rb = new(Buffer)
	wb = largeWriter{err: want}
	rb.WriteString("hello, world.")
	if _, err := Copy(wb, rb); err != want {
		t.Errorf("Copy error: got %v, want %v", err, want)
	}
}

func TestNopCloserWriterToForwarding(t *testing.T) {
	for _, tc := range [...]struct {
		Name string
		r    Reader
	}{
		{"not a WriterTo", Reader(nil)},
		{"a WriterTo", struct {
			Reader
			WriterTo
		}{}},
	} {