Simplify how we filter out hidden tests.

Sources/Testing/Running/Configuration.swift

@@ -118,38 +118,48 @@ public struct Configuration: Sendable {
   /// - Returns: A Boolean value representing if the test satisfied the filter.
   public typealias TestFilter = @Sendable (Test) -> Bool
 
+  /// Storage for ``testFilter``.
+  private var _testFilter: TestFilter = { !$0.isHidden }
+
   /// The test filter to which tests should be filtered when run.
-  public var testFilter: TestFilter?
+  public var testFilter: TestFilter {
+    get {
+      _testFilter
+    }
+    set {
+      // By default, the test filter should always filter out hidden tests. This
+      // is the appropriate behavior for external clients of this SPI. If the
+      // testing library needs to enable hidden tests in its own test targets,
+      // it should use setTestFilter(toMatch:includeHiddenTests:) instead.
+      _testFilter = { test in
+        !test.isHidden && newValue(test)
+      }
+    }
+  }
 
   /// Filter tests to run to those specified via a set of test IDs.
   ///
   /// - Parameters:
-  ///   - selection: A set of test IDs to be filtered. If `nil`, the current
-  ///     selection is cleared.
+  ///   - selection: A set of test IDs to be filtered.
   ///
   /// By default, all tests are run and no filter is set.
-  public mutating func setTestFilter(toMatch selection: Set<Test.ID>?) {
-    setTestFilter(toMatch: selection.map(Test.ID.Selection.init), includeHiddenTests: false)
+  public mutating func setTestFilter(toMatch selection: Set<Test.ID>) {
+    setTestFilter(toMatch: Test.ID.Selection(testIDs: selection), includeHiddenTests: false)
   }
 
-  /// Filter tests to run to those specified via a Test.ID.Selection instance.
+  /// Filter tests to run to those specified by a selection of test IDs.
   ///
   /// - Parameters:
-  ///   - selection: A selection of test IDs to be filtered. If `nil`, the
-  ///     current selection is cleared.
-  ///   - includeHiddenTests: If false, a test annotated with the `.hidden` trait will not be included, even if its ID is present in `selection`.
+  ///   - selection: A selection of test IDs to be filtered.
+  ///   - includeHiddenTests: If false, a test annotated with the `.hidden`
+  ///     trait will not be included, even if its ID is present in `selection`.
   ///
   /// By default, all tests are run and no filter is set.
-  mutating func setTestFilter(toMatch selection: Test.ID.Selection?, includeHiddenTests: Bool) {
-    if let selection {
-      testFilter = { test in
-        if includeHiddenTests || !test.isHidden {
-          return selection.contains(test)
-        }
-        return false
-      }
+  mutating func setTestFilter(toMatch selection: Test.ID.Selection, includeHiddenTests: Bool) {
+    if includeHiddenTests {
+      _testFilter = selection.contains
     } else {
-      testFilter = nil
+      testFilter = selection.contains
     }
   }
 }

Sources/Testing/Running/EntryPoint.swift

@@ -193,10 +193,6 @@ func configurationForSwiftPMEntryPoint(withArguments args: [String]) throws -> C
       }
     }
   }
-  filters.append { test in
-    // Don't run the fixture tests in the testing library's own test targets.
-    !test.isHidden
-  }
   configuration.testFilter = { [filters] test in
     filters.allSatisfy { filter in
       filter(test)

Sources/Testing/Running/Runner.Plan.swift

@@ -122,23 +122,6 @@ extension Runner.Plan {
     }
   }
 
-  /// Determine if a test is included in the selected test IDs, if any are
-  /// configured.
-  ///
-  /// - Parameters:
-  ///   - test: The test to query.
-  ///   - filter: The filter to decide if the test is included.
-  ///
-  /// - Returns: Whether or not the specified test is selected. If
-  ///   `selectedTests` is `nil`, `test` is considered selected if it is not
-  ///   hidden.
-  private static func _isTestIncluded(_ test: Test, using filter: Configuration.TestFilter?) -> Bool {
-    guard let filter else {
-      return !test.isHidden
-    }
-    return filter(test)
-  }
-
   /// Construct a graph of runner plan steps for the specified tests.
   ///
   /// - Parameters:
@@ -176,7 +159,7 @@ extension Runner.Plan {
     // zip() near the end of the function.
     testGraph = testGraph.mapValues { test in
       test.flatMap { test in
-        _isTestIncluded(test, using: configuration.testFilter) ? test : nil
+        configuration.testFilter(test) ? test : nil
       }
     }
 

Sources/Testing/Running/XCTestScaffold.swift

@@ -213,7 +213,7 @@ public enum XCTestScaffold: Sendable {
     if let tags {
       // Check if the test's tags intersect the set of selected tags. If there
       // was a previous filter function, it must also pass.
-      let oldTestFilter = configuration.testFilter ?? { _ in true }
+      let oldTestFilter = configuration.testFilter
       configuration.testFilter = { test in
         !tags.isDisjoint(with: test.tags) && oldTestFilter(test)
       }

Tests/TestingTests/RunnerTests.swift

@@ -49,7 +49,7 @@ struct NeverRunTests {
 final class RunnerTests: XCTestCase {
   func testDefaultInit() async throws {
     let runner = await Runner()
-    XCTAssertFalse(runner.tests.contains { $0.isHidden })
+    XCTAssertFalse(runner.tests.contains(where: \.isHidden))
   }
 
   func testTestsProperty() async throws {