[Concurrency] Nest stack traffic in withValue.

Because `_taskLocalValuePush` and `_taskLocalValuePop` can result in calls
to `swift_task_alloc` and `swift_task_dealloc` respectively, and because
the compiler hasn't been taught about that (e.g.
`SILInstruction::isAllocatingStack`,
`SILInstruction::isDeallocatingStack`, etc), calling them (push and pop)
from a function which makes use the stack for dynamically sized
allocations can result in violations of stack discipline of the form

```
swift_task_alloc // allocates %ptr_1
copy_value_witness // copies into %ptr_1
swift_task_localValuePush // calls swift_task_alloc and allocates %ptr_2
swift_task_dealloc // deallocates %ptr_1
swift_task_localValuePop // calls swift_task_dealloc and deallocates %ptr_2
```

Avoid the problem by not allocating dynamically sized stack space in the
function which calls `_taskLocalValuePush` and `_taskLocalValuePop`.
Split the calls to those functions into `withValueImpl` function which
takes its argument `__owned`.  Call that function from `withValue`,
ensuring that the necessary copy (to account for the fact that withValue
takes its argument `__guaranteed` but `_taskLocalValuePush` takes its
`__owned`) and associated stack traffic occur in `withValue`.

Still, allow `withValueImpl` to be inlined.  The stack nesting will be
preserved across it.

rdar://107275872

Author: nate-chandler

stdlib/public/BackDeployConcurrency/TaskLocal.swift

@@ -141,18 +141,41 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
   @_backDeploy(before: SwiftStdlib 5.8)
   public func withValue<R>(_ valueDuringOperation: Value, operation: () async throws -> R,
                            file: String = #file, line: UInt = #line) async rethrows -> R {
+    return try await withValueImpl(valueDuringOperation, operation: operation, file: file, line: line)
+  }
+
+  /// Implementation for withValue that consumes valueDuringOperation.
+  ///
+  /// Because _taskLocalValuePush and _taskLocalValuePop involve calls to
+  /// swift_task_alloc/swift_task_dealloc respectively unbeknownst to the
+  /// compiler, compiler-emitted calls to swift_task_de/alloc must be avoided
+  /// in a function that calls them.
+  ///
+  /// A copy of valueDuringOperation is required because withValue borrows its
+  /// argument but _taskLocalValuePush consumes its.  Because
+  /// valueDuringOperation is of generic type, its size is not generally known,
+  /// so such a copy entails a stack allocation and a copy to that allocation.
+  /// That stack traffic gets lowered to calls to
+  /// swift_task_alloc/swift_task_deallloc.
+  ///
+  /// Split the calls _taskLocalValuePush/Pop from the compiler-emitted calls
+  /// to swift_task_de/alloc for the copy as follows:
+  /// - withValue contains the compiler-emitted calls swift_task_de/alloc.
+  /// - withValueImpl contains the calls to _taskLocalValuePush/Pop
+  @inlinable
+  @discardableResult
+  @_unsafeInheritExecutor
+  @available(SwiftStdlib 5.1, *) // back deploy requires we declare the availability explicitly on this method
+  @_backDeploy(before: SwiftStdlib 5.9)
+  internal func withValueImpl<R>(_ valueDuringOperation: __owned Value, operation: () async throws -> R,
+                                 file: String = #fileID, line: UInt = #line) async 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(key: key, value: valueDuringOperation)
-    do {
-      let result = try await operation()
-      _taskLocalValuePop()
-      return result
-    } catch {
-      _taskLocalValuePop()
-      throw error
-    }
+    _taskLocalValuePush(key: key, value: consume valueDuringOperation)
+    defer { _taskLocalValuePop() }
+
+    return try await operation()
   }
 
   /// Binds the task-local to the specific value for the duration of the

stdlib/public/Concurrency/TaskLocal.swift

@@ -141,18 +141,41 @@ public final class TaskLocal<Value: Sendable>: Sendable, CustomStringConvertible
   @_backDeploy(before: SwiftStdlib 5.8)
   public func withValue<R>(_ valueDuringOperation: Value, operation: () async throws -> R,
                            file: String = #fileID, line: UInt = #line) async rethrows -> R {
+    return try await withValueImpl(valueDuringOperation, operation: operation, file: file, line: line)
+  }
+
+  /// Implementation for withValue that consumes valueDuringOperation.
+  ///
+  /// Because _taskLocalValuePush and _taskLocalValuePop involve calls to
+  /// swift_task_alloc/swift_task_dealloc respectively unbeknownst to the
+  /// compiler, compiler-emitted calls to swift_task_de/alloc must be avoided
+  /// in a function that calls them.
+  ///
+  /// A copy of valueDuringOperation is required because withValue borrows its
+  /// argument but _taskLocalValuePush consumes its.  Because
+  /// valueDuringOperation is of generic type, its size is not generally known,
+  /// so such a copy entails a stack allocation and a copy to that allocation.
+  /// That stack traffic gets lowered to calls to
+  /// swift_task_alloc/swift_task_deallloc.
+  ///
+  /// Split the calls _taskLocalValuePush/Pop from the compiler-emitted calls
+  /// to swift_task_de/alloc for the copy as follows:
+  /// - withValue contains the compiler-emitted calls swift_task_de/alloc.
+  /// - withValueImpl contains the calls to _taskLocalValuePush/Pop
+  @inlinable
+  @discardableResult
+  @_unsafeInheritExecutor
+  @available(SwiftStdlib 5.1, *) // back deploy requires we declare the availability explicitly on this method
+  @_backDeploy(before: SwiftStdlib 5.9)
+  internal func withValueImpl<R>(_ valueDuringOperation: __owned Value, operation: () async throws -> R,
+                                 file: String = #fileID, line: UInt = #line) async 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(key: key, value: valueDuringOperation)
-    do {
-      let result = try await operation()
-      _taskLocalValuePop()
-      return result
-    } catch {
-      _taskLocalValuePop()
-      throw error
-    }
+    _taskLocalValuePush(key: key, value: consume valueDuringOperation)
+    defer { _taskLocalValuePop() }
+
+    return try await operation()
   }
 
   /// Binds the task-local to the specific value for the duration of the

validation-test/Concurrency/rdar107275872.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift -O -Xfrontend -disable-availability-checking %s -parse-as-library -module-name main -o %t/main
+// RUN: %target-codesign %t/main
+// RUN: %target-run %t/main | %FileCheck %s
+
+// REQUIRES: objc_interop
+// REQUIRES: concurrency
+// REQUIRES: executable_test
+// REQUIRES: concurrency_runtime
+
+import Foundation
+
+@main struct M {
+  @TaskLocal static var v: UUID = UUID()
+  static func test(_ t: UUID) async {
+    await Self.$v.withValue(t) {
+      await Task.sleep(1)
+      print(Self.$v.get())
+    }
+  }
+  static func main() async {
+    // CHECK: before
+    print("before")
+    await test(UUID())
+    // CHECK: after
+    print("after")
+  }
+}