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> {
64 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
68 fn write(&mut self, buf: &[u8]) -> Result<usize, ::std::io::Error> {
69 self.0.write_all(buf)?;
73 fn flush(&mut self) -> Result<(), ::std::io::Error> {
78 pub(crate) struct VecWriter(pub Vec<u8>);
79 impl Writer for VecWriter {
81 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
82 self.0.extend_from_slice(buf);
86 fn size_hint(&mut self, size: usize) {
87 self.0.reserve_exact(size);
91 /// Writer that only tracks the amount of data written - useful if you need to calculate the length
92 /// of some data when serialized but don't yet need the full data.
93 pub(crate) struct LengthCalculatingWriter(pub usize);
94 impl Writer for LengthCalculatingWriter {
96 fn write_all(&mut self, buf: &[u8]) -> Result<(), ::std::io::Error> {
101 fn size_hint(&mut self, _size: usize) {}
104 /// Essentially std::io::Take but a bit simpler and with a method to walk the underlying stream
105 /// forward to ensure we always consume exactly the fixed length specified.
106 pub(crate) struct FixedLengthReader<R: Read> {
111 impl<R: Read> FixedLengthReader<R> {
112 pub fn new(read: R, total_bytes: u64) -> Self {
113 Self { read, bytes_read: 0, total_bytes }
117 pub fn bytes_remain(&mut self) -> bool {
118 self.bytes_read != self.total_bytes
122 pub fn eat_remaining(&mut self) -> Result<(), DecodeError> {
123 ::std::io::copy(self, &mut ::std::io::sink()).unwrap();
124 if self.bytes_read != self.total_bytes {
125 Err(DecodeError::ShortRead)
131 impl<R: Read> Read for FixedLengthReader<R> {
133 fn read(&mut self, dest: &mut [u8]) -> Result<usize, ::std::io::Error> {
134 if self.total_bytes == self.bytes_read {
137 let read_len = cmp::min(dest.len() as u64, self.total_bytes - self.bytes_read);
138 match self.read.read(&mut dest[0..(read_len as usize)]) {
140 self.bytes_read += v as u64;
149 /// A Read which tracks whether any bytes have been read at all. This allows us to distinguish
150 /// between "EOF reached before we started" and "EOF reached mid-read".
151 pub(crate) struct ReadTrackingReader<R: Read> {
155 impl<R: Read> ReadTrackingReader<R> {
156 pub fn new(read: R) -> Self {
157 Self { read, have_read: false }
160 impl<R: Read> Read for ReadTrackingReader<R> {
162 fn read(&mut self, dest: &mut [u8]) -> Result<usize, ::std::io::Error> {
163 match self.read.read(dest) {
166 self.have_read = true;
174 /// A trait that various rust-lightning types implement allowing them to be written out to a Writer
176 /// (C-not exported) as we only export serialization to/from byte arrays instead
177 pub trait Writeable {
178 /// Writes self out to the given Writer
179 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error>;
181 /// Writes self out to a Vec<u8>
182 fn encode(&self) -> Vec<u8> {
183 let mut msg = VecWriter(Vec::new());
184 self.write(&mut msg).unwrap();
188 /// Writes self out to a Vec<u8>
189 fn encode_with_len(&self) -> Vec<u8> {
190 let mut msg = VecWriter(Vec::new());
191 0u16.write(&mut msg).unwrap();
192 self.write(&mut msg).unwrap();
193 let len = msg.0.len();
194 msg.0[..2].copy_from_slice(&(len as u16 - 2).to_be_bytes());
198 /// Gets the length of this object after it has been serialized. This can be overridden to
199 /// optimize cases where we prepend an object with its length.
200 // Note that LLVM optimizes this away in most cases! Check that it isn't before you override!
202 fn serialized_length(&self) -> usize {
203 let mut len_calc = LengthCalculatingWriter(0);
204 self.write(&mut len_calc).expect("No in-memory data may fail to serialize");
209 impl<'a, T: Writeable> Writeable for &'a T {
210 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> { (*self).write(writer) }
213 /// A trait that various rust-lightning types implement allowing them to be read in from a Read
215 /// (C-not exported) as we only export serialization to/from byte arrays instead
219 /// Reads a Self in from the given Read
220 fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError>;
223 /// A trait that various higher-level rust-lightning types implement allowing them to be read in
224 /// from a Read given some additional set of arguments which is required to deserialize.
