[FEA]: Implement _CCCL_MAYBE_UNUSED?

[davebayer]

Is this a duplicate?

• I confirmed there appear to be no duplicate issues for this request and that I agree to the Code of Conduct

Area

General CCCL

Is your feature request related to a problem? Please describe.

In many places in the code, there are various implementations of the [[maybe_unused]] behaviour which is unfortunately unavailable before C++17. My idea is to implement _CCCL_MAYBE_UNUSED macro that would be used in these cases instead.

Describe the solution you'd like

The macro could be implemented like this:

#if _CCCL_STD_VER >= 2017 && _CCCL_HAS_CPP_ATTRIBUTE(maybe_unused)
#  define _CCCL_MAYBE_UNUSED [[maybe_unused]]
#elif defined(_CCCL_COMPILER_MSVC)
  // MSVC has no equivalent to [[maybe_unused]], this is the best we can do
# define _CCCL_MAYBE_UNUSED _CCCL_DIAG_PUSH _CCCL_DIAG_SUPPRESS_MSVC(4100 4101 4189 4505) _CCCL_DIAG_POP
#else // ^^^ _CCCL_COMPILER_MSVC ^^^ / vvv !_CCCL_COMPILER_MSVC vvv
# define _CCCL_MAYBE_UNUSED __attribute__((__unused__))
#endif // ^^^ !_CCCL_COMPILER_MSVC ^^^

In C++17 we can just use the [[maybe_unused]] attribute. Otherwise with GCC, Clang, NVHPC and Intel compilers its easy, we can just use __attribute__((unused)). However, MSVC does not implement such behaviour. I tried to improvise with diagnostics - allowing and disabling the diagnostic warnings seems to work, with the fact that the disabled warnings are applied to the whole line instad of a single statement.

Here is a link with some test code: https://godbolt.org/z/jrnE8eq7e

I am mostly focusing on unused variables and functions, [[maybe_unused]] can be used in many other places.

Describe alternatives you've considered

No response

Additional context

No response

[fbusato]

+1. I recently needed this feature in CUB here

[bernhardmgruber]

FYI: We are going to require C++17 in Thrust and CUB soon, so maybe don't spend too much time coming up with workarounds there.