Re: [PATCH] revision: use C99 declaration of variable in for() loop

On Wed, Nov 17, 2021 at 11:03:58AM +0000, Phillip Wood wrote:

Hi Junio

On 15/11/2021 06:27, Junio C Hamano wrote:

quoted

There are certain C99 features that might be nice to use in our code
base, but we've hesitated to do so in order to avoid breaking
compatibility with older compilers. But we don't actually know if
people are even using pre-C99 compilers these days.

One way to figure that out is to introduce a very small use of a
feature, and see if anybody complains, and we've done so to probe
the portability for a few features like "trailing comma in enum
declaration", "designated initializer for struct", and "designated
initializer for array".  A few years ago, we tried to use a handy

     for (int i = 0; i < n; i++)
	use(i);

to introduce a new variable valid only in the loop, but found that
some compilers we cared about didn't like it back then.  Two years
is a long-enough time, so let's try it agin.

If this patch can survive a few releases without complaint, then we
can feel more confident that variable declaration in for() loop is
supported by the compilers our user base use.  And if we do get
complaints, then we'll have gained some data and we can easily
revert this patch.

I like the idea of using a specific test balloon for the features that we
want to use but wont this one break the build for anyone doing 'make
DEVELOPER=1' because -Wdeclaration-after-statement will error out. I think
we could wrap the loop in gcc's warning pragmas to avoid that.

The scope of the loop variable is limited to the loop, so I don't
think this is considered as declaration after statement, just like
other variable declarations in limited scopes that are abundant in
Git's codebase, e.g.:

  printf("...");
  if (var) {
      int a;
      ...
  }

FWIW, I've spent some time with Compiler Explorer compiling a for loop
initial declaration after a statement with '-std=c99 -Werror
-Wdeclaration-after-statement', and none of them complained (though
there were some that didn't understand the '-std=c99' or '-Wdecl...'
options or couldn't compile it for some other reason).