Add a test trait for dependency key. (#308)

* Add a test trait for dependency key.

* wip

* wip

* Update Sources/DependenciesTestSupport/TestTrait.swift

* wip

* wip

---------

Co-authored-by: Stephen Celis <stephen@stephencelis.com>

Package.swift

@@ -50,6 +50,7 @@ let package = Package(
       dependencies: [
         "Dependencies",
         "DependenciesMacros",
+        .product(name: "ConcurrencyExtras", package: "swift-concurrency-extras"),
         .product(name: "IssueReportingTestSupport", package: "xctest-dynamic-overlay"),
       ]
     ),

Package@swift-6.0.swift

@@ -53,6 +53,7 @@ let package = Package(
       name: "DependenciesTestSupport",
       dependencies: [
         "Dependencies",
+        .product(name: "ConcurrencyExtras", package: "swift-concurrency-extras"),
         .product(name: "IssueReportingTestSupport", package: "xctest-dynamic-overlay"),
       ]
     ),

Sources/DependenciesTestSupport/TestTrait.swift

@@ -1,4 +1,5 @@
-#if canImport(Testing)
+#if canImport(Testing) && compiler(>=6)
+  import ConcurrencyExtras
   import Dependencies
   import Testing
 
@@ -40,11 +41,47 @@
     /// - Parameters:
     ///   - keyPath: A key path to a dependency value.
     ///   - value: A dependency value to override for the test.
-    public static func dependency<Value: Sendable>(
+    public static func dependency<Value>(
       _ keyPath: WritableKeyPath<DependencyValues, Value> & Sendable,
-      _ value: Value
+      _ value: sending Value
     ) -> Self {
-      Self { $0[keyPath: keyPath] = value }
+      Self { [uncheckedValue = UncheckedSendable(value)] in
+        $0[keyPath: keyPath] = uncheckedValue.wrappedValue
+      }
+    }
+
+    /// A trait that overrides a test's or suite's dependency.
+    ///
+    /// Useful for overriding a dependency in a test without incurring the nesting and
+    /// indentation of ``withDependencies(_:operation:)-4uz6m``.
+    ///
+    /// ```swift
+    /// struct Client: DependencyKey { … }
+    /// @Test(
+    ///   .dependency(Client.mock)
+    /// )
+    /// func feature() {
+    ///   // ...
+    /// }
+    /// ```
+    ///
+    /// > Important: Due to [a Swift bug](https://github.com/swiftlang/swift/issues/76409), it is
+    /// > not possible to specify a closure directly inside a `@Suite` or `@Test` macro:
+    /// >
+    /// > ```swift
+    /// > @Suite(
+    /// >   .dependency(Client { _ in .mock })  // 🛑
+    /// > )
+    /// > struct FeatureTests { /* ... */ }
+    /// > ```
+    ///
+    /// - Parameters:
+    ///   - keyPath: A key path to a dependency value.
+    ///   - value: A dependency value to override for the test.
+    public static func dependency<Value: TestDependencyKey>(
+      _ value: Value
+    ) -> Self where Value == Value.Value {
+      Self { $0[Value.self] = value }
     }
 
     /// A trait that overrides a test's or suite's dependencies.

Tests/DependenciesTests/SwiftTestingTests.swift

@@ -1,3 +1,5 @@
+import ConcurrencyExtras
+
 #if canImport(Testing)
   import Dependencies
   import DependenciesTestSupport
@@ -78,6 +80,18 @@
         }
       }
     }
+
+    private static let mockClient = Client { 42 }
+    @Test(.dependency(mockClient))
+    func dependencyKeyTypeTrait() {
+      @Dependency(Client.self) var client
+      #expect(client.increment() == 42)
+    }
+
+    @Test(.dependency(\.classClient, ClassClient()))
+    func dependencyKeyNonSendableValue() {
+      // NB: This test is to prove this trait compiles with a non-sendable type.
+    }
   }
 
   private struct Client: TestDependencyKey {
@@ -92,4 +106,19 @@
       }
     }
   }
+
+  class ClassClient {
+    var count = 0
+  }
+  extension DependencyValues {
+    var classClient: ClassClient {
+      get { self[ClassClientKey.self].wrappedValue }
+      set { self[ClassClientKey.self] = UncheckedSendable(newValue) }
+    }
+  }
+  enum ClassClientKey: TestDependencyKey {
+    static var testValue: UncheckedSendable<ClassClient> {
+      UncheckedSendable(ClassClient())
+    }
+  }
 #endif