226 /// (C-not exported) as we only export serialization to/from byte arrays instead
227 pub trait ReadableArgs<P>
230 /// Reads a Self in from the given Read
231 fn read<R: Read>(reader: &mut R, params: P) -> Result<Self, DecodeError>;
234 /// A trait that various rust-lightning types implement allowing them to (maybe) be read in from a Read
236 /// (C-not exported) as we only export serialization to/from byte arrays instead
237 pub trait MaybeReadable
240 /// Reads a Self in from the given Read
241 fn read<R: Read>(reader: &mut R) -> Result<Option<Self>, DecodeError>;
244 pub(crate) struct OptionDeserWrapper<T: Readable>(pub Option<T>);
245 impl<T: Readable> Readable for OptionDeserWrapper<T> {
247 fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError> {
248 Ok(Self(Some(Readable::read(reader)?)))
252 const MAX_ALLOC_SIZE: u64 = 64*1024;
254 pub(crate) struct VecWriteWrapper<'a, T: Writeable>(pub &'a Vec<T>);
255 impl<'a, T: Writeable> Writeable for VecWriteWrapper<'a, T> {
257 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
258 (self.0.len() as u64).write(writer)?;
259 for ref v in self.0.iter() {
265 pub(crate) struct VecReadWrapper<T: Readable>(pub Vec<T>);
266 impl<T: Readable> Readable for VecReadWrapper<T> {
268 fn read<R: Read>(reader: &mut R) -> Result<Self, DecodeError> {
269 let count: u64 = Readable::read(reader)?;
270 let mut values = Vec::with_capacity(cmp::min(count, MAX_ALLOC_SIZE / (core::mem::size_of::<T>() as u64)) as usize);
272 match Readable::read(reader) {
273 Ok(v) => { values.push(v); },
274 Err(e) => return Err(e),
281 pub(crate) struct U48(pub u64);
282 impl Writeable for U48 {
284 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
285 writer.write_all(&be48_to_array(self.0))
288 impl Readable for U48 {
290 fn read<R: Read>(reader: &mut R) -> Result<U48, DecodeError> {
291 let mut buf = [0; 6];
292 reader.read_exact(&mut buf)?;
293 Ok(U48(slice_to_be48(&buf)))
297 /// Lightning TLV uses a custom variable-length integer called BigSize. It is similar to Bitcoin's
298 /// variable-length integers except that it is serialized in big-endian instead of little-endian.
300 /// Like Bitcoin's variable-length integer, it exhibits ambiguity in that certain values can be
301 /// encoded in several different ways, which we must check for at deserialization-time. Thus, if
302 /// you're looking for an example of a variable-length integer to use for your own project, move
303 /// along, this is a rather poor design.
304 pub(crate) struct BigSize(pub u64);
305 impl Writeable for BigSize {
307 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
310 (self.0 as u8).write(writer)
313 0xFDu8.write(writer)?;
314 (self.0 as u16).write(writer)
316 0x10000...0xFFFFFFFF => {
317 0xFEu8.write(writer)?;
318 (self.0 as u32).write(writer)
321 0xFFu8.write(writer)?;
322 (self.0 as u64).write(writer)
327 impl Readable for BigSize {
329 fn read<R: Read>(reader: &mut R) -> Result<BigSize, DecodeError> {
330 let n: u8 = Readable::read(reader)?;
333 let x: u64 = Readable::read(reader)?;
335 Err(DecodeError::InvalidValue)
341 let x: u32 = Readable::read(reader)?;
343 Err(DecodeError::InvalidValue)
345 Ok(BigSize(x as u64))
349 let x: u16 = Readable::read(reader)?;
351 Err(DecodeError::InvalidValue)
353 Ok(BigSize(x as u64))
356 n => Ok(BigSize(n as u64))
361 /// In TLV we occasionally send fields which only consist of, or potentially end with, a
362 /// variable-length integer which is simply truncated by skipping high zero bytes. This type
363 /// encapsulates such integers implementing Readable/Writeable for them.
