1 // This file is Copyright its original authors, visible in version control
4 // This file is licensed under the Apache License, Version 2.0 <LICENSE-APACHE
5 // or http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your option.
7 // You may not use this file except in accordance with one or both of these
10 //! A very simple serialization framework which is used to serialize/deserialize messages as well
11 //! as ChannelsManagers and ChannelMonitors.
14 use std::io::{Read, Write};
15 use std::collections::HashMap;
20 use bitcoin::secp256k1::Signature;
21 use bitcoin::secp256k1::key::{PublicKey, SecretKey};
22 use bitcoin::secp256k1::constants::{PUBLIC_KEY_SIZE, SECRET_KEY_SIZE, COMPACT_SIGNATURE_SIZE};
23 use bitcoin::blockdata::script::Script;
24 use bitcoin::blockdata::transaction::{OutPoint, Transaction, TxOut};
25 use bitcoin::consensus;
26 use bitcoin::consensus::Encodable;
27 use bitcoin::hashes::sha256d::Hash as Sha256dHash;
28 use bitcoin::hash_types::{Txid, BlockHash};
29 use core::marker::Sized;
30 use ln::msgs::DecodeError;
31 use ln::{PaymentPreimage, PaymentHash, PaymentSecret};
33 use util::byte_utils::{be48_to_array, slice_to_be48};
35 /// serialization buffer size
36 pub const MAX_BUF_SIZE: usize = 64 * 1024;
38 /// A trait that is similar to std::io::Write but has one extra function which can be used to size
39 /// buffers being written into.
40 /// An impl is provided for any type that also impls std::io::Write which simply ignores size
43 /// (C-not exported) as we only export serialization to/from byte arrays instead
45 /// Writes the given buf out. See std::io::Write::write_all for more
46 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error>;
47 /// Hints that data of the given size is about the be written. This may not always be called
48 /// prior to data being written and may be safely ignored.
49 fn size_hint(&mut self, size: usize);
52 impl<W: Write> Writer for W {
54 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
55 <Self as ::std::io::Write>::write_all(self, buf)
58 fn size_hint(&mut self, _size: usize) { }
61 pub(crate) struct WriterWriteAdaptor<'a, W: Writer + 'a>(pub &'a mut W);
62 impl<'a, W: Writer + 'a> Write for WriterWriteAdaptor<'a, W> {
63 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
66 fn write(&mut self, buf: &[u8]) -> Result<usize, ::std::io::Error> {
67 self.0.write_all(buf)?;
70 fn flush(&mut self) -> Result<(), ::std::io::Error> {
75 pub(crate) struct VecWriter(pub Vec<u8>);
76 impl Writer for VecWriter {
77 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
78 self.0.extend_from_slice(buf);
81 fn size_hint(&mut self, size: usize) {
82 self.0.reserve_exact(size);
86 /// Writer that only tracks the amount of data written - useful if you need to calculate the length
87 /// of some data when serialized but don't yet need the full data.
88 pub(crate) struct LengthCalculatingWriter(pub usize);
89 impl Writer for LengthCalculatingWriter {
91 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
96 fn size_hint(&mut self, _size: usize) {}
99 /// Essentially std::io::Take but a bit simpler and with a method to walk the underlying stream
100 /// forward to ensure we always consume exactly the fixed length specified.
101 pub(crate) struct FixedLengthReader<R: Read> {
106 impl<R: Read> FixedLengthReader<R> {
107 pub fn new(read: R, total_bytes: u64) -> Self {
108 Self { read, bytes_read: 0, total_bytes }
111 pub fn bytes_remain(&mut self) -> bool {
112 self.bytes_read != self.total_bytes
115 pub fn eat_remaining(&mut self) -> Result<(), DecodeError> {
116 ::std::io::copy(self, &mut ::std::io::sink()).unwrap();
117 if self.bytes_read != self.total_bytes {
118 Err(DecodeError::ShortRead)
124 impl<R: Read> Read for FixedLengthReader<R> {
125 fn read(&mut self, dest: &mut [u8]) -> Result<usize, ::std::io::Error> {
126 if self.total_bytes == self.bytes_read {
129 let read_len = cmp::min(dest.len() as u64, self.total_bytes - self.bytes_read);
130 match self.read.read(&mut dest[0..(read_len as usize)]) {
132 self.bytes_read += v as u64;
141 /// A Read which tracks whether any bytes have been read at all. This allows us to distinguish
142 /// between "EOF reached before we started" and "EOF reached mid-read".
