compiler/rustc_serialize/src/opaque.rs - third_party/github.com/rust-lang/rust - Git at Google

 use crate::leb128;
 use crate::serialize::{Decodable, Decoder, Encodable, Encoder};
 use std::fs::File;
 use std::io::{self, Write};
 use std::marker::PhantomData;
 use std::ops::Range;
 use std::path::Path;
 use std::path::PathBuf;

 // This code is very hot and uses lots of arithmetic, avoid overflow checks for performance.
 // See https://github.com/rust-lang/rust/pull/119440#issuecomment-1874255727
 use crate::int_overflow::DebugStrictAdd;

 // -----------------------------------------------------------------------------
 // Encoder
 // -----------------------------------------------------------------------------

 pub type FileEncodeResult = Result<usize, (PathBuf, io::Error)>;

 pub const MAGIC_END_BYTES: &[u8] = b"rust-end-file";

 /// The size of the buffer in `FileEncoder`.
 const BUF_SIZE: usize = 8192;

 /// `FileEncoder` encodes data to file via fixed-size buffer.
 ///
 /// There used to be a `MemEncoder` type that encoded all the data into a
 /// `Vec`. `FileEncoder` is better because its memory use is determined by the
 /// size of the buffer, rather than the full length of the encoded data, and
 /// because it doesn't need to reallocate memory along the way.
 pub struct FileEncoder {
     // The input buffer. For adequate performance, we need to be able to write
     // directly to the unwritten region of the buffer, without calling copy_from_slice.
     // Note that our buffer is always initialized so that we can do that direct access
     // without unsafe code. Users of this type write many more than BUF_SIZE bytes, so the
     // initialization is approximately free.
     buf: Box<[u8; BUF_SIZE]>,
     buffered: usize,
     flushed: usize,
     file: File,
     // This is used to implement delayed error handling, as described in the
     // comment on `trait Encoder`.
     res: Result<(), io::Error>,
     path: PathBuf,
     #[cfg(debug_assertions)]
     finished: bool,
 }

 impl FileEncoder {
     pub fn new<P: AsRef<Path>>(path: P) -> io::Result<Self> {
         // File::create opens the file for writing only. When -Zmeta-stats is enabled, the metadata
         // encoder rewinds the file to inspect what was written. So we need to always open the file
         // for reading and writing.
         let file =
             File::options().read(true).write(true).create(true).truncate(true).open(&path)?;

         Ok(FileEncoder {
             buf: vec![0u8; BUF_SIZE].into_boxed_slice().try_into().unwrap(),
             path: path.as_ref().into(),
             buffered: 0,
             flushed: 0,
             file,
             res: Ok(()),
             #[cfg(debug_assertions)]
             finished: false,
         })
     }

     #[inline]
     pub fn position(&self) -> usize {
         // Tracking position this way instead of having a `self.position` field
         // means that we only need to update `self.buffered` on a write call,
         // as opposed to updating `self.position` and `self.buffered`.
         self.flushed.debug_strict_add(self.buffered)
     }

     #[cold]
     #[inline(never)]
     pub fn flush(&mut self) {
         #[cfg(debug_assertions)]
         {
             self.finished = false;
         }
         if self.res.is_ok() {
             self.res = self.file.write_all(&self.buf[..self.buffered]);
         }
         self.flushed += self.buffered;
         self.buffered = 0;
     }

     pub fn file(&self) -> &File {
         &self.file
     }

     pub fn path(&self) -> &Path {
         &self.path
     }

     #[inline]
     fn buffer_empty(&mut self) -> &mut [u8] {
         // SAFETY: self.buffered is inbounds as an invariant of the type
         unsafe { self.buf.get_unchecked_mut(self.buffered..) }
     }

     #[cold]
     #[inline(never)]
     fn write_all_cold_path(&mut self, buf: &[u8]) {
         self.flush();
         if let Some(dest) = self.buf.get_mut(..buf.len()) {
             dest.copy_from_slice(buf);
             self.buffered += buf.len();
         } else {
             if self.res.is_ok() {
                 self.res = self.file.write_all(buf);
             }
             self.flushed += buf.len();
         }
     }

