Organize discovered tests in a runner plan based on the module they're implemented in

Sources/Testing/Running/Runner.Plan.swift

@@ -93,6 +93,11 @@ extension Runner {
       stepGraph.compactMap(\.value).sorted { $0.test.sourceLocation < $1.test.sourceLocation }
     }
 
+    /// The tests this runner plan contains.
+    public var tests: some Collection<Test> {
+      steps.lazy.map(\.test)
+    }
+
     /// Initialize an instance of this type with the specified graph of test
     /// plan steps.
     ///

Sources/Testing/Running/Runner.swift

@@ -15,7 +15,7 @@ public struct Runner: Sendable {
   public var plan: Plan
 
   /// The set of tests this runner will run.
-  public var tests: [Test] { plan.steps.map(\.test) }
+  public var tests: [Test] { .init(plan.tests) }
 
   /// The runner's configuration.
   public var configuration: Configuration

Sources/Testing/Test.ID.swift

@@ -10,7 +10,15 @@
 
 extension Test: Identifiable {
   public struct ID: Sendable, Equatable, Hashable {
-    /// The name of the module containing the corresponding test.
+    /// The name of the module in which this test is defined.
+    ///
+    /// This may be different than the name of the module this test's containing
+    /// suite type is declared in. For example, if the test is defined in an
+    /// extension of a type declared in an imported module, the value of this
+    /// property on the ID of the containing suite will be the name of the
+    /// imported module, but the value of this property for the ID of the test
+    /// within that extension will be the name of the module which declares the
+    /// extension.
     public var moduleName: String
 
     /// The fully qualified name components (other than the module name) used to
@@ -123,6 +131,8 @@ extension Test: Identifiable {
     var result = containingTypeInfo.map(ID.init)
       ?? ID(moduleName: sourceLocation.moduleName, nameComponents: [], sourceLocation: nil)
 
+    result.moduleName = sourceLocation.moduleName
+
     if !isSuite {
       result.nameComponents.append(name)
       result.sourceLocation = sourceLocation

Tests/TestingTests/Test.IDTests.swift

@@ -0,0 +1,46 @@
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 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(ForToolsIntegrationOnly) import Testing
+
+@Suite("Test.ID Tests")
+struct Test_IDTests {
+  @Test func topmostSuiteInCurrentModule() async throws {
+    let plan = await Runner.Plan(selecting: SomeSuite.self)
+
+    let suiteID = try #require(plan.tests.first { $0.name == "SomeSuite" }?.id)
+    #expect(suiteID.moduleName == .currentModuleName())
+
+    let functionID = try #require(plan.tests.first { $0.name == "example()" }?.id)
+    #expect(functionID.moduleName == .currentModuleName())
+  }
+
+  @Test func topmostSuiteInDifferentModule() async throws {
+    let plan = await Runner.Plan(selecting: String.AnotherSuite.self)
+
+    let suiteID = try #require(plan.tests.first { $0.name == "AnotherSuite" }?.id)
+    #expect(suiteID.moduleName == .currentModuleName())
+
+    let functionID = try #require(plan.tests.first { $0.name == "example()" }?.id)
+    #expect(functionID.moduleName == .currentModuleName())
+  }
+}
+
+// MARK: - Fixtures
+
+@Suite(.hidden) struct SomeSuite {
+  @Test func example() {}
+}
+
+extension String {
+  @Suite(.hidden) struct AnotherSuite {
+    @Test func example() {}
+  }
+}

Tests/TestingTests/TestSupport/TestingAdditions.swift

@@ -66,12 +66,16 @@ func testFunction(named name: String, in containingType: Any.Type) async -> Test
 ///
 /// - Parameters:
 ///   - containingType: The type containing the tests that should be run.
+///   - fileID: The `#fileID` string whose module should be used to locate
+///     the test suite to run. If `nil`, the module which declares
+///     `containingType` is used. The default value is the file ID of the file
+///     in which this method is called.
 ///   - configuration: The configuration to use for running.
 ///
 /// Any tests defined within `containingType` are also run. If no test is found
 /// representing that type, nothing is run.
-func runTest(for containingType: Any.Type, configuration: Configuration = .init()) async {
-  let plan = await Runner.Plan(selecting: containingType, configuration: configuration)
+func runTest(for containingType: Any.Type, inModuleOf fileID: String? = #fileID, configuration: Configuration = .init()) async {
+  let plan = await Runner.Plan(selecting: containingType, inModuleOf: fileID, configuration: configuration)
   let runner = Runner(plan: plan, configuration: configuration)
   await runner.run()
 }
@@ -123,11 +127,20 @@ extension Runner.Plan {
   ///
   /// - Parameters:
   ///   - containingType: The suite type this plan should select.
+  ///   - fileID: The `#fileID` string whose module should be used to locate
+  ///     the test suite to select. If `nil`, the module which declares
+  ///     `containingType` is used. The default value is the file ID of the file
+  ///     in which this initializer is called.
   ///   - configuration: The configuration to use for planning.
-  init(selecting containingType: Any.Type, configuration: Configuration = .init()) async {
+  init(selecting containingType: Any.Type, inModuleOf fileID: String? = #fileID, configuration: Configuration = .init()) async {
+    var testID = Test.ID(type: containingType)
+
+    if let fileID {
+      testID.moduleName = String(fileID[..<fileID.lastIndex(of: "/")!])
+    }
+
     var configuration = configuration
-    let selection = [Test.ID(type: containingType)]
-    configuration.setTestFilter(toInclude: selection, includeHiddenTests: true)
+    configuration.setTestFilter(toInclude: [testID], includeHiddenTests: true)
 
     await self.init(configuration: configuration)
   }
@@ -409,3 +422,17 @@ extension SourceContext {
     self.init(backtrace: .current(), sourceLocation: sourceLocation)
   }
 }
+
+extension String {
+  /// The name of the module this method is called from.
+  ///
+  /// - Parameters:
+  ///   - fileID: The `#fileID` of the file in which this method is called,
+  ///     which is used to determine the module name.
+  ///
+  /// - Returns: A string containing the name of the module this method is
+  ///   called from.
+  static func currentModuleName(from fileID: String = #fileID) -> String {
+    String(fileID[..<fileID.lastIndex(of: "/")!])
+  }
+}