143 pub(crate) struct ReadTrackingReader<R: Read> {
147 impl<R: Read> ReadTrackingReader<R> {
148 pub fn new(read: R) -> Self {
149 Self { read, have_read: false }
152 impl<R: Read> Read for ReadTrackingReader<R> {
153 fn read(&mut self, dest: &mut [u8]) -> Result<usize, ::std::io::Error> {
154 match self.read.read(dest) {
157 self.have_read = true;
165 /// A trait that various rust-lightning types implement allowing them to be written out to a Writer
167 /// (C-not exported) as we only export serialization to/from byte arrays instead
168 pub trait Writeable {
169 /// Writes self out to the given Writer
170 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error>;
172 /// Writes self out to a Vec<u8>
173 fn encode(&self) -> Vec<u8> {
174 let mut msg = VecWriter(Vec::new());
175 self.write(&mut msg).unwrap();
179 /// Writes self out to a Vec<u8>
180 fn encode_with_len(&self) -> Vec<u8> {
181 let mut msg = VecWriter(Vec::new());
182 0u16.write(&mut msg).unwrap();
183 self.write(&mut msg).unwrap();
184 let len = msg.0.len();
185 msg.0[..2].copy_from_slice(&(len as u16 - 2).to_be_bytes());
189 /// Gets the length of this object after it has been serialized. This can be overridden to
190 /// optimize cases where we prepend an object with its length.
191 // Note that LLVM optimizes this away in most cases! Check that it isn't before you override!
193 fn serialized_length(&self) -> usize {
194 let mut len_calc = LengthCalculatingWriter(0);
195 self.write(&mut len_calc).expect("No in-memory data may fail to serialize");
200 impl<'a, T: Writeable> Writeable for &'a T {
201 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> { (*self).write(writer) }
204 /// A trait that various rust-lightning types implement allowing them to be read in from a Read
206 /// (C-not exported) as we only export serialization to/from byte arrays instead
210 /// Reads a Self in from the given Read
211 fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError>;
214 /// A trait that various higher-level rust-lightning types implement allowing them to be read in
215 /// from a Read given some additional set of arguments which is required to deserialize.
217 /// (C-not exported) as we only export serialization to/from byte arrays instead
218 pub trait ReadableArgs<P>
221 /// Reads a Self in from the given Read
222 fn read<R: Read>(reader: &mut R, params: P) -> Result<Self, DecodeError>;
225 /// A trait that various rust-lightning types implement allowing them to (maybe) be read in from a Read
227 /// (C-not exported) as we only export serialization to/from byte arrays instead
228 pub trait MaybeReadable
231 /// Reads a Self in from the given Read
232 fn read<R: Read>(reader: &mut R) -> Result<Option<Self>, DecodeError>;
235 pub(crate) struct OptionDeserWrapper<T: Readable>(pub Option<T>);
236 impl<T: Readable> Readable for OptionDeserWrapper<T> {
237 fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError> {
238 Ok(Self(Some(Readable::read(reader)?)))
242 const MAX_ALLOC_SIZE: u64 = 64*1024;
244 pub(crate) struct VecWriteWrapper<'a, T: Writeable>(pub &'a Vec<T>);
245 impl<'a, T: Writeable> Writeable for VecWriteWrapper<'a, T> {
246 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
247 (self.0.len() as u64).write(writer)?;
248 for ref v in self.0.iter() {
254 pub(crate) struct VecReadWrapper<T: Readable>(pub Vec<T>);
255 impl<T: Readable> Readable for VecReadWrapper<T> {
256 fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError> {
257 let count: u64 = Readable::read(reader)?;
258 let mut values = Vec::with_capacity(cmp::min(count, MAX_ALLOC_SIZE / (core::mem::size_of::<T>() as u64)) as usize);
260 match Readable::read(reader) {
261 Ok(v) => { values.push(v); },
262 Err(e) => return Err(e),
269 pub(crate) struct U48(pub u64);
270 impl Writeable for U48 {
272 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
273 writer.write_all(&be48_to_array(self.0))
276 impl Readable for U48 {
278 fn read<R: Read>(reader: &mut R) -> Result<U48, DecodeError> {
279 let mut buf = [0; 6];
280 reader.read_exact(&mut buf)?;
281 Ok(U48(slice_to_be48(&buf)))
285 /// Lightning TLV uses a custom variable-length integer called BigSize. It is similar to Bitcoin's
286 /// variable-length integers except that it is serialized in big-endian instead of little-endian.
