Pathname#add_trailing_separator

static VALUE
add_trailing_separator(VALUE self, VALUE path)
{
    if (RSTRING_LEN(check_strpath(path)) <= 0) return path;
    rb_encoding *enc = rb_enc_get(path);
    const char *name = RSTRING_PTR(path);
    const char *end = RSTRING_END(path);
    const char *top = rb_enc_path_skip_prefix(name, end, enc);
    if (top < end && isdirsep(end[-1])) {
        if (end[-1] == '/' || rb_enc_prev_char(top, end, end, enc) == end - 1)
            return path;
    }
    return rb_str_cat_cstr(rb_str_dup(path), "/");
}