[clang] [clang][headers] Including stddef.h always redefines NULL (PR #99727)

[Richard Smith via cfe-commits]

================
@@ -49,7 +48,24 @@
 #define __need_rsize_t
 #endif
 #define __need_wchar_t
+#if !defined(__STDDEF_H) || __has_feature(modules)
+/*
+ * __stddef_null.h is special when building without modules: if __need_NULL is
+ * set, then it will unconditionally redefine NULL. To avoid stepping on client
+ * definitions of NULL, __need_NULL should only be set the first time this
+ * header is included, that is when __STDDEF_H is not defined. However, when
+ * building with modules, this header is a textual header and needs to
+ * unconditionally include __stdef_null.h to support multiple submodules
+ * exporting _Builtin_stddef.null. Take module SM with submodules A and B, whose
+ * headers both include stddef.h When SM.A builds, __STDDEF_H will be defined.
+ * When SM.B builds, the definition from SM.A will leak when building without
+ * local submodule visibility. stddef.h wouldn't include __stddef_null.h, and
+ * SM.B wouldn't import _Builtin_stddef.null, and SM.B's `export *` wouldn't
+ * export NULL as expected. When building with modules, always include
+ * __stddef_null.h so that everything works as expected.
+ */
 #define __need_NULL
+#endif
----------------
zygoloid wrote:

OK, I see: we want a plain include of `<stddef.h>` to define `NULL` only the first time. In non-modules builds, we get this by checking for `__STDDEF_H`, and in modules builds we get this because `__stddef_null.h` is a modular header. (And checking for `__STDDEF_H` isn't sufficient for a modules but non-LSV build because it might leak from a different submodule.) So it doesn't matter whether we define `__need_NULL` for a modules LSV build; it does the same thing either way.

It looks like we would give different behavior in a modules build for something like:
```
#define __need_NULL
#include <stddef.h>
#undef NULL

#include <stddef.h>
```
... which normally results in `NULL` being defined, but with any flavor of modules, results in `NULL` *not* being defined -- because we build `__stddef_null.h` as a modular header, even though it's really not (it's supposed to redefine `NULL` each time it's included). But that's a pre-existing issue.

In any case, I think I'm convinced this change is correct. Thanks for bearing with me :)

https://github.com/llvm/llvm-project/pull/99727