     #[inline]
     fn write_all(&mut self, buf: &[u8]) {
         #[cfg(debug_assertions)]
         {
             self.finished = false;
         }
         if let Some(dest) = self.buffer_empty().get_mut(..buf.len()) {
             dest.copy_from_slice(buf);
             self.buffered = self.buffered.debug_strict_add(buf.len());
         } else {
             self.write_all_cold_path(buf);
         }
     }

     /// Write up to `N` bytes to this encoder.
     ///
     /// This function can be used to avoid the overhead of calling memcpy for writes that
     /// have runtime-variable length, but are small and have a small fixed upper bound.
     ///
     /// This can be used to do in-place encoding as is done for leb128 (without this function
     /// we would need to write to a temporary buffer then memcpy into the encoder), and it can
     /// also be used to implement the varint scheme we use for rmeta and dep graph encoding,
     /// where we only want to encode the first few bytes of an integer. Copying in the whole
     /// integer then only advancing the encoder state for the few bytes we care about is more
     /// efficient than calling [`FileEncoder::write_all`], because variable-size copies are
     /// always lowered to `memcpy`, which has overhead and contains a lot of logic we can bypass
     /// with this function. Note that common architectures support fixed-size writes up to 8 bytes
     /// with one instruction, so while this does in some sense do wasted work, we come out ahead.
     #[inline]
     pub fn write_with<const N: usize>(&mut self, visitor: impl FnOnce(&mut [u8; N]) -> usize) {
         #[cfg(debug_assertions)]
         {
             self.finished = false;
         }
         let flush_threshold = const { BUF_SIZE.checked_sub(N).unwrap() };
         if std::intrinsics::unlikely(self.buffered > flush_threshold) {
             self.flush();
         }
         // SAFETY: We checked above that that N < self.buffer_empty().len(),
         // and if isn't, flush ensures that our empty buffer is now BUF_SIZE.
         // We produce a post-mono error if N > BUF_SIZE.
         let buf = unsafe { self.buffer_empty().first_chunk_mut::<N>().unwrap_unchecked() };
         let written = visitor(buf);
         // We have to ensure that an errant visitor cannot cause self.buffered to exeed BUF_SIZE.
         if written > N {
             Self::panic_invalid_write::<N>(written);
         }
         self.buffered = self.buffered.debug_strict_add(written);
     }

     #[cold]
     #[inline(never)]
     fn panic_invalid_write<const N: usize>(written: usize) {
         panic!("FileEncoder::write_with::<{N}> cannot be used to write {written} bytes");
     }

     /// Helper for calls where [`FileEncoder::write_with`] always writes the whole array.
     #[inline]
     pub fn write_array<const N: usize>(&mut self, buf: [u8; N]) {
         self.write_with(|dest| {
             *dest = buf;
             N
         })
     }

     pub fn finish(&mut self) -> FileEncodeResult {
         self.write_all(MAGIC_END_BYTES);
         self.flush();
         #[cfg(debug_assertions)]
         {
             self.finished = true;
         }
         match std::mem::replace(&mut self.res, Ok(())) {
             Ok(()) => Ok(self.position()),
             Err(e) => Err((self.path.clone(), e)),
         }
     }
 }

 #[cfg(debug_assertions)]
 impl Drop for FileEncoder {
     fn drop(&mut self) {
         if !std::thread::panicking() {
             assert!(self.finished);
         }
     }
 }

 macro_rules! write_leb128 {
     ($this_fn:ident, $int_ty:ty, $write_leb_fn:ident) => {
         #[inline]
         fn $this_fn(&mut self, v: $int_ty) {
             self.write_with(|buf| leb128::$write_leb_fn(buf, v))
         }
     };
 }

 impl Encoder for FileEncoder {
     write_leb128!(emit_usize, usize, write_usize_leb128);
     write_leb128!(emit_u128, u128, write_u128_leb128);
     write_leb128!(emit_u64, u64, write_u64_leb128);
     write_leb128!(emit_u32, u32, write_u32_leb128);

