Re: Add macros for diagnostic control, use them in locale/weightwc.h

[Paul Eggert <eggert@cs.ucla.edu>]

A couple of other thoughts.

First, the syntax DIAG_IGNORE (-Wmaybe-uninitialized, ...) is too clever and 
brittle.  Too clever, because in C code it looks like it's subtracting 
'uninitialized' from the negative of 'Wmaybe'.  Too brittle, because suppose GCC 
adds a warning option syntax with commas in it? 
"-Wsuggest-attribute=format,noreturn", say?  Then DIAG_IGNORE 
(-Wsuggest-attribute=format,noreturn, ...) won't work because of C's 
tokenization rules.  For both of these reasions it would be better to use a 
string, e.g., DIAG_IGNORE ("-Wmaybe-uninitialized", ...).

More important, I looked through some of the Gnulib and GNU utilities code that 
deals with working around bugs in GCC's diagnostics generation, and 
-Wmaybe-uninitialized in particular has caused us so much grief that we 
typically silence it in a different way, which I should mention.  We use a macro 
definition like this:

   #ifdef lint
   # define IF_LINT(Code) Code
   #else
   # define IF_LINT(Code) /* empty */
   #endif

and then write declarations like this:

   off_t size IF_LINT ( = 0);

when GCC wrongly complains that 'size' may be used unininitialized.  That way, 
people who want to use -Wmaybe-uninitialized can get clean compiles by compiling 
with '-Wmaybe-uninitialized -Dlint', and people who don't care about this stuff 
can omit the options and get slightly more-efficient code.  Obviously there are 
some pitfalls in this approach but overall its benefits outweigh its cost for 
us.  In particular, it lets us carefully isolate the uninitialized-warning bug, 
whereas the DIAG_IGNORE approach is more of a blunderbuss that disables the 
warning in a too-large region because of GCC's bugs.