[PATCH 5/5] CodingGuidelines: recommend against unportable C99 struct syntax

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Per 33665d98e6b (reftable: make assignments portable to AIX xlc
v12.01, 2022-03-28) forms like ".a.b = *c" can be replaced by using
".a = { .b = *c }" instead.

We'll probably allow these sooner than later, but since the workaround
is trivial let's note it among the C99 features we'd like to hold off
on for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/CodingGuidelines | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index 893f960231f..65b608ca0a2 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -242,6 +242,10 @@ For C programs:
      printf("%"PRIuMAX, (uintmax_t)v); These days the MSVC version we
      rely on supports %z, but the C library used by MinGW does not.
 
+   . Shorthand like ".a.b = *c" in struct assignments is known to trip
+     up an older IBM XLC version, use ".a = { .b = *c }" instead. See
+     the 33665d98e6b portability fix from mid-2022.
+
  - Variables have to be declared at the beginning of the block, before
    the first statement (i.e. -Wdeclaration-after-statement).
 
-- 
2.38.0.971.ge79ff6d20e7