288 /// Like Bitcoin's variable-length integer, it exhibits ambiguity in that certain values can be
289 /// encoded in several different ways, which we must check for at deserialization-time. Thus, if
290 /// you're looking for an example of a variable-length integer to use for your own project, move
291 /// along, this is a rather poor design.
292 pub(crate) struct BigSize(pub u64);
293 impl Writeable for BigSize {
295 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
298 (self.0 as u8).write(writer)
301 0xFDu8.write(writer)?;
302 (self.0 as u16).write(writer)
304 0x10000...0xFFFFFFFF => {
305 0xFEu8.write(writer)?;
306 (self.0 as u32).write(writer)
309 0xFFu8.write(writer)?;
310 (self.0 as u64).write(writer)
315 impl Readable for BigSize {
317 fn read<R: Read>(reader: &mut R) -> Result<BigSize, DecodeError> {
318 let n: u8 = Readable::read(reader)?;
321 let x: u64 = Readable::read(reader)?;
323 Err(DecodeError::InvalidValue)
329 let x: u32 = Readable::read(reader)?;
331 Err(DecodeError::InvalidValue)
333 Ok(BigSize(x as u64))
337 let x: u16 = Readable::read(reader)?;
339 Err(DecodeError::InvalidValue)
341 Ok(BigSize(x as u64))
344 n => Ok(BigSize(n as u64))
349 /// In TLV we occasionally send fields which only consist of, or potentially end with, a
350 /// variable-length integer which is simply truncated by skipping high zero bytes. This type
351 /// encapsulates such integers implementing Readable/Writeable for them.
352 #[cfg_attr(test, derive(PartialEq, Debug))]
353 pub(crate) struct HighZeroBytesDroppedVarInt<T>(pub T);
355 macro_rules! impl_writeable_primitive {
356 ($val_type:ty, $len: expr) => {
357 impl Writeable for $val_type {
359 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
360 writer.write_all(&self.to_be_bytes())
363 impl Writeable for HighZeroBytesDroppedVarInt<$val_type> {
365 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
366 // Skip any full leading 0 bytes when writing (in BE):
367 writer.write_all(&self.0.to_be_bytes()[(self.0.leading_zeros()/8) as usize..$len])
370 impl Readable for $val_type {
372 fn read<R: Read>(reader: &mut R) -> Result<$val_type, DecodeError> {
373 let mut buf = [0; $len];
374 reader.read_exact(&mut buf)?;
375 Ok(<$val_type>::from_be_bytes(buf))
378 impl Readable for HighZeroBytesDroppedVarInt<$val_type> {
380 fn read<R: Read>(reader: &mut R) -> Result<HighZeroBytesDroppedVarInt<$val_type>, DecodeError> {
381 // We need to accept short reads (read_len == 0) as "EOF" and handle them as simply
382 // the high bytes being dropped. To do so, we start reading into the middle of buf
383 // and then convert the appropriate number of bytes with extra high bytes out of
385 let mut buf = [0; $len*2];
386 let mut read_len = reader.read(&mut buf[$len..])?;
387 let mut total_read_len = read_len;
388 while read_len != 0 && total_read_len != $len {
389 read_len = reader.read(&mut buf[($len + total_read_len)..])?;
390 total_read_len += read_len;
392 if total_read_len == 0 || buf[$len] != 0 {
393 let first_byte = $len - ($len - total_read_len);
394 let mut bytes = [0; $len];
395 bytes.copy_from_slice(&buf[first_byte..first_byte + $len]);
396 Ok(HighZeroBytesDroppedVarInt(<$val_type>::from_be_bytes(bytes)))
398 // If the encoding had extra zero bytes, return a failure even though we know
399 // what they meant (as the TLV test vectors require this)
400 Err(DecodeError::InvalidValue)
407 impl_writeable_primitive!(u64, 8);
408 impl_writeable_primitive!(u32, 4);
409 impl_writeable_primitive!(u16, 2);
411 impl Writeable for u8 {
413 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
414 writer.write_all(&[*self])
417 impl Readable for u8 {
419 fn read<R: Read>(reader: &mut R) -> Result<u8, DecodeError> {
420 let mut buf = [0; 1];
421 reader.read_exact(&mut buf)?;
426 impl Writeable for bool {
