[concurrency] Hide Concurrency StackNesting builtins behind a feature flag.

rdar://166244033

include/swift/Basic/Features.def

@@ -277,6 +277,7 @@ LANGUAGE_FEATURE(LifetimeDependenceMutableAccessors, 0, "Support mutable accesso
 LANGUAGE_FEATURE(InoutLifetimeDependence, 0, "Support @_lifetime(&)")
 SUPPRESSIBLE_LANGUAGE_FEATURE(NonexhaustiveAttribute, 487, "Nonexhaustive Enums")
 LANGUAGE_FEATURE(ModuleSelector, 491, "Module selectors (`Module::name` syntax)")
+LANGUAGE_FEATURE(BuiltinConcurrencyStackNesting, 0, "Concurrency Stack Nesting Builtins")
 
 // Swift 6
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)

lib/AST/FeatureSet.cpp

@@ -326,6 +326,10 @@ static bool usesFeatureClosureBodyMacro(Decl *decl) {
   return false;
 }
 
+static bool usesFeatureBuiltinConcurrencyStackNesting(Decl *decl) {
+  return false;
+}
+
 UNINTERESTING_FEATURE(StrictMemorySafety)
 UNINTERESTING_FEATURE(LibraryEvolution)
 UNINTERESTING_FEATURE(SafeInteropWrappers)

stdlib/public/Concurrency/Task+PriorityEscalation.swift

@@ -125,11 +125,30 @@ func __withTaskPriorityEscalationHandler0<T, E>(
   onPriorityEscalated handler0: @Sendable (UInt8, UInt8) -> Void,
   isolation: isolated (any Actor)? = #isolation
 ) async throws(E) -> T {
+#if $BuiltinConcurrencyStackNesting
   let record =
     unsafe Builtin.taskAddPriorityEscalationHandler(handler: handler0)
   defer {
     unsafe Builtin.taskRemovePriorityEscalationHandler(record: record)
   }
+#else
+  let record = unsafe _taskAddPriorityEscalationHandler(handler: handler0)
+  defer { unsafe _taskRemovePriorityEscalationHandler(record: record) }
+#endif
 
   return try await operation()
 }
+
+@usableFromInline
+@available(SwiftStdlib 6.2, *)
+@_silgen_name("swift_task_addPriorityEscalationHandler")
+func _taskAddPriorityEscalationHandler(
+  handler: (UInt8, UInt8) -> Void
+) -> UnsafeRawPointer /*EscalationNotificationStatusRecord*/
+
+@usableFromInline
+@available(SwiftStdlib 6.2, *)
+@_silgen_name("swift_task_removePriorityEscalationHandler")
+func _taskRemovePriorityEscalationHandler(
+  record: UnsafeRawPointer /*EscalationNotificationStatusRecord*/
+)

stdlib/public/Concurrency/TaskCancellation.swift

@@ -84,9 +84,13 @@ public func withTaskCancellationHandler<T>(
 ) async rethrows -> T {
   // unconditionally add the cancellation record to the task.
   // if the task was already cancelled, it will be executed right away.
+#if $BuiltinConcurrencyStackNesting
   let record = unsafe Builtin.taskAddCancellationHandler(handler: handler)
   defer { unsafe Builtin.taskRemoveCancellationHandler(record: record) }
-
+#else
+  let record = unsafe _taskAddCancellationHandler(handler: handler)
+  defer { unsafe _taskRemoveCancellationHandler(record: record) }
+#endif
   return try await operation()
 }
 
@@ -105,9 +109,13 @@ public func _unsafeInheritExecutor_withTaskCancellationHandler<T>(
 ) async rethrows -> T {
   // unconditionally add the cancellation record to the task.
   // if the task was already cancelled, it will be executed right away.
+#if $BuiltinConcurrencyStackNesting
   let record = unsafe Builtin.taskAddCancellationHandler(handler: handler)
   defer { unsafe Builtin.taskRemoveCancellationHandler(record: record) }
-
+#else
+  let record = unsafe _taskAddCancellationHandler(handler: handler)
+  defer { unsafe _taskRemoveCancellationHandler(record: record) }
+#endif
   return try await operation()
 }
 
@@ -163,3 +171,16 @@ public struct CancellationError: Error {
   // no extra information, cancellation is intended to be light-weight
   public init() {}
 }
+
+@usableFromInline
+@available(SwiftStdlib 5.1, *)
+@_silgen_name("swift_task_addCancellationHandler")
+func _taskAddCancellationHandler(handler: () -> Void) -> UnsafeRawPointer /*CancellationNotificationStatusRecord*/
+
+@usableFromInline
+@available(SwiftStdlib 5.1, *)
+@_silgen_name("swift_task_removeCancellationHandler")
+func _taskRemoveCancellationHandler(
+  record: UnsafeRawPointer /*CancellationNotificationStatusRecord*/
+)
+

stdlib/public/Concurrency/TaskLocal.swift

@@ -259,8 +259,13 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
                                  operation: () async throws -> R,
                                  isolation: isolated (any Actor)?,
                                  file: String = #fileID, line: UInt = #line) async rethrows -> R {
+#if $BuiltinConcurrencyStackNesting
     Builtin.taskLocalValuePush(key, consume valueDuringOperation)
     defer { Builtin.taskLocalValuePop() }
+#else
+    _taskLocalValuePush(key: key, value: consume valueDuringOperation)
+    defer { _taskLocalValuePop() }
+#endif
 
     return try await operation()
   }
@@ -275,8 +280,13 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
     operation: () async throws -> R,
     file: String = #fileID, line: UInt = #line
   ) async rethrows -> R {
+#if $BuiltinConcurrencyStackNesting
     Builtin.taskLocalValuePush(key, consume valueDuringOperation)
     defer { Builtin.taskLocalValuePop() }
+#else
+    _taskLocalValuePush(key: key, value: consume valueDuringOperation)
+    defer { _taskLocalValuePop() }
+#endif
 
     return try await operation()
   }
@@ -299,8 +309,13 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
   @discardableResult
   public func withValue<R>(_ valueDuringOperation: Value, operation: () throws -> R,
                            file: String = #fileID, line: UInt = #line) rethrows -> R {
+#if $BuiltinConcurrencyStackNesting
     Builtin.taskLocalValuePush(key, valueDuringOperation)
     defer { Builtin.taskLocalValuePop() }
+#else
+    _taskLocalValuePush(key: key, value: valueDuringOperation)
+    defer { _taskLocalValuePop() }
+#endif
 
     return try operation()
   }
@@ -344,6 +359,19 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
 
 // ==== ------------------------------------------------------------------------
 
+@available(SwiftStdlib 5.1, *)
+@usableFromInline
+@_silgen_name("swift_task_localValuePush")
+func _taskLocalValuePush<Value>(
+  key: Builtin.RawPointer/*: Key*/,
+  value: __owned Value
+) // where Key: TaskLocal
+
+@available(SwiftStdlib 5.1, *)
+@usableFromInline
+@_silgen_name("swift_task_localValuePop")
+func _taskLocalValuePop()
+
 @available(SwiftStdlib 5.1, *)
 @_silgen_name("swift_task_localValueGet")
 func _taskLocalValueGet(