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

From: SZEDER Gábor <[email-masked]>
Date: Wed, 17 Nov 2021 23:30:02 +0100

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:
> > 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).