[libcxx-commits] [libcxx] [libc++] Enable _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION for Bionic (PR #218055)

[Nick Desaulniers via libcxx-commits]

https://github.com/nickdesaulniers approved this pull request.

Looks good to me.  I asked Gemini what unit tests for this might look like; consider adding them:

---
AI comment below:

Purpose: standard C++ does not require mutex to have a trivial destructor (and platforms like Darwin intentionally do not support it due to D64298):
```c++
    //===----------------------------------------------------------------------===//
    //
    // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
    // See https://llvm.org/LICENSE.txt for license information.
    // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
    //
    //===----------------------------------------------------------------------===//
    
    // UNSUPPORTED: no-threads
    
    // <mutex>
    
    // class mutex;
    
    // Test that std::mutex has trivial destruction when and only when
    // _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION is enabled.
    
    #include <mutex>
    #include <type_traits>
    
    #include "test_macros.h"

    #if _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION
    static_assert(std::is_trivially_destructible<std::mutex>::value, "");
    #else
    static_assert(!std::is_trivially_destructible<std::mutex>::value, "");
    #endif
```

Purpose: Verifies that static/global instances do not trigger exit-time destructor
  diagnostics on platforms with _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION:
```c++
    //===----------------------------------------------------------------------===//
    //
    // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
    // See https://llvm.org/LICENSE.txt for license information.
    // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
    //
    //===----------------------------------------------------------------------===//

    // UNSUPPORTED: no-threads
    // ADDITIONAL_COMPILE_FLAGS: -Wexit-time-destructors -Werror

    // <mutex>

    #include <mutex>

    #if _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION
    static std::mutex global_mutex;
    #endif

    int main(int, char**) {
    #if _LIBCPP_HAS_TRIVIAL_MUTEX_DESTRUCTION
      static std::mutex local_mutex;
      (void)local_mutex;
    #endif
      return 0;
    }
```

https://github.com/llvm/llvm-project/pull/218055