Fix an issue where Swift complained that the sent object in transactions could not be passed across actor boundaries

Firestore/CHANGELOG.md

@@ -1,5 +1,7 @@
 # Unreleased
 - [feature] Add `Pipeline` support.
+- [fixed] Fixed an issue where the returned object in transaction blocks could not
+  pass across actor boundaries in Swift 6 (#15467).
 
 # 12.4.0
 - [fixed] Implemented an internal workaround to fix

Firestore/Swift/Source/AsyncAwait/Firestore+AsyncAwait.swift

@@ -102,7 +102,7 @@ public extension Firestore {
   /// explicitly specified in the `updateBlock` parameter.
   /// - Returns Returns the value returned in the `updateBlock` parameter if no errors occurred.
   func runTransaction(_ updateBlock: @escaping (Transaction, NSErrorPointer)
-    -> Any?) async throws -> Any? {
+    -> sending Any?) async throws -> sending Any? {
     // This needs to be wrapped in order to express a nullable return value upon success.
     // See https://github.com/firebase/firebase-ios-sdk/issues/9426 for more details.
     return try await withCheckedThrowingContinuation { continuation in

Firestore/Swift/Source/Stages.swift

@@ -26,7 +26,8 @@ import Foundation
 protocol Stage {
   var name: String { get }
   var bridge: StageBridge { get }
-  /// The `errorMessage` defaults to `nil`. Errors during stage construction are captured and thrown later when `execute()` is called.
+  /// The `errorMessage` defaults to `nil`. Errors during stage construction are captured and thrown
+  /// later when `execute()` is called.
   var errorMessage: String? { get }
 }