     #[inline]
     fn emit_u16(&mut self, v: u16) {
         self.write_array(v.to_le_bytes());
     }

     #[inline]
     fn emit_u8(&mut self, v: u8) {
         self.write_array([v]);
     }

     write_leb128!(emit_isize, isize, write_isize_leb128);
     write_leb128!(emit_i128, i128, write_i128_leb128);
     write_leb128!(emit_i64, i64, write_i64_leb128);
     write_leb128!(emit_i32, i32, write_i32_leb128);

     #[inline]
     fn emit_i16(&mut self, v: i16) {
         self.write_array(v.to_le_bytes());
     }

     #[inline]
     fn emit_raw_bytes(&mut self, s: &[u8]) {
         self.write_all(s);
     }
 }

 // -----------------------------------------------------------------------------
 // Decoder
 // -----------------------------------------------------------------------------

 // Conceptually, `MemDecoder` wraps a `&[u8]` with a cursor into it that is always valid.
 // This is implemented with three pointers, two which represent the original slice and a
 // third that is our cursor.
 // It is an invariant of this type that start <= current <= end.
 // Additionally, the implementation of this type never modifies start and end.
 pub struct MemDecoder<'a> {
     start: *const u8,
     current: *const u8,
     end: *const u8,
     _marker: PhantomData<&'a u8>,
 }

 impl<'a> MemDecoder<'a> {
     #[inline]
     pub fn new(data: &'a [u8], position: usize) -> Result<MemDecoder<'a>, ()> {
         let data = data.strip_suffix(MAGIC_END_BYTES).ok_or(())?;
         let Range { start, end } = data.as_ptr_range();
         Ok(MemDecoder { start, current: data[position..].as_ptr(), end, _marker: PhantomData })
     }

     #[inline]
     pub fn split_at(&self, position: usize) -> MemDecoder<'a> {
         assert!(position <= self.len());
         // SAFETY: We checked above that this offset is within the original slice
         let current = unsafe { self.start.add(position) };
         MemDecoder { start: self.start, current, end: self.end, _marker: PhantomData }
     }

     #[inline]
     pub fn len(&self) -> usize {
         // SAFETY: This recovers the length of the original slice, only using members we never modify.
         unsafe { self.end.sub_ptr(self.start) }
     }

     #[inline]
     pub fn remaining(&self) -> usize {
         // SAFETY: This type guarantees current <= end.
         unsafe { self.end.sub_ptr(self.current) }
     }

     #[cold]
     #[inline(never)]
     fn decoder_exhausted() -> ! {
         panic!("MemDecoder exhausted")
     }

     #[inline]
     pub fn read_array<const N: usize>(&mut self) -> [u8; N] {
         self.read_raw_bytes(N).try_into().unwrap()
     }

     /// While we could manually expose manipulation of the decoder position,
     /// all current users of that method would need to reset the position later,
     /// incurring the bounds check of set_position twice.
     #[inline]
     pub fn with_position<F, T>(&mut self, pos: usize, func: F) -> T
     where
         F: Fn(&mut MemDecoder<'a>) -> T,
     {
         struct SetOnDrop<'a, 'guarded> {
             decoder: &'guarded mut MemDecoder<'a>,
             current: *const u8,
         }
         impl Drop for SetOnDrop<'_, '_> {
             fn drop(&mut self) {
                 self.decoder.current = self.current;
             }
         }

         if pos >= self.len() {
             Self::decoder_exhausted();
         }
         let previous = self.current;
         // SAFETY: We just checked if this add is in-bounds above.
         unsafe {
             self.current = self.start.add(pos);
         }
         let guard = SetOnDrop { current: previous, decoder: self };
         func(guard.decoder)
     }
 }

 macro_rules! read_leb128 {
     ($this_fn:ident, $int_ty:ty, $read_leb_fn:ident) => {
         #[inline]
         fn $this_fn(&mut self) -> $int_ty {
             leb128::$read_leb_fn(self)
         }
     };
 }

 impl<'a> Decoder for MemDecoder<'a> {
     read_leb128!(read_usize, usize, read_usize_leb128);
     read_leb128!(read_u128, u128, read_u128_leb128);
     read_leb128!(read_u64, u64, read_u64_leb128);
     read_leb128!(read_u32, u32, read_u32_leb128);

