[RFC/PATCH] CodingGuidelines: spell out post-C89 rules

[Junio C Hamano <gitster@pobox.com>]

Even though we have been sticking to C89, there are a few handy
features we borrow from more recent C language in our codebase after
trying them in weather balloons and saw that nobody screamed.

Spell them out.

While at it, extend the existing variable declaration rule a bit to
read better with the newly spelled out rule for the for loop.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Documentation/CodingGuidelines | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index 1169ff6c8e..53903b14c8 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -195,10 +195,24 @@ For C programs:
    by e.g. "echo DEVELOPER=1 >>config.mak".
 
  - We try to support a wide range of C compilers to compile Git with,
-   including old ones. That means that you should not use C99
-   initializers, even if a lot of compilers grok it.
+   including old ones. That means that you should not use certain C99
+   features, even if your compiler groks it.  There are a few
+   exceptions:
 
- - Variables have to be declared at the beginning of the block.
+   . since early 2012 with e1327023ea, we have been using an enum
+     definition whose last element is followed by a comma.
+
+   . since mid 2017 with cbc0f81d and 512f41cf, we have been using
+     designated initializers for struct and array.
+
+   These used to be forbidden, but we have not heard breakage report,
+   so they are assumed to be safe.
+
+ - Variables have to be declared at the beginning of the block, before
+   the first statement (i.e. -Wdeclaration-after-statement).
+
+ - Declaring a variable in the for loop "for (int i = 0; i < 10; i++)"
+   is still not allowed in this codebase.
 
  - NULL pointers shall be written as NULL, not as 0.