[PATCH v2] add DEVELOPER makefile knob to check for acknowledged warnings

[larsxschneider@gmail.com]

From: Lars Schneider <larsxschneider@gmail.com>

We assume Git developers have a reasonably modern compiler and recommend
them to enable the DEVELOPER makefile knob to ensure their patches are
clear of all compiler warnings the Git core project cares about.

Enable the DEVELOPER makefile knob in the Travis-CI build.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
---

This patch is the successor of "[PATCH v1] travis-ci: override CFLAGS properly,
add -Wdeclaration-after-statement" [1] which enables compiler warnings for the
Travis-CI build.

Peff suggested to codify the knowledge about the compiler warnings the Git
project cares about [2] which I have done here.

The only problem is the "-Wold-style-declaration" compiler warning as this is
only supported by GCC and not Clang. Should we ignore that warning or would you
prefer to detect the GCC compiler and add the warning? The Linux kernel project
does a similar thing [3].


Thanks,
Lars


[1] http://thread.gmane.org/gmane.comp.version-control.git/285752
[2] http://article.gmane.org/gmane.comp.version-control.git/285761
[3] https://github.com/torvalds/linux/blob/6dc390ad61ac8dfca5fa9b0823981fb6f7ec17a0/Makefile#L303-L306


 .travis.yml                    |  2 +-
 Documentation/CodingGuidelines |  4 ++++
 Makefile                       | 12 ++++++++++++
 3 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/.travis.yml b/.travis.yml
index c3bf9c6..168ae21 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -15,12 +15,12 @@ addons:

 env:
   global:
+    - DEVELOPER=1
     - P4_VERSION="15.2"
     - GIT_LFS_VERSION="1.1.0"
     - DEFAULT_TEST_TARGET=prove
     - GIT_PROVE_OPTS="--timer --jobs 3"
     - GIT_TEST_OPTS="--verbose --tee"
-    - CFLAGS="-g -O2 -Wall -Werror"
     - GIT_TEST_CLONE_2GB=YesPlease
     # t9810 occasionally fails on Travis CI OS X
     # t9816 occasionally fails with "TAP out of sequence errors" on Travis CI OS X
diff --git a/Documentation/CodingGuidelines b/Documentation/CodingGuidelines
index c6e536f..1c676c2 100644
--- a/Documentation/CodingGuidelines
+++ b/Documentation/CodingGuidelines
@@ -171,6 +171,10 @@ For C programs:

  - We try to keep to at most 80 characters per line.

+ - As a Git developer we assume you have a reasonably modern compiler
+   and we recommend you to enable the DEVELOPER makefile knob to
+   ensure your patch is clear of all compiler warnings we care about.
+
  - 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.
diff --git a/Makefile b/Makefile
index fd19b54..9d4614d 100644
--- a/Makefile
+++ b/Makefile
@@ -380,6 +380,18 @@ ALL_CFLAGS = $(CPPFLAGS) $(CFLAGS)
 ALL_LDFLAGS = $(LDFLAGS)
 STRIP ?= strip

+ifdef DEVELOPER
+	CFLAGS +=	-Werror \
+				-Wdeclaration-after-statement \
+				-Wno-format-zero-length \
+				-Wold-style-definition \
+				-Woverflow \
+				-Wpointer-arith \
+				-Wstrict-prototypes \
+				-Wunused \
+				-Wvla
+endif
+
 # Create as necessary, replace existing, make ranlib unneeded.
 ARFLAGS = rcs

--
2.5.1