Add a special file for mutex on unavailable platforms

Author: Azoy

stdlib/public/Synchronization/CMakeLists.txt

@@ -92,6 +92,8 @@ add_swift_target_library(swiftSynchronization ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES
     ${SWIFT_SYNCHRONIZATION_WASM_SOURCES}
   SWIFT_SOURCES_DEPENDS_WINDOWS
     ${SWIFT_SYNCHRONIZATION_WINDOWS_SOURCES}
+  SWIFT_SOURCES_DEPENDS_FREESTANDING
+    Mutex/MutexUnavailable.swift
 
   SWIFT_MODULE_DEPENDS_OSX
     Darwin

stdlib/public/Synchronization/Mutex/MutexUnavailable.swift

@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// This source file is part of the Swift Atomics open source project
+//
+// Copyright (c) 2024 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
+/// A synchronization primitive that protects shared mutable state via
+/// mutual exclusion.
+///
+/// The `Mutex` type offers non-recursive exclusive access to the state
+/// it is protecting by blocking threads attempting to acquire the lock.
+/// Only one execution context at a time has access to the value stored
+/// within the `Mutex` allowing for exclusive access.
+///
+/// An example use of `Mutex` in a class used simultaneously by many
+/// threads protecting a `Dictionary` value:
+///
+///     class Manager {
+///       let cache = Mutex<[Key: Resource]>([:])
+///
+///       func saveResouce(_ resource: Resouce, as key: Key) {
+///         cache.withLock {
+///           $0[key] = resource
+///         }
+///       }
+///     }
+///
+@available(unavailable, *, message: "Mutex is not available on this platform")
+@frozen
+@_staticExclusiveOnly
+public struct Mutex<Value: ~Copyable>: ~Copyable {}