TaskLocal test trait

Sources/Testing/CMakeLists.txt

@@ -130,6 +130,7 @@ add_library(Testing
   Traits/Tags/Tag+Macro.swift
   Traits/Tags/Tag+Predefined.swift
   Traits/TimeLimitTrait.swift
+  Traits/TaskLocalTrait.swift
   Traits/Trait.swift)
 target_link_libraries(Testing PRIVATE
   _TestDiscovery

Sources/Testing/Traits/TaskLocalTrait.swift

@@ -0,0 +1,55 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+extension Trait {
+  /// Constructs a trait that binds a task local value for the duration of a test
+  /// or suite.
+  ///
+  /// - Parameters:
+  ///   - taskLocal: The task local to bind the value to.
+  ///   - value: The value to bind to `taskLocal`.
+  ///
+  ///     This value will only be evaluated if the test this
+  ///     trait is applied to runs, and it will be unbound from
+  ///     the task local and released once the trait is finished
+  ///     providing scope for the test.
+  ///
+  /// - Note: You must define the task local outside the test target where the trait is used.
+  public static func taskLocal<Value: Sendable>(
+    _ taskLocal: TaskLocal<Value>,
+    _ value: @autoclosure @escaping @Sendable () throws -> Value
+  ) -> Self
+  where Self == TaskLocalTrait<Value> {
+    TaskLocalTrait(taskLocal: taskLocal, value: value)
+  }
+}
+
+/// A type that that binds a task local value for the duration of a test or suite.
+///
+/// To add this trait to a test, use ``Trait/taskLocal(_:_:)``.
+public struct TaskLocalTrait<Value: Sendable>: SuiteTrait, TestTrait, TestScoping {
+  /// This trait's task local.
+  fileprivate var taskLocal: TaskLocal<Value>
+
+  /// This trait's value.
+  fileprivate var value: @Sendable () throws -> Value
+
+  public var isRecursive: Bool { true }
+
+  public func provideScope(
+    for test: Test,
+    testCase: Test.Case?,
+    performing function: @Sendable () async throws -> Void
+  ) async throws {
+    try await taskLocal.withValue(value()) {
+      try await function()
+    }
+  }
+}

Tests/TestingTests/Traits/TaskLocalTraitTests.swift

@@ -0,0 +1,41 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2026 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+//
+
+@testable @_spi(Experimental) @_spi(ForToolsIntegrationOnly) import Testing
+
+@Suite("Task Local Tests", .tags(.traitRelated))
+struct TaskLocalTests {
+  @Test(
+    ".taskLocal trait",
+    .taskLocal(local, true)
+  )
+  func taskLocalBinding() throws {
+    #expect(local.wrappedValue == true)
+  }
+
+  @Suite(.serialized, .taskLocal(stateLocal, State())) struct MutableLocal {
+    @Test func run1() {
+      #expect(stateLocal.wrappedValue.count == 0)
+      stateLocal.wrappedValue.count += 1
+      #expect(stateLocal.wrappedValue.count == 1)
+    }
+    @Test func run2() {
+      #expect(stateLocal.wrappedValue.count == 0)
+      stateLocal.wrappedValue.count += 1
+      #expect(stateLocal.wrappedValue.count == 1)
+    }
+  }
+}
+
+private let local = TaskLocal(wrappedValue: false)
+private class State: @unchecked Sendable {
+  var count = 0
+}
+private let stateLocal = TaskLocal(wrappedValue: State())