Should STL errors use `pragma message` in order to make the output prettier/more obvious?

[strega-nil-ms]

Currently, our STL errors look like:

#error STL1003: Unexpected compiler, expected C++ compiler.

This results in the following output from MSVC:

C:\Users\nimazzuc\projects\stl\out\x64\out/inc\yvals_core.h(687): fatal error C1189: #error STL1003: Unexpected compiler, expected C++ compiler.

This is as opposed to our STL warnings, which result in much prettier output:

C:\Users\nimazzuc\projects\stl\out\x64\out/inc\yvals.h(238): warning STL4000: _STATIC_CPPLIB is deprecated and will be REMOVED.

Do we wish to use pragma message instead, so that we get prettier output?

Something like:

// yvals_core.h
// preprocessor fun
#define _STL_PRAGMA(PRAGMA) _Pragma(#PRAGMA)
#define _STL_PRAGMA_MESSAGE(MESSAGE) _STL_PRAGMA(message(MESSAGE))

// note: clang-format does weird things to this
// clang-format off
#define _ERROR_MESSAGE(NUMBER, MESSAGE) \
    _STL_PRAGMA_MESSAGE(__FILE__ "(" _CRT_STRINGIZE(__LINE__) "): error STL" #NUMBER ": " MESSAGE) \
    static_assert(false, "Error in C++ Standard Library usage.")
// clang-format on

called like:

#ifndef __cplusplus
_ERROR_MESSAGE(1003, "Unexpected compiler, expected C++ compiler.");
#endif

resulting in the following output:

PS C:\..\nimazzuc\projects\stl-tests> cl /c test.cxx
Microsoft (R) C/C++ Optimizing Compiler Version 19.33.31517 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test.cxx
test.cxx(12): error STL1004: Unexpected compiler, expected C++ compiler.
test.cxx(12): error C2338: static_assert failed: 'Error in C++ Standard Library usage.'

which I at least believe is far more readable (and is probably better for VS diagnostics as well)

(if we do make this change, we should do the same pragma preprocessor nonsense for _WARNING_MESSAGE

[CaseyCarter]

I'm concerned about using non-preprocessor machinery (static_assert) to enforce error-ness for the "this isn't a C++ compiler" case. rc/midl/moc all run the preprocessor, but they don't know what a static_assert is, so I think this damages the existing behavior.

For any other STL messages emitted when we know we're being parsed by a C++ compiler (or at least something that proclaims itself to be such) I think this is fine.

[strega-nil]

@CaseyCarter one thing I'll say is that rc/midl/moc should, in theory, just ignore the static_assert, so it won't be damaging at all to leave it in; adding a #error to that specifically would be reasonable though.

Also it frustrates me that _Pragma(error) is not a thing.

[StephanTLavavej]

@CaseyCarter and I talked about this and we agree with you here that this would be a desirable change for consistency and user experience. Casey was convinced after looking at the code that rc/midl/moc/etc will indeed not be affected as we near-immediately prevent them from seeing the vast majority of STL header contents:

STL/stl/inc/yvals_core.h

Lines 10 to 24 in 04ee878

	// All STL headers avoid exposing their contents when included by various
	// non-C++-compiler tools to avoid breaking builds when we use newer language
	// features in the headers than such tools understand.
	#if defined(RC_INVOKED) || defined(Q_MOC_RUN) || defined(__midl)
	#define _STL_COMPILER_PREPROCESSOR 0
	#else
	#define _STL_COMPILER_PREPROCESSOR 1
	#endif
	#endif // _STL_COMPILER_PREPROCESSOR
	#if _STL_COMPILER_PREPROCESSOR
	#ifndef __cplusplus
	#error STL1003: Unexpected compiler, expected C++ compiler.
	#endif // __cplusplus