No it's a good point actually, and it pays to know your context. Frankly, looking at the 118 lines in assert.h, it does considerably more than I really need.

Instead of this:

  assert(buf->pos < buf->str->len);

I could just do this instead:

  if (buf->pos >= buf->str->len) die("bad pos");

If the die message is unique, I don't really need to include __FILE__, __LINE__, and the expression itself in it.

I could even do this, though it's probably overkill for something that should never happen anyway:

  if (buf->pos >= buf->str->len)
      die("bad pos %d %d", buf->pos, buf->str->len);