Array#compact!

static VALUE
rb_ary_compact_bang(VALUE ary)
{
    VALUE *p, *t, *end;
    long n;

    rb_ary_modify(ary);
    p = t = (VALUE *)RARRAY_CONST_PTR_TRANSIENT(ary); /* WB: no new reference */
    end = p + RARRAY_LEN(ary);

    while (t < end) {
        if (NIL_P(*t)) t++;
        else *p++ = *t++;
    }
    n = p - RARRAY_CONST_PTR_TRANSIENT(ary);
    if (RARRAY_LEN(ary) == n) {
        return Qnil;
    }
    ary_resize_smaller(ary, n);

    return ary;
}