Add Embedded Swift support to the _TestDiscovery target.

This PR adds preliminary/experimental support for Embedded Swift _to the `_TestDiscovery` target only_ when building Swift Testing as a package. To try it out, you must set the environment variable `SWT_EMBEDDED` to `true` before building.

Tested with the following incantation using the 2025-03-28 main-branch toolchain:

```sh
SWT_EMBEDDED=1 swift build --target _TestDiscovery --triple arm64-apple-macosx
```

Author: grynspan

Diff for: Package.swift

@@ -20,17 +20,53 @@ let git = Context.gitInformation
 /// distribution as a package dependency.
 let buildingForDevelopment = (git?.currentTag == nil)
 
+/// Whether or not this package is being built for Embedded Swift.
+///
+/// This value is `true` if `SWT_EMBEDDED` is set in the environment to `true`
+/// when `swift build` is invoked. This inference is experimental and is subject
+/// to change in the future.
+///
+/// - Bug: There is currently no way for us to tell if we are being asked to
+/// 	build for an Embedded Swift target at the package manifest level.
+///   ([swift-syntax-#8431](https://github.com/swiftlang/swift-package-manager/issues/8431))
+let buildingForEmbedded: Bool = {
+  guard let envvar = Context.environment["SWT_EMBEDDED"] else {
+    return false
+  }
+  let result = Bool(envvar) ?? ((Int(envvar) ?? 0) != 0)
+  if result {
+    print("Building for Embedded Swift...")
+  }
+  return result
+}()
+
 let package = Package(
   name: "swift-testing",
 
-  platforms: [
-    .macOS(.v10_15),
-    .iOS(.v13),
-    .watchOS(.v6),
-    .tvOS(.v13),
-    .macCatalyst(.v13),
-    .visionOS(.v1),
-  ],
+  platforms: {
+    if !buildingForEmbedded {
+      [
+        .macOS(.v10_15),
+        .iOS(.v13),
+        .watchOS(.v6),
+        .tvOS(.v13),
+        .macCatalyst(.v13),
+        .visionOS(.v1),
+      ]
+    } else {
+      // Open-source main-branch toolchains (currently required to build this
+      // package for Embedded Swift) have higher Apple platform deployment
+      // targets than we would otherwise require.
+      [
+        .macOS(.v14),
+        .iOS(.v18),
+        .watchOS(.v10),
+        .tvOS(.v18),
+        .macCatalyst(.v18),
+        .visionOS(.v1),
+      ]
+    }
+  }(),
 
   products: {
     var result = [Product]()
@@ -185,6 +221,31 @@ package.targets.append(contentsOf: [
 ])
 #endif
 
+extension BuildSettingCondition {
+  /// Creates a build setting condition that evaluates to `true` for Embedded
+  /// Swift.
+  ///
+  /// - Parameters:
+  /// 	- nonEmbeddedCondition: The value to return if the target is not
+  ///   	Embedded Swift. If `nil`, the build condition evaluates to `false`.
+  ///
+  /// - Returns: A build setting condition that evaluates to `true` for Embedded
+  /// 	Swift or is equal to `nonEmbeddedCondition` for non-Embedded Swift.
+  static func whenEmbedded(or nonEmbeddedCondition: @autoclosure () -> Self? = nil) -> Self? {
+    if !buildingForEmbedded {
+      if let nonEmbeddedCondition = nonEmbeddedCondition() {
+        nonEmbeddedCondition
+      } else {
+        // The caller did not supply a fallback.
+        .when(platforms: [])
+      }
+    } else {
+      // Enable unconditionally because the target is Embedded Swift.
+      nil
+    }
+  }
+}
+
 extension Array where Element == PackageDescription.SwiftSetting {
   /// Settings intended to be applied to every Swift target in this package.
   /// Analogous to project-level build settings in an Xcode project.
@@ -197,6 +258,7 @@ extension Array where Element == PackageDescription.SwiftSetting {
 
     result += [
       .enableUpcomingFeature("ExistentialAny"),
+      .enableExperimentalFeature("Embedded", .whenEmbedded()),
 
       .enableExperimentalFeature("AccessLevelOnImport"),
       .enableUpcomingFeature("InternalImportsByDefault"),
@@ -214,11 +276,14 @@ extension Array where Element == PackageDescription.SwiftSetting {
 
       .define("SWT_TARGET_OS_APPLE", .when(platforms: [.macOS, .iOS, .macCatalyst, .watchOS, .tvOS, .visionOS])),
 
-      .define("SWT_NO_EXIT_TESTS", .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android])),
-      .define("SWT_NO_PROCESS_SPAWNING", .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android])),
-      .define("SWT_NO_SNAPSHOT_TYPES", .when(platforms: [.linux, .custom("freebsd"), .openbsd, .windows, .wasi, .android])),
-      .define("SWT_NO_DYNAMIC_LINKING", .when(platforms: [.wasi])),
-      .define("SWT_NO_PIPES", .when(platforms: [.wasi])),
+      .define("SWT_NO_EXIT_TESTS", .whenEmbedded(or: .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android]))),
+      .define("SWT_NO_PROCESS_SPAWNING", .whenEmbedded(or: .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android]))),
+      .define("SWT_NO_SNAPSHOT_TYPES", .whenEmbedded(or: .when(platforms: [.linux, .custom("freebsd"), .openbsd, .windows, .wasi, .android]))),
+      .define("SWT_NO_DYNAMIC_LINKING", .whenEmbedded(or: .when(platforms: [.wasi]))),
+      .define("SWT_NO_PIPES", .whenEmbedded(or: .when(platforms: [.wasi]))),
+
+      .define("SWT_NO_LEGACY_TEST_DISCOVERY", .whenEmbedded()),
+      .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),
     ]
 
