True, symlinks complicate things.

We have to be careful about doing too much (adding edge cases and complexity), though in this case how about:

    canon_src = canonicalize(src);
    canon_dst = canonicalize(dst);
    int ret = rename(src, dst);
    if (ret == EXDEV) {
      /* Note if there are symlinks being recreated
         between the canonicalize() and rename() above,
         then it's better to not fall back to a
         cross device copy anyway.  */
      if (common_prefix(canon_src, canon_dst))
          ret = EINVAL; /* Treat like "copy into self" */
    }