if (c == ESC)
  {
    *r++ = '\\';
    c = 'e';
  }
      else if (c == '\\' || c == '"')
  *r++ = '\\';

      *r++ = (unsigned char)c;
    }
  *r = '\0';

How does that happen? Because people end up mixing tabs and spaces. Some lines are indented with spaces, some with tabs, too many with tabs intermixed with spaces (I mean things like <space><tab><space>). I'm not even sure how people end up doing the last one, but it's common:

gdb 8.3 include/libiberty.h line 543 has 15 spaces, then a tab, then 10 spaces, then 2 tabs, then 3 spaces.

binutils 2.32 gas/itbl-parse.c line 119 has 1 tab, then 4 spaces, then 1 tab, then 4 spaces, then 1 tab, then 4 spaces, then 1 tab, then 4 spaces, then 1 tab, then 4 spaces, then 1 tab, then 4 spaces, then 1 tab, then 4 spaces.

binutils 2.32 zlib/make_vms.com line 831 has 6 spaces, then a tab, then 2 spaces, then 3 tabs, then 5 spaces.

I'm sure they added tabs to give the convenience of being able to adjust the indentation to what I like, but in reality they just force me to change it to what others like to even read a file. Using hard-tabs seems like naive idealism. I haven't yet seen a project that uses hard-tabs consistently. It's either consistent spaces or tabs intermixed with spaces. At least, that's been my experience.