Simplify nodiscard macros

[StephanTLavavej]

This is a prelude to using [[nodiscard]] literally everywhere, which will be a huge mechanical change.

Commits

• Simplify nodiscard macro definitions.
  • MSVC, Clang, and EDG always support the most advanced form. I'm removing the control macro _HAS_NODISCARD from <vcruntime.h>.
• _NODISCARD_CTOR_MSG => _NODISCARD_MSG
• _NODISCARD_MSG(_Msg) => [[nodiscard(_Msg)]]
• Improve message wrapping.
• Replace _NODISCARD_CTOR with slightly more informative messages.
• Reported VSO-2696854 "C1XX should support [[fallthrough]] in C++14 mode".
  • Clang and EDG already do.
• Replace _NODISCARD_CTOR with _NODISCARD.
  • If compiler warnings aren't sufficiently clear that they're complaining about unused temporary objects, the compiler should be improved.
• _NODISCARD_CTOR_LOCK is now identical to _NODISCARD_LOCK.

__has_cpp_attribute(nodiscard) Investigation

Click to expand:

C:\Temp>type meow.cpp

#ifndef __has_cpp_attribute
static_assert(false, "#ifndef __has_cpp_attribute, this is not C++.");
#elif __has_cpp_attribute(nodiscard) >= 201907L
static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
#elif __has_cpp_attribute(nodiscard) >= 201603L
static_assert(false, "__has_cpp_attribute(nodiscard) >= 201603L, only nodiscard-msg available.");
#else
static_assert(false, "C++ but really old, nodiscard-msg/ctor unavailable.");
#endif

C:\Temp>for %I in (/std:c++14 /std:c++17 /std:c++20) do (cl /EHsc /nologo /W4 /c %I meow.cpp)

C:\Temp>(cl /EHsc /nologo /W4 /c /std:c++14 meow.cpp )
meow.cpp
meow.cpp(4): error C2338: static assertion failed: '__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.'

C:\Temp>(cl /EHsc /nologo /W4 /c /std:c++17 meow.cpp )
meow.cpp
meow.cpp(4): error C2338: static assertion failed: '__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.'

C:\Temp>(cl /EHsc /nologo /W4 /c /std:c++20 meow.cpp )
meow.cpp
meow.cpp(4): error C2338: static assertion failed: '__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.'

C:\Temp>for %I in (/std:c++14 /std:c++17 /std:c++20) do (clang-cl /EHsc /nologo /W4 /c %I meow.cpp)

C:\Temp>(clang-cl /EHsc /nologo /W4 /c /std:c++14 meow.cpp )
meow.cpp(4,15): error: static assertion failed: __has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
      available.
    4 | static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
      |               ^~~~~
1 error generated.

C:\Temp>(clang-cl /EHsc /nologo /W4 /c /std:c++17 meow.cpp )
meow.cpp(4,15): error: static assertion failed: __has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
      available.
    4 | static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
      |               ^~~~~
1 error generated.

C:\Temp>(clang-cl /EHsc /nologo /W4 /c /std:c++20 meow.cpp )
meow.cpp(4,15): error: static assertion failed: __has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
      available.
    4 | static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
      |               ^~~~~
1 error generated.

C:\Temp>for %I in (/std:c++14 /std:c++17 /std:c++20) do (cl /EHsc /nologo /W4 /c /BE %I meow.cpp)

C:\Temp>(cl /EHsc /nologo /W4 /c /BE /std:c++14 meow.cpp )
meow.cpp
"meow.cpp", line 4: error: static assertion failed with
          "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
          available."
  static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
  ^


C:\Temp>(cl /EHsc /nologo /W4 /c /BE /std:c++17 meow.cpp )
meow.cpp
"meow.cpp", line 4: error: static assertion failed with
          "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
          available."
  static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
  ^


C:\Temp>(cl /EHsc /nologo /W4 /c /BE /std:c++20 meow.cpp )
meow.cpp
"meow.cpp", line 4: error: static assertion failed with
          "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully
          available."
  static_assert(false, "__has_cpp_attribute(nodiscard) >= 201907L, nodiscard-msg/ctor fully available.");
  ^