     #[inline]
     fn read_u16(&mut self) -> u16 {
         u16::from_le_bytes(self.read_array())
     }

     #[inline]
     fn read_u8(&mut self) -> u8 {
         if self.current == self.end {
             Self::decoder_exhausted();
         }
         // SAFETY: This type guarantees current <= end, and we just checked current == end.
         unsafe {
             let byte = *self.current;
             self.current = self.current.add(1);
             byte
         }
     }

     read_leb128!(read_isize, isize, read_isize_leb128);
     read_leb128!(read_i128, i128, read_i128_leb128);
     read_leb128!(read_i64, i64, read_i64_leb128);
     read_leb128!(read_i32, i32, read_i32_leb128);

     #[inline]
     fn read_i16(&mut self) -> i16 {
         i16::from_le_bytes(self.read_array())
     }

     #[inline]
     fn read_raw_bytes(&mut self, bytes: usize) -> &'a [u8] {
         if bytes > self.remaining() {
             Self::decoder_exhausted();
         }
         // SAFETY: We just checked if this range is in-bounds above.
         unsafe {
             let slice = std::slice::from_raw_parts(self.current, bytes);
             self.current = self.current.add(bytes);
             slice
         }
     }

     #[inline]
     fn peek_byte(&self) -> u8 {
         if self.current == self.end {
             Self::decoder_exhausted();
         }
         // SAFETY: This type guarantees current is inbounds or one-past-the-end, which is end.
         // Since we just checked current == end, the current pointer must be inbounds.
         unsafe { *self.current }
     }

     #[inline]
     fn position(&self) -> usize {
         // SAFETY: This type guarantees start <= current
         unsafe { self.current.sub_ptr(self.start) }
     }
 }

 // Specializations for contiguous byte sequences follow. The default implementations for slices
 // encode and decode each element individually. This isn't necessary for `u8` slices when using
 // opaque encoders and decoders, because each `u8` is unchanged by encoding and decoding.
 // Therefore, we can use more efficient implementations that process the entire sequence at once.

 // Specialize encoding byte slices. This specialization also applies to encoding `Vec<u8>`s, etc.,
 // since the default implementations call `encode` on their slices internally.
 impl Encodable<FileEncoder> for [u8] {
     fn encode(&self, e: &mut FileEncoder) {
         Encoder::emit_usize(e, self.len());
         e.emit_raw_bytes(self);
     }
 }

 // Specialize decoding `Vec<u8>`. This specialization also applies to decoding `Box<[u8]>`s, etc.,
 // since the default implementations call `decode` to produce a `Vec<u8>` internally.
 impl<'a> Decodable<MemDecoder<'a>> for Vec<u8> {
     fn decode(d: &mut MemDecoder<'a>) -> Self {
         let len = Decoder::read_usize(d);
         d.read_raw_bytes(len).to_owned()
     }
 }

 /// An integer that will always encode to 8 bytes.
 pub struct IntEncodedWithFixedSize(pub u64);

 impl IntEncodedWithFixedSize {
     pub const ENCODED_SIZE: usize = 8;
 }

 impl Encodable<FileEncoder> for IntEncodedWithFixedSize {
     #[inline]
     fn encode(&self, e: &mut FileEncoder) {
         let _start_pos = e.position();
         e.write_array(self.0.to_le_bytes());
         let _end_pos = e.position();
         debug_assert_eq!((_end_pos - _start_pos), IntEncodedWithFixedSize::ENCODED_SIZE);
     }
 }

 impl<'a> Decodable<MemDecoder<'a>> for IntEncodedWithFixedSize {
     #[inline]
     fn decode(decoder: &mut MemDecoder<'a>) -> IntEncodedWithFixedSize {
         let bytes = decoder.read_array::<{ IntEncodedWithFixedSize::ENCODED_SIZE }>();
         IntEncodedWithFixedSize(u64::from_le_bytes(bytes))
     }
 }
	use crate::leb128;
	use crate::serialize::{Decodable, Decoder, Encodable, Encoder};
	use std::fs::File;
	use std::io::{self, Write};
	use std::marker::PhantomData;
	use std::ops::Range;
	use std::path::Path;
	use std::path::PathBuf;

