Pathname#chop_basename

static VALUE
chop_basename(VALUE self, VALUE path)
{
    rb_len_t baselen, alllen = RSTRING_LEN(check_strpath(path));
    if (alllen <= 0) return Qnil;
    rb_encoding *enc = rb_enc_get(path);
    const char *name = RSTRING_PTR(path);
    const char *base = ruby_enc_find_basename(name, &baselen, &alllen, enc);
    if (baselen < 1) return Qnil;
    if (baselen == 1 && isdirsep(*base)) return Qnil;
    RUBY_ASSERT(base >= name);
    RUBY_ASSERT(base <= RSTRING_END(path));
    VALUE dir = rb_str_subseq(path, 0, base - name);
    VALUE basename = rb_enc_str_new(base, alllen, enc);
    RB_GC_GUARD(path);
    return rb_assoc_new(dir, basename);
}