Add a couple of workarounds for Swift on Windows

The C++ Interop efforts in Swift currently have some limitations.  In
particular, it cannot support trivial types with non-trivial
destructors.  As a workaround, provide a copy constructor which can be
used by the Swift interop while using the regular semantics for all
other cases.

A second issue arises in the handling of futures.  Unfortunately, it is
not currently possible to pass an indirect block parameter which
prevents the construction of a callback.  Workaround this by providing
an inline shim to use a direct parameter (i.e. indirect value through a
pointer) which then allows a callback to be formed.

Both of these items are being tracked upstream but seem to be
potentially sufficient to enable the use of Swift for using the C++ SDK
for desktop scenarios.

Author: compnerd

app/src/include/firebase/future.h

@@ -407,6 +407,11 @@ class Future : public FutureBase {
   /// when you set up the callback.
   typedef void (*TypedCompletionCallback)(const Future<ResultType>& result_data,
                                           void* user_data);
+#if defined(__swift__)
+  // TODO(apple/swift#67662) indirect block parameters are unsupported
+  typedef void (*TypedCompletionCallback_SwiftWorkaround)(
+      const Future<ResultType>* result_data, void* user_data);
+#endif
 
   /// Construct a future.
   Future() {}
@@ -464,6 +469,16 @@ class Future : public FutureBase {
   inline void OnCompletion(TypedCompletionCallback callback,
                            void* user_data) const;
 
+#if defined(__swift__)
+  // TODO(apple/swift#67662) indirect block parameters are unsupported
+  inline void OnCompletion_SwiftWorkaround(
+      TypedCompletionCallback_SwiftWorkaround callback, void *user_data) const {
+    OnCompletion([callback, user_data](const Future<ResultType>& future) {
+      callback(&future, user_data);
+    });
+  }
+#endif
+
 #if defined(FIREBASE_USE_STD_FUNCTION) || defined(DOXYGEN)
   /// Register a single callback that will be called at most once, when the
   /// future is completed.

auth/CMakeLists.txt

@@ -51,6 +51,7 @@ build_flatbuffers("${flatbuffer_schemas}"
 # Common source files used by all platforms
 set(common_SRCS
     src/auth.cc
+    src/auth_swift.cc
     src/credential.cc
     src/common.cc
     src/common.h

auth/src/auth_swift.cc

@@ -0,0 +1,26 @@
+/*
+ * Copyright 2023 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#define __swift__ 50000
+#include "auth/src/include/firebase/auth.h"
+
+#if FIREBASE_PLATFORM_WINDOWS
+namespace firebase {
+namespace auth {
+Auth::Auth(const Auth &) noexcept = default;
+}
+} // namespace firebase
+#endif

auth/src/include/firebase/auth.h

@@ -147,6 +147,13 @@ class Auth {
 
   ~Auth();
 
+#if defined(__swift__)
+#if FIREBASE_PLATFORM_WINDOWS
+  // TODO(apple/swift#67288) support trivial C++ types with non-trivial dtors
+  Auth(const Auth&) noexcept;
+#endif
+#endif
+
   /// Synchronously gets the cached current user, or returns an object where
   /// is_valid() == false if there is none.
   ///