const TypeInfo* typeInfo;
   int32_t bodyOffset;
@@ -67,8 +69,7 @@ static void DeallocImp(id self, SEL _cmd) {
 }
 
 static void AddDeallocMethod(Class clazz) {
-  Class nsObjectClass = objc_getClass("NSObject");
-  RuntimeAssert(nsObjectClass != nullptr, "NSObject class not found");
+  Class nsObjectClass = Kotlin_Interop_getObjCClass("NSObject");
 
   SEL deallocSelector = sel_registerName("dealloc");
   Method nsObjectDeallocMethod = class_getInstanceMethod(nsObjectClass, deallocSelector);
@@ -136,7 +137,7 @@ void* CreateKotlinObjCClass(const KotlinObjCClassInfo* info) {
     className = classNameBuffer;
   }
 
-  Class superclass = objc_getClass(info->superclassName);
+  Class superclass = Kotlin_Interop_getObjCClass(info->superclassName);
   Class newClass = objc_allocateClassPair(superclass, className, sizeof(struct KotlinClassData));
   RuntimeAssert(newClass != nullptr, "Failed to allocate Objective-C class");
 
@@ -205,6 +206,10 @@ void Kotlin_objc_release(id ptr) {
   objc_release(ptr);
 }
 
+Class Kotlin_objc_lookUpClass(const char* name) {
+  return objc_lookUpClass(name);
+}
+
 } // extern "C"
 
 #else  // KONAN_OBJC_INTEROP
@@ -236,6 +241,10 @@ void Kotlin_objc_release(void* ptr) {
   RuntimeAssert(false, "Objective-C interop is disabled");
 }
 
+void* Kotlin_objc_lookUpClass(const char* name) {
+  RuntimeAssert(false, "Objective-C interop is disabled");
+}
+
 } // extern "C"
 
 #endif // KONAN_OBJC_INTEROP