Relax the alignment requirement for DiscoverableAsTestContent.Context. (#1076)

This PR allows `DiscoverableAsTestContent.Context` to be less-aligned
than `UInt` so long as its stride remains the same. It also removes the
sneaky conformance of `ExitTest` to `DiscoverableAsTestContent`, opting
instead to use an internal type. Since the conformance to
`DiscoverableAsTestContent` and the implementation of `__store()` form a
closed system (where all type information is controlled by Swift Testing
at runtime), we can do this without breaking any ABI. I've updated
ABI/TestContent.md to remove some of the relevant implementation
details.

### Checklist:

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

---------

Co-authored-by: Stuart Montgomery <smontgomery@apple.com>

Author: grynspan

Documentation/ABI/TestContent.md

@@ -149,25 +149,12 @@ The fourth argument to this function, `reserved`, is reserved for future use.
 Accessor functions should assume it is `0` and must not access it.
 
 The concrete Swift type of the value written to `outValue`, the type pointed to
-by `type`, and the value pointed to by `hint` depend on the kind of record:
+by `type`, and the value pointed to by `hint` depend on the kind of record.
 
-- For test or suite declarations (kind `0x74657374`), the accessor produces a
-  structure of type `Testing.Test.Generator` that the testing library can use
-  to generate the corresponding test[^notAccessorSignature].
-
-  [^notAccessorSignature]: This level of indirection is necessary because
-    loading a test or suite declaration is an asynchronous operation, but C
-    functions cannot be `async`.
-
-  Test content records of this kind do not specify a type for `hint`. Always
-  pass `nil`.
-
-- For exit test declarations (kind `0x65786974`), the accessor produces a
-  structure describing the exit test (of type `Testing.ExitTest`.)
-
-  Test content records of this kind accept a `hint` of type `Testing.ExitTest.ID`.
-  They only produce a result if they represent an exit test declared with the
-  same ID (or if `hint` is `nil`.)
+The record kinds defined by Swift Testing (kinds `0x74657374` and `0x65786974`)
+make use of the `DiscoverableAsTestContent` protocol in the `_TestDiscovery`
+module and do not publicly expose the types of their accessor functions'
+arguments. Do not call the accessor functions for these records directly.
 