     return result
@@ -271,11 +336,14 @@ extension Array where Element == PackageDescription.CXXSetting {
     var result = Self()
 
     result += [
-      .define("SWT_NO_EXIT_TESTS", .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android])),
-      .define("SWT_NO_PROCESS_SPAWNING", .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android])),
-      .define("SWT_NO_SNAPSHOT_TYPES", .when(platforms: [.linux, .custom("freebsd"), .openbsd, .windows, .wasi, .android])),
-      .define("SWT_NO_DYNAMIC_LINKING", .when(platforms: [.wasi])),
-      .define("SWT_NO_PIPES", .when(platforms: [.wasi])),
+      .define("SWT_NO_EXIT_TESTS", .whenEmbedded(or: .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android]))),
+      .define("SWT_NO_PROCESS_SPAWNING", .whenEmbedded(or: .when(platforms: [.iOS, .watchOS, .tvOS, .visionOS, .wasi, .android]))),
+      .define("SWT_NO_SNAPSHOT_TYPES", .whenEmbedded(or: .when(platforms: [.linux, .custom("freebsd"), .openbsd, .windows, .wasi, .android]))),
+      .define("SWT_NO_DYNAMIC_LINKING", .whenEmbedded(or: .when(platforms: [.wasi]))),
+      .define("SWT_NO_PIPES", .whenEmbedded(or: .when(platforms: [.wasi]))),
+
+      .define("SWT_NO_LEGACY_TEST_DISCOVERY", .whenEmbedded()),
+      .define("SWT_NO_LIBDISPATCH", .whenEmbedded()),
     ]
 
     // Capture the testing library's version as a C++ string constant.

Diff for: Sources/_TestDiscovery/DiscoverableAsTestContent.swift

@@ -48,8 +48,10 @@ public protocol DiscoverableAsTestContent: Sendable, ~Copyable {
 #endif
 }
 
+#if !SWT_NO_LEGACY_TEST_DISCOVERY
 extension DiscoverableAsTestContent where Self: ~Copyable {
   public static var _testContentTypeNameHint: String {
     "__🟡$"
   }
 }
+#endif

Diff for: Sources/_TestDiscovery/TestContentKind.swift

@@ -52,9 +52,11 @@ extension TestContentKind: Equatable, Hashable {
   }
 }
 
+#if !hasFeature(Embedded)
 // MARK: - Codable
 
 extension TestContentKind: Codable {}
+#endif
 
 // MARK: - ExpressibleByStringLiteral, ExpressibleByIntegerLiteral
 

Diff for: Sources/_TestDiscovery/TestContentRecord.swift

@@ -139,6 +139,34 @@ public struct TestContentRecord<T> where T: DiscoverableAsTestContent & ~Copyabl
   /// The type of the `hint` argument to ``load(withHint:)``.
   public typealias Hint = T.TestContentAccessorHint
 
+  /// Invoke an accessor function to load a test content record.
+  ///
+  /// - Parameters:
+  /// 	- accessor: The accessor function to call.
+  ///   - typeAddress: A pointer to the type of test content record.
+  ///   - hint: An optional hint value.
+  ///
+  /// - Returns: An instance of the test content type `T`, or `nil` if the
+  ///   underlying test content record did not match `hint` or otherwise did not
+  ///   produce a value.
+  ///
+  /// Do not call this function directly. Instead, call ``load(withHint:)``.
+  private static func _load(using accessor: _TestContentRecordAccessor, withTypeAt typeAddress: UnsafeRawPointer, withHint hint: Hint? = nil) -> T? {
+    withUnsafeTemporaryAllocation(of: T.self, capacity: 1) { buffer in
+      let initialized = if let hint {
+        withUnsafePointer(to: hint) { hint in
+          accessor(buffer.baseAddress!, typeAddress, hint, 0)
+        }
+      } else {
+        accessor(buffer.baseAddress!, typeAddress, nil, 0)
+      }
+      guard initialized else {
+        return nil
+      }
+      return buffer.baseAddress!.move()
+    }
+  }
+
   /// Load the value represented by this record.
   ///
   /// - Parameters:
@@ -157,21 +185,14 @@ public struct TestContentRecord<T> where T: DiscoverableAsTestContent & ~Copyabl
       return nil
     }
 
-    return withUnsafePointer(to: T.self) { type in
-      withUnsafeTemporaryAllocation(of: T.self, capacity: 1) { buffer in
-        let initialized = if let hint {
-          withUnsafePointer(to: hint) { hint in
-            accessor(buffer.baseAddress!, type, hint, 0)
-          }
-        } else {
-          accessor(buffer.baseAddress!, type, nil, 0)
-        }
-        guard initialized else {
-          return nil
-        }
-        return buffer.baseAddress!.move()
-      }
+#if !hasFeature(Embedded)
+    return withUnsafePointer(to: T.self) { typeAddress in
+      Self._load(using: accessor, withTypeAt: typeAddress, withHint: hint)
     }
+#else
+    let typeAddress = UnsafeRawPointer(bitPattern: UInt(T.testContentKind.rawValue)).unsafelyUnwrapped
+    return Self._load(using: accessor, withTypeAt: typeAddress, withHint: hint)
+#endif
   }
 }
 
@@ -188,7 +209,11 @@ extension TestContentRecord: Sendable where Context: Sendable {}
 
 extension TestContentRecord: CustomStringConvertible {
   public var description: String {
+#if !hasFeature(Embedded)
     let typeName = String(describing: Self.self)
+#else
+    let typeName = "TestContentRecord"
+#endif
     switch _recordStorage {
     case let .atAddress(recordAddress):
       let recordAddress = imageAddress.map { imageAddress in