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gh-159464: Implement Py_UNUSED using [[maybe_unused]] when available #159467

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gh-159464: Implement Py_UNUSED using [[maybe_unused]] when available #159467

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4 changes: 4 additions & 0 deletions Doc/c-api/intro.rst
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Expand Up @@ -227,6 +227,10 @@ The following macros are for common tasks not specific to Python.
Use this for unused arguments in a function definition to silence compiler
warnings. Example: ``int func(int a, int Py_UNUSED(b)) { return a; }``.

The macro mangles *name* so that it cannot be used, and when possible,
applies the C23/C++17 attribute ``[[maybe_unused]]`` or a compiler-specific
equivalent.

.. versionadded:: 3.4

.. c:macro:: Py_GCC_ATTRIBUTE(name)
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4 changes: 3 additions & 1 deletion Include/pymacro.h
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Expand Up @@ -254,7 +254,9 @@
*
* int func(int a, int Py_UNUSED(b)) { return a; }
*/
#if defined(__GNUC__) || defined(__clang__)
#if _Py__has_c_cpp_attribute(maybe_unused)
# define Py_UNUSED(name) _unused_ ## name _Py_C_ATTRIBUTE(maybe_unused)
#elif defined(__GNUC__) || defined(__clang__)
# define Py_UNUSED(name) _unused_ ## name __attribute__((unused))
#elif defined(_MSC_VER)
// Disable warning C4100: unreferenced formal parameter,
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46 changes: 46 additions & 0 deletions Include/pyport.h
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Expand Up @@ -27,6 +27,52 @@
# define _Py__has_attribute(x) 0
#endif

// Same check for C23 [[attribute]].
#ifdef __has_c_attribute
# define _Py__has_c_attribute(X) __has_c_attribute(X)
#else
# define _Py__has_c_attribute(X) 0
#endif

// Same check for C++17 [[attribute]].
#if defined(__cpp_attributes) && defined(__has_cpp_attribute)
# define _Py__has_cpp_attribute(X) __has_cpp_attribute(X)
#else
# define _Py__has_cpp_attribute(X) 0
#endif

// Combined check for C & C++ [[attribute]]. Only use if attribute works
// the same way in both languages.
#ifdef __cplusplus
# define _Py__has_c_cpp_attribute(X) _Py__has_cpp_attribute(X)
#else
# define _Py__has_c_cpp_attribute(X) _Py__has_c_attribute(X)
#endif

// _Py_C_ATTRIBUTE(attribute): wrapper for [[attribute]]
#if defined(__GNUC__) && !defined(__clang__) && defined(__STDC_VERSION__) && __STDC_VERSION__ < 202311L
// In GCC pedantic mode before C23, `__has_c_attribute` is defined but using
// C attributes emits warnings. Suppress them.
# define _Py_C_ATTRIBUTE(ATTR) [[__extension__ ATTR]]
#elif defined(__clang__) && defined(__STDC_VERSION__) && __STDC_VERSION__ < 202311L
// Similar for clang and C23. It's not clear how to suppress this,
// so pretend the feature is missing.
# undef _Py__has_cpp_attribute
# undef _Py__has_c_cpp_attribute
# define _Py__has_cpp_attribute(X) 0
# define _Py__has_c_cpp_attribute(X) 0
# define _Py_C_ATTRIBUTE(ATTR)
#elif defined(__clang__) && defined(__cplusplus) && __cplusplus < 201703L
// Same for clang and C++17.
# undef _Py__has_c_attribute
# undef _Py__has_c_cpp_attribute
# define _Py__has_c_attribute(X) 0
# define _Py__has_c_cpp_attribute(X) 0
# define _Py_C_ATTRIBUTE(ATTR)
#else
# define _Py_C_ATTRIBUTE(ATTR) [[ATTR]]
#endif

// Macro to use C++ static_cast<> in the Python C API.
#ifdef __cplusplus
# define _Py_STATIC_CAST(type, expr) static_cast<type>(expr)
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