Move wrapper type into Test, make it an enum for ABIness, update documentation to tell devs not to use our typealias because it'll almost certainly break them in the future

Author: grynspan

Documentation/ABI/TestContent.md

@@ -63,6 +63,17 @@ struct SWTTestContentRecord {
 };
 ```
 
+Do not use the `__TestContentRecord` typealias defined in the testing library.
+This type exists to support the testing library's macros and may change in the
+future (e.g. to accomodate a generic argument or to make use of one of the
+reserved fields.)
+
+Instead, define your own copy of this type where needed—you can copy the
+definition above _verbatim_. If your test record type's `context` field (as
+described below) is a pointer type, make sure to change its type in your version
+of `TestContentRecord` accordingly so that, on systems with pointer
+authentication enabled, the pointer is correctly resigned at load time.
+
 ### Record content
 
 #### The kind field
@@ -79,6 +90,9 @@ record's kind is a 32-bit unsigned value. The following kinds are defined:
 <!-- When adding cases to this enumeration, be sure to also update the
 corresponding enumeration in TestContentGeneration.swift. -->
 
+If a test content record's `kind` field equals `0x00000000`, the values of all
+other fields in that record are undefined.
+
 #### The accessor field
 
 The function `accessor` is a C function. When called, it initializes the memory

Sources/Testing/Test+Discovery.swift

@@ -10,28 +10,28 @@
 
 private import _TestingInternals
 
-/// A type that encapsulates test content records that produce instances of
-/// ``Test``.
-///
-/// This type is necessary because such test content records produce an indirect
-/// `async` accessor function rather than directly producing instances of
-/// ``Test``, but functions are non-nominal types and cannot directly conform to
-/// protocols.
-///
-/// - Note: This helper type must have the exact in-memory layout of the `async`
-///   accessor function. Do not add any additional stored properties. The layout
-///   of this type is _de facto_ [guaranteed](https://github.com/swiftlang/swift/blob/main/docs/ABI/TypeLayout.rst)
-///   by the Swift ABI.
-/* @frozen */ private struct _TestRecord: TestContent {
-  static var testContentKind: UInt32 {
-    0x74657374
-  }
+extension Test {
+  /// A type that encapsulates test content records that produce instances of
+  /// ``Test``.
+  ///
+  /// This type is necessary because such test content records produce an
+  /// indirect `async` accessor function rather than directly producing
+  /// instances of ``Test``, but functions are non-nominal types and cannot
+  /// directly conform to protocols.
+  ///
+  /// - Note: This helper type must have the exact in-memory layout of the
+  ///   `async` accessor function. Do not add any additional cases or associated
+  ///   values. The layout of this type is [guaranteed](https://github.com/swiftlang/swift/blob/main/docs/ABI/TypeLayout.rst#fragile-enum-layout)
+  ///   by the Swift ABI.
+  /* @frozen */ private enum _Record: TestContent {
+    static var testContentKind: UInt32 {
+      0x74657374
+    }
 
-  /// This instance's actual (asynchronous) accessor function.
-  var asyncAccessor: @Sendable () async -> Test
-}
+    /// The actual (asynchronous) accessor function.
+    case generator(@Sendable () async -> Test)
+  }
 
-extension Test {
   /// All available ``Test`` instances in the process, according to the runtime.
   ///
   /// The order of values in this sequence is unspecified.
@@ -58,7 +58,12 @@ extension Test {
       // Walk all test content and gather generator functions, then call them in
       // a task group and collate their results.
       if useNewMode {
-        let generators = _TestRecord.allTestContentRecords().lazy.compactMap { $0.load()?.asyncAccessor }
+        let generators = _Record.allTestContentRecords().lazy.compactMap { record in
+          if case let .generator(generator) = record.load() {
+            return generator
+          }
+          return nil // currently unreachable, but not provably so
+        }
         await withTaskGroup(of: Self.self) { taskGroup in
           for generator in generators {
             taskGroup.addTask(operation: generator)