	// This code is very hot and uses lots of arithmetic, avoid overflow checks for performance.
	// See https://github.com/rust-lang/rust/pull/119440#issuecomment-1874255727
	use crate::int_overflow::DebugStrictAdd;

	// -----------------------------------------------------------------------------
	// Encoder
	// -----------------------------------------------------------------------------

	pub type FileEncodeResult = Result<usize, (PathBuf, io::Error)>;

	pub const MAGIC_END_BYTES: &[u8] = b"rust-end-file";

	/// The size of the buffer in `FileEncoder`.
	const BUF_SIZE: usize = 8192;

	/// `FileEncoder` encodes data to file via fixed-size buffer.
	///
	/// There used to be a `MemEncoder` type that encoded all the data into a
	/// `Vec`. `FileEncoder` is better because its memory use is determined by the
	/// size of the buffer, rather than the full length of the encoded data, and
	/// because it doesn't need to reallocate memory along the way.
	pub struct FileEncoder {
	// The input buffer. For adequate performance, we need to be able to write
	// directly to the unwritten region of the buffer, without calling copy_from_slice.
	// Note that our buffer is always initialized so that we can do that direct access
	// without unsafe code. Users of this type write many more than BUF_SIZE bytes, so the
	// initialization is approximately free.
	buf: Box<[u8; BUF_SIZE]>,
	buffered: usize,
	flushed: usize,
	file: File,
	// This is used to implement delayed error handling, as described in the
	// comment on `trait Encoder`.
	res: Result<(), io::Error>,
	path: PathBuf,
	#[cfg(debug_assertions)]
	finished: bool,
	}

	impl FileEncoder {
	pub fn new<P: AsRef<Path>>(path: P) -> io::Result<Self> {
	// File::create opens the file for writing only. When -Zmeta-stats is enabled, the metadata
	// encoder rewinds the file to inspect what was written. So we need to always open the file
	// for reading and writing.
	let file =
	File::options().read(true).write(true).create(true).truncate(true).open(&path)?;

	Ok(FileEncoder {
	buf: vec![0u8; BUF_SIZE].into_boxed_slice().try_into().unwrap(),
	path: path.as_ref().into(),
	buffered: 0,
	flushed: 0,
	file,
	res: Ok(()),
	#[cfg(debug_assertions)]
	finished: false,
	})
	}

	#[inline]
	pub fn position(&self) -> usize {
	// Tracking position this way instead of having a `self.position` field
	// means that we only need to update `self.buffered` on a write call,
	// as opposed to updating `self.position` and `self.buffered`.
	self.flushed.debug_strict_add(self.buffered)
	}

	#[cold]
	#[inline(never)]
	pub fn flush(&mut self) {
	#[cfg(debug_assertions)]
	{
	self.finished = false;
	}
	if self.res.is_ok() {
	self.res = self.file.write_all(&self.buf[..self.buffered]);
	}
	self.flushed += self.buffered;
	self.buffered = 0;
	}

	pub fn file(&self) -> &File {
	&self.file
	}

	pub fn path(&self) -> &Path {
	&self.path
	}

	#[inline]
	fn buffer_empty(&mut self) -> &mut [u8] {
	// SAFETY: self.buffered is inbounds as an invariant of the type
	unsafe { self.buf.get_unchecked_mut(self.buffered..) }
	}

	#[cold]
	#[inline(never)]
	fn write_all_cold_path(&mut self, buf: &[u8]) {
	self.flush();
	if let Some(dest) = self.buf.get_mut(..buf.len()) {
	dest.copy_from_slice(buf);
	self.buffered += buf.len();
	} else {
	if self.res.is_ok() {
	self.res = self.file.write_all(buf);
	}
	self.flushed += buf.len();
	}
	}