 > [!WARNING]
 > Calling code should use [`withUnsafeTemporaryAllocation(of:capacity:_:)`](https://developer.apple.com/documentation/swift/withunsafetemporaryallocation(of:capacity:_:))
@@ -274,7 +261,8 @@ extension FoodTruckDiagnostic: DiscoverableAsTestContent {
 ```
 
 If you customize `TestContentContext`, be aware that the type you specify must
-have the same stride and alignment as `UInt`.
+have the same stride as `UInt` and must have an alignment less than or equal to
+that of `UInt`.
 
 When you are done configuring your type's protocol conformance, you can then
 enumerate all test content records matching it as instances of

Sources/Testing/Discovery+Macro.swift

@@ -8,15 +8,6 @@
 // See https://swift.org/CONTRIBUTORS.txt for Swift project authors
 //
 
-@_spi(Experimental) @_spi(ForToolsIntegrationOnly) internal import _TestDiscovery
-
-/// A shadow declaration of `_TestDiscovery.DiscoverableAsTestContent` that
-/// allows us to add public conformances to it without causing the
-/// `_TestDiscovery` module to appear in `Testing.private.swiftinterface`.
-///
-/// This protocol is not part of the public interface of the testing library.
-protocol DiscoverableAsTestContent: _TestDiscovery.DiscoverableAsTestContent, ~Copyable {}
-
 /// The type of the accessor function used to access a test content record.
 ///
 /// The signature of this function type must match that of the corresponding

Sources/Testing/ExitTests/ExitTest.swift

@@ -66,15 +66,18 @@ public struct ExitTest: Sendable, ~Copyable {
   @_spi(ForToolsIntegrationOnly)
   public var id: ID
 
-  /// The body closure of the exit test.
+  /// An exit test body function.
   ///
   /// - Parameters:
   ///   - exitTest: The exit test to which this body closure belongs.
+  fileprivate typealias Body = @Sendable (_ exitTest: inout Self) async throws -> Void
+
+  /// The body closure of the exit test.
   ///
   /// Do not invoke this closure directly. Instead, invoke ``callAsFunction()``
   /// to run the exit test. Running the exit test will always terminate the
   /// current process.
-  fileprivate var body: @Sendable (_ exitTest: inout Self) async throws -> Void = { _ in }
+  fileprivate var body: Body = { _ in }
 
   /// Storage for ``observedValues``.
   ///
@@ -275,12 +278,34 @@ extension ExitTest {
 
 // MARK: - Discovery
 
-extension ExitTest: DiscoverableAsTestContent {
-  fileprivate static var testContentKind: TestContentKind {
-    "exit"
-  }
+extension ExitTest {
+  /// A type representing an exit test as a test content record.
+  fileprivate struct Record: Sendable, DiscoverableAsTestContent {
+    static var testContentKind: TestContentKind {
+      "exit"
+    }
+
+    typealias TestContentAccessorHint = ID
 
-  fileprivate typealias TestContentAccessorHint = ID
+    /// The ID of the represented exit test.
+    var id: ExitTest.ID
+
+    /// The body of the represented exit test.
+    var body: ExitTest.Body
+
+    /// The set of values captured in the parent process before the exit test is
+    /// called.
+    var capturedValues = [CapturedValue]()
+
+    /// Make the exit test represented by this instance.
+    ///
+    /// - Returns: A new exit test as represented by this instance.
+    func makeExitTest() -> ExitTest {
+      var exitTest = ExitTest(id: id, body: body)
+      exitTest.capturedValues = capturedValues
+      return exitTest
+    }
+  }
 
   /// Store the exit test into the given memory.
   ///
@@ -305,9 +330,7 @@ extension ExitTest: DiscoverableAsTestContent {
   ) -> CBool where repeat each T: Codable & Sendable {
 #if !hasFeature(Embedded)
     // Check that the type matches.
-    let callerExpectedType = TypeInfo(describing: typeAddress.load(as: Any.Type.self))
-    let selfType = TypeInfo(describing: Self.self)
-    guard callerExpectedType == selfType else {
+    guard typeAddress.load(as: Any.Type.self) == Record.self else {
       return false
     }
 #endif
@@ -320,15 +343,15 @@ extension ExitTest: DiscoverableAsTestContent {
 
     // Wrap the body function in a thunk that decodes any captured state and
     // passes it along.
-    let body: @Sendable (inout Self) async throws -> Void = { exitTest in
+    let body: ExitTest.Body = { exitTest in
       let values: (repeat each T) = try exitTest.capturedValues.takeCapturedValues()
       try await body(repeat each values)
     }
 
-    // Construct and return the instance.
-    var exitTest = Self(id: id, body: body)
-    exitTest.capturedValues = Array(repeat (each T).self)
-    outValue.initializeMemory(as: Self.self, to: exitTest)
+    // Construct and return the record.
+    var record = Record(id: id, body: body)
+    record.capturedValues = Array(repeat (each T).self)
+    outValue.initializeMemory(as: Record.self, to: record)
     return true
   }
 }
@@ -343,16 +366,16 @@ extension ExitTest {
   /// - Returns: The specified exit test function, or `nil` if no such exit test
   ///   could be found.
   public static func find(identifiedBy id: ExitTest.ID) -> Self? {
-    for record in Self.allTestContentRecords() {
-      if let exitTest = record.load(withHint: id) {
+    for record in Record.allTestContentRecords() {
+      if let exitTest = record.load(withHint: id)?.makeExitTest() {
         return exitTest
       }
     }
 
 #if !SWT_NO_LEGACY_TEST_DISCOVERY
     // Call the legacy lookup function that discovers tests embedded in types.
-    for record in Self.allTypeMetadataBasedTestContentRecords() {
-      if let exitTest = record.load(withHint: id) {
+    for record in Record.allTypeMetadataBasedTestContentRecords() {
+      if let exitTest = record.load(withHint: id)?.makeExitTest() {
         return exitTest
       }
     }

Sources/_TestDiscovery/TestContentRecord.swift

@@ -52,7 +52,7 @@ extension DiscoverableAsTestContent where Self: ~Copyable {
   /// ([swift-#79667](https://github.com/swiftlang/swift/issues/79667))
   fileprivate static func validateMemoryLayout() {
     precondition(MemoryLayout<TestContentContext>.stride == MemoryLayout<UInt>.stride, "'\(self).TestContentContext' aka '\(TestContentContext.self)' must have the same stride as 'UInt'.")
-    precondition(MemoryLayout<TestContentContext>.alignment == MemoryLayout<UInt>.alignment, "'\(self).TestContentContext' aka '\(TestContentContext.self)' must have the same alignment as 'UInt'.")
+    precondition(MemoryLayout<TestContentContext>.alignment <= MemoryLayout<UInt>.alignment, "'\(self).TestContentContext' aka '\(TestContentContext.self)' must have an alignment less than or equal to that of 'UInt'.")
   }
 }
 

Tests/TestingTests/DiscoveryTests.swift

@@ -59,7 +59,7 @@ struct DiscoveryTests {
 #endif
 
 #if !SWT_NO_DYNAMIC_LINKING && hasFeature(SymbolLinkageMarkers)
-  struct MyTestContent: Testing.DiscoverableAsTestContent {
+  struct MyTestContent: DiscoverableAsTestContent {
     typealias TestContentAccessorHint = UInt32
 
     var value: UInt32