[TaskLocals] set task local value in synchronous function

stdlib/public/Concurrency/TaskLocal.swift

@@ -103,6 +103,29 @@ extension TaskLocal {
   }
 }
 
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+extension UnsafeCurrentTask {
+
+  /// Allows for executing a synchronous `body` while binding a task-local value
+  /// in the current task.
+  ///
+  /// This function MUST NOT be invoked by any other task than the current task
+  /// represented by this object.
+  @discardableResult
+  public func withTaskLocal<Value: Sendable, R>(
+      _ access: TaskLocal<Value>.Access, boundTo valueDuringBody: Value,
+      do body: () throws -> R,
+      file: String = #file, line: UInt = #line) rethrows -> R {
+    // check if we're not trying to bind a value from an illegal context; this may crash
+    _checkIllegalTaskLocalBindingWithinWithTaskGroup(file: file, line: line)
+
+    _taskLocalValuePush(self._task, key: access.key, value: valueDuringBody)
+    defer { _taskLocalValuePop(_task) }
+
+    return try body()
+  }
+}
+
 // ==== ------------------------------------------------------------------------
 
 @available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)

test/Concurrency/Runtime/async_task_locals_synchronous_bind.swift

@@ -0,0 +1,66 @@
+// RUN: %target-run-simple-swift(-Xfrontend -enable-experimental-concurrency -parse-as-library %import-libdispatch) | %FileCheck %s
+
+// REQUIRES: executable_test
+// REQUIRES: concurrency
+// REQUIRES: libdispatch
+
+// rdar://76038845
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+enum TL {
+  @TaskLocal(default: 0)
+  static var number
+}
+
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+@discardableResult
+func printTaskLocal<V>(
+    _ key: TaskLocal<V>.Access,
+    _ expected: V? = nil,
+    file: String = #file, line: UInt = #line
+) -> V? {
+  let value = key.get()
+  print("\(key) (\(value)) at \(file):\(line)")
+  if let expected = expected {
+    assert("\(expected)" == "\(value)",
+        "Expected [\(expected)] but found: \(value), at \(file):\(line)")
+  }
+  return expected
+}
+
+// ==== ------------------------------------------------------------------------
+
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+func synchronous_bind() async {
+
+  func synchronous() {
+    printTaskLocal(TL.number) // CHECK: TaskLocal<Int>.Access (1111)
+
+    withUnsafeCurrentTask { task in
+      guard let task = task else {
+        fatalError()
+      }
+
+      task.withTaskLocal(TL.number, boundTo: 2222) {
+        printTaskLocal(TL.number) // CHECK: TaskLocal<Int>.Access (2222)
+      }
+
+      printTaskLocal(TL.number) // CHECK: TaskLocal<Int>.Access (1111)
+    }
+
+    printTaskLocal(TL.number) // CHECK: TaskLocal<Int>.Access (1111)
+  }
+
+  await TL.number.withValue(1111) {
+    synchronous()
+  }
+}
+
+@available(macOS 9999, iOS 9999, watchOS 9999, tvOS 9999, *)
+@main struct Main {
+  static func main() async {
+    await synchronous_bind()
+  }
+}