364 #[cfg_attr(test, derive(PartialEq, Debug))]
365 pub(crate) struct HighZeroBytesDroppedVarInt<T>(pub T);
367 macro_rules! impl_writeable_primitive {
368 ($val_type:ty, $len: expr) => {
369 impl Writeable for $val_type {
371 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
372 writer.write_all(&self.to_be_bytes())
375 impl Writeable for HighZeroBytesDroppedVarInt<$val_type> {
377 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
378 // Skip any full leading 0 bytes when writing (in BE):
379 writer.write_all(&self.0.to_be_bytes()[(self.0.leading_zeros()/8) as usize..$len])
382 impl Readable for $val_type {
384 fn read<R: Read>(reader: &mut R) -> Result<$val_type, DecodeError> {
385 let mut buf = [0; $len];
386 reader.read_exact(&mut buf)?;
387 Ok(<$val_type>::from_be_bytes(buf))
390 impl Readable for HighZeroBytesDroppedVarInt<$val_type> {
392 fn read<R: Read>(reader: &mut R) -> Result<HighZeroBytesDroppedVarInt<$val_type>, DecodeError> {
393 // We need to accept short reads (read_len == 0) as "EOF" and handle them as simply
394 // the high bytes being dropped. To do so, we start reading into the middle of buf
395 // and then convert the appropriate number of bytes with extra high bytes out of
397 let mut buf = [0; $len*2];
398 let mut read_len = reader.read(&mut buf[$len..])?;
399 let mut total_read_len = read_len;
400 while read_len != 0 && total_read_len != $len {
401 read_len = reader.read(&mut buf[($len + total_read_len)..])?;
402 total_read_len += read_len;
404 if total_read_len == 0 || buf[$len] != 0 {
405 let first_byte = $len - ($len - total_read_len);
406 let mut bytes = [0; $len];
407 bytes.copy_from_slice(&buf[first_byte..first_byte + $len]);
408 Ok(HighZeroBytesDroppedVarInt(<$val_type>::from_be_bytes(bytes)))
410 // If the encoding had extra zero bytes, return a failure even though we know
411 // what they meant (as the TLV test vectors require this)
412 Err(DecodeError::InvalidValue)
419 impl_writeable_primitive!(u64, 8);
420 impl_writeable_primitive!(u32, 4);
421 impl_writeable_primitive!(u16, 2);
423 impl Writeable for u8 {
425 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
426 writer.write_all(&[*self])
429 impl Readable for u8 {
431 fn read<R: Read>(reader: &mut R) -> Result<u8, DecodeError> {
432 let mut buf = [0; 1];
433 reader.read_exact(&mut buf)?;
438 impl Writeable for bool {
440 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
441 writer.write_all(&[if *self {1} else {0}])
444 impl Readable for bool {
446 fn read<R: Read>(reader: &mut R) -> Result<bool, DecodeError> {
447 let mut buf = [0; 1];
448 reader.read_exact(&mut buf)?;
449 if buf[0] != 0 && buf[0] != 1 {
450 return Err(DecodeError::InvalidValue);
457 macro_rules! impl_array {
459 impl Writeable for [u8; $size]
462 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
467 impl Readable for [u8; $size]
470 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
471 let mut buf = [0u8; $size];
472 r.read_exact(&mut buf)?;
479 //TODO: performance issue with [u8; size] with impl_array!()
480 impl_array!(3); // for rgb
481 impl_array!(4); // for IPv4
482 impl_array!(10); // for OnionV2
483 impl_array!(16); // for IPv6
484 impl_array!(32); // for channel id & hmac
485 impl_array!(PUBLIC_KEY_SIZE); // for PublicKey
486 impl_array!(COMPACT_SIGNATURE_SIZE); // for Signature
487 impl_array!(1300); // for OnionPacket.hop_data
490 impl<K, V> Writeable for HashMap<K, V>
491 where K: Writeable + Eq + Hash,
495 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
496 (self.len() as u16).write(w)?;
497 for (key, value) in self.iter() {
505 impl<K, V> Readable for HashMap<K, V>
506 where K: Readable + Eq + Hash,
510 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
511 let len: u16 = Readable::read(r)?;
512 let mut ret = HashMap::with_capacity(len as usize);
514 ret.insert(K::read(r)?, V::read(r)?);
521 impl Writeable for Vec<u8> {
523 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
524 (self.len() as u16).write(w)?;
529 impl Readable for Vec<u8> {
531 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
532 let len: u16 = Readable::read(r)?;
533 let mut ret = Vec::with_capacity(len as usize);
534 ret.resize(len as usize, 0);
535 r.read_exact(&mut ret)?;
539 impl Writeable for Vec<Signature> {
541 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
542 (self.len() as u16).write(w)?;
543 for e in self.iter() {
550 impl Readable for Vec<Signature> {
552 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
553 let len: u16 = Readable::read(r)?;
554 let byte_size = (len as usize)
555 .checked_mul(COMPACT_SIGNATURE_SIZE)
556 .ok_or(DecodeError::BadLengthDescriptor)?;
557 if byte_size > MAX_BUF_SIZE {
558 return Err(DecodeError::BadLengthDescriptor);
560 let mut ret = Vec::with_capacity(len as usize);
561 for _ in 0..len { ret.push(Signature::read(r)?); }
566 impl Writeable for Script {
567 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
568 (self.len() as u16).write(w)?;
569 w.write_all(self.as_bytes())
573 impl Readable for Script {
574 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
575 let len = <u16 as Readable>::read(r)? as usize;
576 let mut buf = vec![0; len];
577 r.read_exact(&mut buf)?;
578 Ok(Script::from(buf))
582 impl Writeable for PublicKey {
583 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
584 self.serialize().write(w)
587 fn serialized_length(&self) -> usize {
592 impl Readable for PublicKey {
593 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
594 let buf: [u8; PUBLIC_KEY_SIZE] = Readable::read(r)?;
595 match PublicKey::from_slice(&buf) {
597 Err(_) => return Err(DecodeError::InvalidValue),
602 impl Writeable for SecretKey {
603 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
604 let mut ser = [0; SECRET_KEY_SIZE];
605 ser.copy_from_slice(&self[..]);
609 fn serialized_length(&self) -> usize {
614 impl Readable for SecretKey {
615 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
616 let buf: [u8; SECRET_KEY_SIZE] = Readable::read(r)?;
617 match SecretKey::from_slice(&buf) {
619 Err(_) => return Err(DecodeError::InvalidValue),
624 impl Writeable for Sha256dHash {
625 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
626 w.write_all(&self[..])