428 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
429 writer.write_all(&[if *self {1} else {0}])
432 impl Readable for bool {
434 fn read<R: Read>(reader: &mut R) -> Result<bool, DecodeError> {
435 let mut buf = [0; 1];
436 reader.read_exact(&mut buf)?;
437 if buf[0] != 0 && buf[0] != 1 {
438 return Err(DecodeError::InvalidValue);
445 macro_rules! impl_array {
447 impl Writeable for [u8; $size]
450 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
455 impl Readable for [u8; $size]
458 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
459 let mut buf = [0u8; $size];
460 r.read_exact(&mut buf)?;
467 //TODO: performance issue with [u8; size] with impl_array!()
468 impl_array!(3); // for rgb
469 impl_array!(4); // for IPv4
470 impl_array!(10); // for OnionV2
471 impl_array!(16); // for IPv6
472 impl_array!(32); // for channel id & hmac
473 impl_array!(PUBLIC_KEY_SIZE); // for PublicKey
474 impl_array!(COMPACT_SIGNATURE_SIZE); // for Signature
475 impl_array!(1300); // for OnionPacket.hop_data
478 impl<K, V> Writeable for HashMap<K, V>
479 where K: Writeable + Eq + Hash,
483 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
484 (self.len() as u16).write(w)?;
485 for (key, value) in self.iter() {
493 impl<K, V> Readable for HashMap<K, V>
494 where K: Readable + Eq + Hash,
498 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
499 let len: u16 = Readable::read(r)?;
500 let mut ret = HashMap::with_capacity(len as usize);
502 ret.insert(K::read(r)?, V::read(r)?);
509 impl Writeable for Vec<u8> {
511 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
512 (self.len() as u16).write(w)?;
517 impl Readable for Vec<u8> {
519 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
520 let len: u16 = Readable::read(r)?;
521 let mut ret = Vec::with_capacity(len as usize);
522 ret.resize(len as usize, 0);
523 r.read_exact(&mut ret)?;
527 impl Writeable for Vec<Signature> {
529 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
530 (self.len() as u16).write(w)?;
531 for e in self.iter() {
538 impl Readable for Vec<Signature> {
540 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
541 let len: u16 = Readable::read(r)?;
542 let byte_size = (len as usize)
543 .checked_mul(COMPACT_SIGNATURE_SIZE)
544 .ok_or(DecodeError::BadLengthDescriptor)?;
545 if byte_size > MAX_BUF_SIZE {
546 return Err(DecodeError::BadLengthDescriptor);
548 let mut ret = Vec::with_capacity(len as usize);
549 for _ in 0..len { ret.push(Signature::read(r)?); }
554 impl Writeable for Script {
555 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
556 (self.len() as u16).write(w)?;
557 w.write_all(self.as_bytes())
561 impl Readable for Script {
562 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
563 let len = <u16 as Readable>::read(r)? as usize;
564 let mut buf = vec![0; len];
565 r.read_exact(&mut buf)?;
566 Ok(Script::from(buf))
570 impl Writeable for PublicKey {
571 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
572 self.serialize().write(w)
575 fn serialized_length(&self) -> usize {
580 impl Readable for PublicKey {
581 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
582 let buf: [u8; PUBLIC_KEY_SIZE] = Readable::read(r)?;
583 match PublicKey::from_slice(&buf) {
585 Err(_) => return Err(DecodeError::InvalidValue),
590 impl Writeable for SecretKey {
591 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
592 let mut ser = [0; SECRET_KEY_SIZE];
593 ser.copy_from_slice(&self[..]);
597 fn serialized_length(&self) -> usize {
602 impl Readable for SecretKey {
603 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
604 let buf: [u8; SECRET_KEY_SIZE] = Readable::read(r)?;
605 match SecretKey::from_slice(&buf) {
607 Err(_) => return Err(DecodeError::InvalidValue),
612 impl Writeable for Sha256dHash {
613 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
614 w.write_all(&self[..])
