fix: Replace async let with Task to work around Swift 6.3 regression (swiftlang/swift#87481)

Backport of firebase#15991 to 12.11.0.
Fixes SIGABRT: freed pointer was not the last allocation in release builds.

Changes:
- FunctionsContext.swift: 3 async let -> Task {} + .value
- AuthBackend.swift: 2 async let -> Task {} + .value

Author: vadimf

FirebaseAuth/Sources/Swift/Backend/AuthBackend.swift

@@ -64,11 +64,16 @@ final class AuthBackend: AuthBackendProtocol {
                       httpMethod: String,
                       contentType: String,
                       requestConfiguration: AuthRequestConfiguration) async -> URLRequest {
+    // Previously, this section uses `async let`, but that was changed for a
+    // `Task`-based approach to work around a Swift 6.3 regression in Xcode 26.4.
+    // - Context: https://github.com/firebase/firebase-ios-sdk/issues/15974
     // Kick off tasks for the async header values.
-    async let heartbeatsHeaderValue = requestConfiguration.heartbeatLogger?.asyncHeaderValue()
-    async let appCheckTokenHeaderValue = requestConfiguration.appCheck?
-      .getToken(forcingRefresh: false)
-
+    let heartbeatsHeaderValue = Task {
+      await requestConfiguration.heartbeatLogger?.asyncHeaderValue()
+    }
+    let appCheckTokenHeaderValue = Task {
+      await requestConfiguration.appCheck?.getToken(forcingRefresh: false)
+    }
     var request = URLRequest(url: url)
     request.setValue(contentType, forHTTPHeaderField: "Content-Type")
     let additionalFrameworkMarker = requestConfiguration.additionalFrameworkMarker
@@ -86,8 +91,8 @@ final class AuthBackend: AuthBackendProtocol {
       request.setValue(languageCode, forHTTPHeaderField: "X-Firebase-Locale")
     }
     // Wait for the async header values.
-    await request.setValue(heartbeatsHeaderValue, forHTTPHeaderField: "X-Firebase-Client")
-    if let tokenResult = await appCheckTokenHeaderValue {
+    await request.setValue(heartbeatsHeaderValue.value, forHTTPHeaderField: "X-Firebase-Client")
+    if let tokenResult = await appCheckTokenHeaderValue.value {
       if let error = tokenResult.error {
         AuthLog.logWarning(code: "I-AUT000018",
                            message: "Error getting App Check token; using placeholder " +

FirebaseFunctions/Sources/Internal/FunctionsContext.swift

@@ -37,17 +37,20 @@ struct FunctionsContextProvider: Sendable {
   }
 
   func context(options: HTTPSCallableOptions?) async throws -> FunctionsContext {
-    async let authToken = auth?.getToken(forcingRefresh: false)
-    async let appCheckToken = getAppCheckToken(options: options)
-    async let limitedUseAppCheckToken = getLimitedUseAppCheckToken(options: options)
+    // Previously, this section uses `async let`, but that was changed for a
+    // `Task`-based approach to work around Swift regression in Xcode 26.4.
+    // - Context: https://github.com/firebase/firebase-ios-sdk/issues/15974
+    let authToken = Task { try await auth?.getToken(forcingRefresh: false) }
+    let appCheckToken = Task { await getAppCheckToken(options: options) }
+    let limitedUseAppCheckToken = Task { await getLimitedUseAppCheckToken(options: options) }
 
     // Only `authToken` is throwing, but the formatter script removes the `try`
     // from `try authToken` and puts it in front of the initializer call.
-    return try await FunctionsContext(
-      authToken: authToken,
+    return try FunctionsContext(
+      authToken: await authToken.value,
       fcmToken: messaging?.fcmToken,
-      appCheckToken: appCheckToken,
-      limitedUseAppCheckToken: limitedUseAppCheckToken
+      appCheckToken: await appCheckToken.value,
+      limitedUseAppCheckToken: await limitedUseAppCheckToken.value
     )
   }