630 impl Readable for Sha256dHash {
631 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
632 use bitcoin::hashes::Hash;
634 let buf: [u8; 32] = Readable::read(r)?;
635 Ok(Sha256dHash::from_slice(&buf[..]).unwrap())
639 impl Writeable for Signature {
640 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
641 self.serialize_compact().write(w)
644 fn serialized_length(&self) -> usize {
645 COMPACT_SIGNATURE_SIZE
649 impl Readable for Signature {
650 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
651 let buf: [u8; COMPACT_SIGNATURE_SIZE] = Readable::read(r)?;
652 match Signature::from_compact(&buf) {
654 Err(_) => return Err(DecodeError::InvalidValue),
659 impl Writeable for PaymentPreimage {
660 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
665 impl Readable for PaymentPreimage {
666 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
667 let buf: [u8; 32] = Readable::read(r)?;
668 Ok(PaymentPreimage(buf))
672 impl Writeable for PaymentHash {
673 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
678 impl Readable for PaymentHash {
679 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
680 let buf: [u8; 32] = Readable::read(r)?;
685 impl Writeable for PaymentSecret {
686 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
691 impl Readable for PaymentSecret {
692 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
693 let buf: [u8; 32] = Readable::read(r)?;
694 Ok(PaymentSecret(buf))
698 impl<T: Writeable> Writeable for Option<T> {
699 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
701 None => 0u8.write(w)?,
703 BigSize(data.serialized_length() as u64 + 1).write(w)?;
711 impl<T: Readable> Readable for Option<T>
713 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
714 match BigSize::read(r)?.0 {
717 let mut reader = FixedLengthReader::new(r, len - 1);
718 Ok(Some(Readable::read(&mut reader)?))
724 impl Writeable for Txid {
725 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
726 w.write_all(&self[..])
730 impl Readable for Txid {
731 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
732 use bitcoin::hashes::Hash;
734 let buf: [u8; 32] = Readable::read(r)?;
735 Ok(Txid::from_slice(&buf[..]).unwrap())
739 impl Writeable for BlockHash {
740 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
741 w.write_all(&self[..])
745 impl Readable for BlockHash {
746 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
747 use bitcoin::hashes::Hash;
749 let buf: [u8; 32] = Readable::read(r)?;
750 Ok(BlockHash::from_slice(&buf[..]).unwrap())
754 impl Writeable for OutPoint {
755 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
762 impl Readable for OutPoint {
763 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
764 let txid = Readable::read(r)?;
765 let vout = Readable::read(r)?;
773 macro_rules! impl_consensus_ser {
774 ($bitcoin_type: ty) => {
775 impl Writeable for $bitcoin_type {
776 fn write<W: Writer>(&self, writer: &mut W) -> Result<(), ::std::io::Error> {
777 match self.consensus_encode(WriterWriteAdaptor(writer)) {
784 impl Readable for $bitcoin_type {
785 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
786 match consensus::encode::Decodable::consensus_decode(r) {
788 Err(consensus::encode::Error::Io(ref e)) if e.kind() == ::std::io::ErrorKind::UnexpectedEof => Err(DecodeError::ShortRead),
789 Err(consensus::encode::Error::Io(e)) => Err(DecodeError::Io(e.kind())),
790 Err(_) => Err(DecodeError::InvalidValue),
796 impl_consensus_ser!(Transaction);
797 impl_consensus_ser!(TxOut);
799 impl<T: Readable> Readable for Mutex<T> {
800 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
801 let t: T = Readable::read(r)?;
805 impl<T: Writeable> Writeable for Mutex<T> {
806 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {
807 self.lock().unwrap().write(w)
811 impl<A: Readable, B: Readable> Readable for (A, B) {
812 fn read<R: Read>(r: &mut R) -> Result<Self, DecodeError> {
813 let a: A = Readable::read(r)?;
814 let b: B = Readable::read(r)?;
818 impl<A: Writeable, B: Writeable> Writeable for (A, B) {
819 fn write<W: Writer>(&self, w: &mut W) -> Result<(), ::std::io::Error> {