diff --git a/Sources/Testing/Support/Locked.swift b/Sources/Testing/Support/Locked.swift
index fbebaa063..601fb14ce 100644
--- a/Sources/Testing/Support/Locked.swift
+++ b/Sources/Testing/Support/Locked.swift
@@ -9,7 +9,9 @@
 //
 
 internal import _TestingInternals
+#if canImport(Synchronization)
 private import Synchronization
+#endif
 
 /// A type that wraps a value requiring access from a synchronous caller during
 /// concurrent execution.
@@ -24,7 +26,7 @@ private import Synchronization
 /// This type is not part of the public interface of the testing library.
 struct Locked<T> {
   /// A type providing storage for the underlying lock and wrapped value.
-#if SWT_TARGET_OS_APPLE && canImport(os)
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
   private typealias _Storage = ManagedBuffer<T, os_unfair_lock_s>
 #elseif !SWT_FIXED_85448 && (os(Linux) || os(Android))
   private final class _Storage: ManagedBuffer<T, pthread_mutex_t> {
@@ -34,7 +36,7 @@ struct Locked<T> {
       }
     }
   }
-#else
+#elseif canImport(Synchronization)
   private final class _Storage {
     let mutex: Mutex<T>
 
@@ -42,6 +44,8 @@ struct Locked<T> {
       mutex = Mutex(rawValue)
     }
   }
+#else
+#error("Platform-specific misconfiguration: no mutex or lock type available")
 #endif
 
   /// Storage for the underlying lock and wrapped value.
@@ -52,7 +56,7 @@ extension Locked: Sendable where T: Sendable {}
 
 extension Locked: RawRepresentable {
   init(rawValue: T) {
-#if SWT_TARGET_OS_APPLE && canImport(os)
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
     _storage = .create(minimumCapacity: 1, makingHeaderWith: { _ in rawValue })
     _storage.withUnsafeMutablePointerToElements { lock in
       lock.initialize(to: .init())
@@ -62,9 +66,11 @@ extension Locked: RawRepresentable {
     _storage.withUnsafeMutablePointerToElements { lock in
       _ = pthread_mutex_init(lock, nil)
     }
-#else
+#elseif canImport(Synchronization)
     nonisolated(unsafe) let rawValue = rawValue
     _storage = _Storage(rawValue)
+#else
+#error("Platform-specific misconfiguration: no mutex or lock type available")
 #endif
   }
 
@@ -91,7 +97,7 @@ extension Locked {
   /// concurrency tools.
   func withLock<R>(_ body: (inout T) throws -> sending R) rethrows -> sending R where R: ~Copyable {
     nonisolated(unsafe) let result: R
-#if SWT_TARGET_OS_APPLE && canImport(os)
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
     result = try _storage.withUnsafeMutablePointers { rawValue, lock in
       os_unfair_lock_lock(lock)
       defer {
@@ -107,10 +113,12 @@ extension Locked {
       }
       return try body(&rawValue.pointee)
     }
-#else
+#elseif canImport(Synchronization)
     result = try _storage.mutex.withLock { rawValue in
       try body(&rawValue)
     }
+#else
+#error("Platform-specific misconfiguration: no mutex or lock type available")
 #endif
     return result
   }
@@ -130,7 +138,7 @@ extension Locked {
   /// concurrency tools.
   func withLockIfAvailable<R>(_ body: (inout T) throws -> sending R) rethrows -> sending R? where R: ~Copyable {
     nonisolated(unsafe) let result: R?
-#if SWT_TARGET_OS_APPLE && canImport(os)
+#if SWT_TARGET_OS_APPLE && !SWT_NO_OS_UNFAIR_LOCK
     result = try _storage.withUnsafeMutablePointers { rawValue, lock in
       guard os_unfair_lock_trylock(lock) else {
         return nil
@@ -150,10 +158,12 @@ extension Locked {
       }
       return try body(&rawValue.pointee)
     }
-#else
+#elseif canImport(Synchronization)
     result = try _storage.mutex.withLockIfAvailable { rawValue in
       return try body(&rawValue)
     }
+#else
+#error("Platform-specific misconfiguration: no mutex or lock type available")
 #endif
     return result
   }
diff --git a/Sources/Testing/Support/Serializer.swift b/Sources/Testing/Support/Serializer.swift
index 96adfca7c..94f7d4f5b 100644
--- a/Sources/Testing/Support/Serializer.swift
+++ b/Sources/Testing/Support/Serializer.swift
@@ -10,9 +10,10 @@
 
 private import _TestingInternals
 
+#if !SWT_NO_UNSTRUCTURED_TASKS
 /// The number of CPU cores on the current system, or `nil` if that
 /// information is not available.
-var cpuCoreCount: Int? {
+private var _cpuCoreCount: Int? {
 #if SWT_TARGET_OS_APPLE || os(Linux) || os(FreeBSD) || os(OpenBSD) || os(Android)
   return Int(sysconf(Int32(_SC_NPROCESSORS_CONF)))
 #elseif os(Windows)
@@ -26,10 +27,11 @@ var cpuCoreCount: Int? {
   return nil
 #endif
 }
+#endif
 
 /// The default parallelization width when parallelized testing is enabled.
 var defaultParallelizationWidth: Int {
-  // cpuCoreCount.map { max(1, $0) * 2 } ?? .max
+  // _cpuCoreCount.map { max(1, $0) * 2 } ?? .max
   .max
 }
 
@@ -45,11 +47,13 @@ final actor Serializer {
   /// The maximum number of work items that may run concurrently.
   nonisolated let maximumWidth: Int
 
+#if !SWT_NO_UNSTRUCTURED_TASKS
   /// The number of scheduled work items, including any currently running.
   private var _currentWidth = 0
 
   /// Continuations for any scheduled work items that haven't started yet.
   private var _continuations = [CheckedContinuation<Void, Never>]()
+#endif
 
   init(maximumWidth: Int = 1) {
     precondition(maximumWidth >= 1, "Invalid serializer width \(maximumWidth).")
@@ -65,6 +69,7 @@ final actor Serializer {
   ///
   /// - Throws: Whatever is thrown by `workItem`.
   func run<R>(_ workItem: @isolated(any) @Sendable () async throws -> R) async rethrows -> R where R: Sendable {
+#if !SWT_NO_UNSTRUCTURED_TASKS
     _currentWidth += 1
     defer {
       // Resume the next scheduled closure.
@@ -86,6 +91,7 @@ final actor Serializer {
         _continuations.append(continuation)
       }
     }
+#endif
 
     return try await workItem()
   }