code: func (m *Quantity) MarshalTo(data []byte) (int, error) {
  	var i int
  	_ = i
  	var l int
  	_ = l
  
  	data[i] = 0xa
  	i++
  	// BEGIN CUSTOM MARSHAL
  	out := m.String()
  	i = encodeVarintGenerated(data, i, uint64(len(out)))
  	i += copy(data[i:], out)
  	// END CUSTOM MARSHAL
  
  	return i, nil
  }
docs: // MarshalTo is a customized version of the generated Protobuf unmarshaler for a struct
  // with a single string field.