Add a test trait for dependency key.

Sources/DependenciesTestSupport/TestTrait.swift

@@ -47,6 +47,40 @@
       Self { $0[keyPath: keyPath] = value }
     }
 
+    /// 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.
     public static func dependencies(
       _ updateValues: @escaping @Sendable (inout DependencyValues) -> Void

Tests/DependenciesTests/SwiftTestingTests.swift

@@ -78,6 +78,13 @@
         }
       }
     }
+
+    private static let mockClient = Client { 42 }
+    @Test(.dependency(mockClient))
+    func dependencyKeyTypeTrait() {
+      @Dependency(Client.self) var client
+      #expect(client.increment() == 42)
+    }
   }
 
   private struct Client: TestDependencyKey {