618 impl Readable for Sha256dHash {
619 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
620 use bitcoin::hashes::Hash;
622 let buf: [u8; 32] = Readable::read(r)?;
623 Ok(Sha256dHash::from_slice(&buf[..]).unwrap())
627 impl Writeable for Signature {
628 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
629 self.serialize_compact().write(w)
632 fn serialized_length(&self) -> usize {
633 COMPACT_SIGNATURE_SIZE
637 impl Readable for Signature {
638 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
639 let buf: [u8; COMPACT_SIGNATURE_SIZE] = Readable::read(r)?;
640 match Signature::from_compact(&buf) {
642 Err(_) => return Err(DecodeError::InvalidValue),
647 impl Writeable for PaymentPreimage {
648 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
653 impl Readable for PaymentPreimage {
654 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
655 let buf: [u8; 32] = Readable::read(r)?;
656 Ok(PaymentPreimage(buf))
660 impl Writeable for PaymentHash {
661 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
666 impl Readable for PaymentHash {
667 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
668 let buf: [u8; 32] = Readable::read(r)?;
673 impl Writeable for PaymentSecret {
674 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
679 impl Readable for PaymentSecret {
680 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
681 let buf: [u8; 32] = Readable::read(r)?;
682 Ok(PaymentSecret(buf))
686 impl<T: Writeable> Writeable for Option<T> {
687 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
689 None => 0u8.write(w)?,
691 BigSize(data.serialized_length() as u64 + 1).write(w)?;
699 impl<T: Readable> Readable for Option<T>
701 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
702 match BigSize::read(r)?.0 {
705 let mut reader = FixedLengthReader::new(r, len - 1);
706 Ok(Some(Readable::read(&mut reader)?))
712 impl Writeable for Txid {
713 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
714 w.write_all(&self[..])
718 impl Readable for Txid {
719 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
720 use bitcoin::hashes::Hash;
722 let buf: [u8; 32] = Readable::read(r)?;
723 Ok(Txid::from_slice(&buf[..]).unwrap())
727 impl Writeable for BlockHash {
728 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
729 w.write_all(&self[..])
733 impl Readable for BlockHash {
734 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
735 use bitcoin::hashes::Hash;
737 let buf: [u8; 32] = Readable::read(r)?;
738 Ok(BlockHash::from_slice(&buf[..]).unwrap())
742 impl Writeable for OutPoint {
743 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
750 impl Readable for OutPoint {
751 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
752 let txid = Readable::read(r)?;
753 let vout = Readable::read(r)?;
761 macro_rules! impl_consensus_ser {
762 ($bitcoin_type: ty) => {
763 impl Writeable for $bitcoin_type {
764 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
765 match self.consensus_encode(WriterWriteAdaptor(writer)) {
772 impl Readable for $bitcoin_type {
773 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
774 match consensus::encode::Decodable::consensus_decode(r) {
776 Err(consensus::encode::Error::Io(ref e)) if e.kind() == ::std::io::ErrorKind::UnexpectedEof => Err(DecodeError::ShortRead),
777 Err(consensus::encode::Error::Io(e)) => Err(DecodeError::Io(e.kind())),
778 Err(_) => Err(DecodeError::InvalidValue),
784 impl_consensus_ser!(Transaction);
785 impl_consensus_ser!(TxOut);
787 impl<T: Readable> Readable for Mutex<T> {
788 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
789 let t: T = Readable::read(r)?;
793 impl<T: Writeable> Writeable for Mutex<T> {
794 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
795 self.lock().unwrap().write(w)
799 impl<A: Readable, B: Readable> Readable for (A, B) {
800 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
801 let a: A = Readable::read(r)?;
802 let b: B = Readable::read(r)?;
806 impl<A: Writeable, B: Writeable> Writeable for (A, B) {
807 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {