<xkeycheck.h>: Fix circular inclusion for Header Units

[StephanTLavavej]

Works towards #60.

Discovered by @olgaark - we've done something weird in the machinery that detects macroized keywords, and this is a problem for building Standard Library Header Units.

We had correctly marked xkeycheck.h as not being automatically importable as a header unit:

STL/stl/inc/header-units.json

Line 126 in 3c13e3e

// "xkeycheck.h", // internal header, provides no machinery, scans for macroized keywords only

And correctly marked yvals_core.h (the STL's central internal header) as being importable (like the vast majority of STL headers):

STL/stl/inc/header-units.json

Line 148 in 3c13e3e

"yvals_core.h"

Next, yvals_core.h defines _STL_COMPILER_PREPROCESSOR:

STL/stl/inc/yvals_core.h

Lines 9 to 18 in 3c13e3e

	#ifndef _STL_COMPILER_PREPROCESSOR
	// 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

And includes vcruntime.h for _HAS_CXX17 etc. before including xkeycheck.h:

STL/stl/inc/yvals_core.h

Lines 341 to 342 in 3c13e3e

	#include <vcruntime.h>
	#include <xkeycheck.h> // The _HAS_CXX tags must be defined before including this.

🪲 Finally, because xkeycheck.h wants _STL_COMPILER_PREPROCESSOR and _HAS_CXX17 etc. it includes yvals_core.h, which is circular inclusion!

STL/stl/inc/xkeycheck.h

Lines 6 to 10 in 3c13e3e

	#pragma once
	#ifndef _XKEYCHECK_H
	#define _XKEYCHECK_H
	#include <yvals_core.h>
	#if _STL_COMPILER_PREPROCESSOR

This wasn't causing problems for ordinary compilation (because yvals_core.h's #pragma once and include guards prevent the recursion), but attempting to build STL Header Units in an optimal way involves "source dependencies" scanning, and this makes yvals_core.h look like it includes itself, which is a problem.

The fix is simple: remove the circular inclusion, and simply comment that xkeycheck.h is super special. Nothing else in the STL includes it, and users should not be including it directly. (I vaguely recall that at one time, every STL header tried to include xkeycheck.h itself, but we centralized this long ago.)

[CaseyCarter]

Instead of assuming, should we #error if _YVALS_CORE_H_ isn't defined? I'm inclined to not check, since (1) as you say elsewhere no user should ever include this and there's no reason to include from elsewhere in the STL, and (2) if someone does somehow manage to include this before yvals_core.h the effect will simply be to disable the contents (_STL_COMPILER_PREPROCESSOR won't be defined). There's no danger of silent bad codegen.

The flip side of this coin is, if this file is only supposed to be included from one place ever, why not inline its contents and delete this file instead of forcing the preprocessor to inline it into yvals_core.h on every compile?

(This is an "approve with suggestions" comment: it's worth discussion, but I certainly won't be bothered if we merge this as is.)

[StephanTLavavej]

I'm inclined to not check

Same. I would add (3) this wouldn't enforce the particular ordering we care about (early enough to protect most of yvals_core.h from macros, but after the definitions of _STL_COMPILER_PREPROCESSOR and _HAS_CXXmeow).

The flip side of this coin is, if this file is only supposed to be included from one place ever, why not inline its contents and delete this file instead of forcing the preprocessor to inline it into yvals_core.h on every compile?

We could do that (as we recently did for <cmath>), but xkeycheck.h is rather large (almost 600 lines, 20 KB) and extremely boring (it should do nothing for builds that aren't doomed); yvals_core.h is already big enough (almost 1400 lines, 67 KB) and contains lots of important stuff.

What I really want is for the compilers to reward keyword macros with doom.

[barcharcraz]

Looks good (xkeycheck sounds like an NSA program doesn't it).