func BenchmarkMmapCopy(b *testing.B) {
	// mmap some big-ish file; will only work on unix-like OSs.
	f, err := Open("/proc/self/exe")
	if err != nil {
		b.Fatalf("Open: %v", err)
	}

	type sectionReaderWithWriterTo struct {
		*io.SectionReader
		*ReaderAt
	}
	reader := &sectionReaderWithWriterTo{
		SectionReader: io.NewSectionReader(f, 0, int64(f.Len())),
		ReaderAt:      f,
	}

	// Sanity check: ensure we will run into the io.Copy optimization when using the NEW code above.
	var _ io.WriterTo = (*sectionReaderWithWriterTo)(nil)

	buf := &bytes.Buffer{}
	// "Hide" the ReaderFrom interface by wrapping the writer.
	// Otherwise we skew the results by optimizing the wrong side.
	writer := struct{ io.Writer }{buf}

	b.ReportAllocs()
	b.ResetTimer()

	for i := 0; i < b.N; i++ {
		_, _ = reader.Seek(0, io.SeekStart)
		buf.Reset()

		n, _ := io.Copy(writer, reader)
		b.SetBytes(n)
	}
}