[Bug middle-end/77696] Confusing wording for -Wformat-overflow

[dmalcolm at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77696

--- Comment #5 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
(In reply to David Malcolm from comment #4)
> In the meantime, I'm going to post some of the other UI ideas for this that
> we've being chatting about, so that they're captured publicly.

Consider this problematic call to sprintf:

$ cat demo.c
#include <stdio.h>

const char *test_1 (const char *msg)
{
  static char buf[16];
  sprintf (buf, "msg: %s\n", msg);
  return buf; 
}

void test_2 ()
{
  test_1 ("this is long enough to cause trouble");
}

Right now, we emit this (this is trunk, plus some fixes for line-
numbering bugs):

$ ./xgcc -B. -c demo.c  -Wall -O2
demo.c: In function ‘test_2’:
demo.c:6:23: warning: ‘%s’ directive writing 36 bytes into a region of size 11
[-Wformat-overflow=]
     6 |   sprintf (buf, "msg: %s\n", msg);
       |                       ^~
demo.c:12:11:
    12 |   test_1 ("this is long enough to cause trouble");
       |           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
demo.c:6:3: note: ‘sprintf’ output 43 bytes into a destination of size 16
     6 |   sprintf (buf, "msg: %s\n", msg);
       |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I brainstormed some ideas on making these kinds of warning easier for
the user to understand.