[PATCH v2] build_bug.h: more user friendly error messages in BUILD_BUG_ON_ZERO()

[Vincent Mailhol]

__BUILD_BUG_ON_ZERO_MSG(), as introduced in [1], makes it possible to
do a static assertions in expressions. The direct benefit is to
provide a meaningful error message instead of the cryptic negative
bitfield size error message currently returned by BUILD_BUG_ON_ZERO():

  ./include/linux/build_bug.h:16:51: error: negative width in bit-field '<anonymous>'
     16 | #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
        |                                                   ^

Get rid of BUILD_BUG_ON_ZERO()'s bitfield size hack. Instead rely on
__BUILD_BUG_ON_ZERO_MSG() which in turn relies on C11's
_Static_assert().

Use some macro magic, similarly to static_assert(), to either use an
optional error message provided by the user or, when omitted, to
produce a default error message by stringifying the tested
expression. With this, for example:

  BUILD_BUG_ON_ZERO(1 > 0)

would now throw:

  ./include/linux/compiler.h:197:62: error: static assertion failed: "1 > 0 is true"
    197 | define __BUILD_BUG_ON_ZERO_MSG(e, msg) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
        |                                                             ^~~~~~~~~~~~~~

Finally, __BUILD_BUG_ON_ZERO_MSG() is already guarded by an:

  #ifdef __CHECKER__

So no need any more for that guard clause for BUILD_BUG_ON_ZERO().
Remove it.

[1] commit d7a516c6eeae ("compiler.h: Fix undefined BUILD_BUG_ON_ZERO()")
Link: https://git.kernel.org/torvalds/c/d7a516c6eeae

Signed-off-by: Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>
---
** Prerequisite **

This patch depends on:

  commit b88937277df ("drm/i915: Convert REG_GENMASK*() to fixed-width GENMASK_U*()")
  Link: https://git.kernel.org/next/linux-next/c/b88937277df

Changelog:

  v1 -> v2:

    - The patch caused an issue because of a conflict in drm/i915:

      Link: https://lore.kernel.org/all/202412080849.sPp82jSi-lkp@xxxxxxxxx/

      Above conflict is indirectly resolved by commit b88937277df
      (c.f. above prerequisite).

      Now that the conflict is resolved, resend the patch.

    - Remove the intermediary __BUILD_BUG_ON_ZERO() macro, instead,
      make __BUILD_BUG_ON_ZERO_MSG() variadic.

  Link to v1: https://lore.kernel.org/all/20241205151316.1480255-2-mailhol.vincent@xxxxxxxxxx/
---
 include/linux/build_bug.h | 10 +++++-----
 include/linux/compiler.h  |  4 ++--
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/include/linux/build_bug.h b/include/linux/build_bug.h
index 3aa3640f8c181f6a54bacffbc43260b57481e67f..2cfbb4c65c784ad82edd1e45c1b4f4c23e78b009 100644
--- a/include/linux/build_bug.h
+++ b/include/linux/build_bug.h
@@ -4,17 +4,17 @@
 
 #include <linux/compiler.h>
 
-#ifdef __CHECKER__
-#define BUILD_BUG_ON_ZERO(e) (0)
-#else /* __CHECKER__ */
 /*
  * Force a compilation error if condition is true, but also produce a
  * result (of value 0 and type int), so the expression can be used
  * e.g. in a structure initializer (or where-ever else comma expressions
  * aren't permitted).
+ *
+ * Take an error message as an optional second argument. If omitted,
+ * default to the stringification of the tested expression.
  */
-#define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); })))
-#endif /* __CHECKER__ */
+#define BUILD_BUG_ON_ZERO(e, ...) \
+	__BUILD_BUG_ON_ZERO_MSG(e, ##__VA_ARGS__, #e " is true")
 
 /* Force a compilation error if a constant expression is not a power of 2 */
 #define __BUILD_BUG_ON_NOT_POWER_OF_2(n)	\
diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 9fc30b6b80c9ef8e53a89f53c16ebbe84e40eedb..48793a7822daad99b27324848d585e3cd9893e71 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -192,9 +192,9 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 })
 
 #ifdef __CHECKER__
-#define __BUILD_BUG_ON_ZERO_MSG(e, msg) (0)
+#define __BUILD_BUG_ON_ZERO_MSG(e, msg, ...) (0)
 #else /* __CHECKER__ */
-#define __BUILD_BUG_ON_ZERO_MSG(e, msg) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
+#define __BUILD_BUG_ON_ZERO_MSG(e, msg, ...) ((int)sizeof(struct {_Static_assert(!(e), msg);}))
 #endif /* __CHECKER__ */
 
 /* &a[0] degrades to a pointer: a different type from an array */

---
base-commit: de305063001d5624138a452bdb2d56e68dc2301c
change-id: 20250327-build_bug-a55c1812ce51

Best regards,
-- 
Vincent Mailhol <mailhol.vincent@xxxxxxxxxx>