	#[inline]
	fn write_all(&mut self, buf: &[u8]) {
	#[cfg(debug_assertions)]
	{
	self.finished = false;
	}
	if let Some(dest) = self.buffer_empty().get_mut(..buf.len()) {
	dest.copy_from_slice(buf);
	self.buffered = self.buffered.debug_strict_add(buf.len());
	} else {
	self.write_all_cold_path(buf);
	}
	}

	/// Write up to `N` bytes to this encoder.
	///
	/// This function can be used to avoid the overhead of calling memcpy for writes that
	/// have runtime-variable length, but are small and have a small fixed upper bound.
	///
	/// This can be used to do in-place encoding as is done for leb128 (without this function
	/// we would need to write to a temporary buffer then memcpy into the encoder), and it can
	/// also be used to implement the varint scheme we use for rmeta and dep graph encoding,
	/// where we only want to encode the first few bytes of an integer. Copying in the whole
	/// integer then only advancing the encoder state for the few bytes we care about is more
	/// efficient than calling [`FileEncoder::write_all`], because variable-size copies are
	/// always lowered to `memcpy`, which has overhead and contains a lot of logic we can bypass
	/// with this function. Note that common architectures support fixed-size writes up to 8 bytes
	/// with one instruction, so while this does in some sense do wasted work, we come out ahead.
	#[inline]
	pub fn write_with<const N: usize>(&mut self, visitor: impl FnOnce(&mut [u8; N]) -> usize) {
	#[cfg(debug_assertions)]
	{
	self.finished = false;
	}
	let flush_threshold = const { BUF_SIZE.checked_sub(N).unwrap() };
	if std::intrinsics::unlikely(self.buffered > flush_threshold) {
	self.flush();
	}
	// SAFETY: We checked above that that N < self.buffer_empty().len(),
	// and if isn't, flush ensures that our empty buffer is now BUF_SIZE.
	// We produce a post-mono error if N > BUF_SIZE.
	let buf = unsafe { self.buffer_empty().first_chunk_mut::<N>().unwrap_unchecked() };
	let written = visitor(buf);
	// We have to ensure that an errant visitor cannot cause self.buffered to exeed BUF_SIZE.
	if written > N {
	Self::panic_invalid_write::<N>(written);
	}
	self.buffered = self.buffered.debug_strict_add(written);
	}

	#[cold]
	#[inline(never)]
	fn panic_invalid_write<const N: usize>(written: usize) {
	panic!("FileEncoder::write_with::<{N}> cannot be used to write {written} bytes");
	}

	/// Helper for calls where [`FileEncoder::write_with`] always writes the whole array.
	#[inline]
	pub fn write_array<const N: usize>(&mut self, buf: [u8; N]) {
	self.write_with(\|dest\| {
	*dest = buf;
	N
	})
	}

	pub fn finish(&mut self) -> FileEncodeResult {
	self.write_all(MAGIC_END_BYTES);
	self.flush();
	#[cfg(debug_assertions)]
	{
	self.finished = true;
	}
	match std::mem::replace(&mut self.res, Ok(())) {
	Ok(()) => Ok(self.position()),
	Err(e) => Err((self.path.clone(), e)),
	}
	}
	}

	#[cfg(debug_assertions)]
	impl Drop for FileEncoder {
	fn drop(&mut self) {
	if !std::thread::panicking() {
	assert!(self.finished);
	}
	}
	}

	macro_rules! write_leb128 {
	($this_fn:ident, $int_ty:ty, $write_leb_fn:ident) => {
	#[inline]
	fn $this_fn(&mut self, v: $int_ty) {
	self.write_with(\|buf\| leb128::$write_leb_fn(buf, v))
	}
	};
	}

	impl Encoder for FileEncoder {
	write_leb128!(emit_usize, usize, write_usize_leb128);
	write_leb128!(emit_u128, u128, write_u128_leb128);
	write_leb128!(emit_u64, u64, write_u64_leb128);
	write_leb128!(emit_u32, u32, write_u32_leb128);

	#[inline]
	fn emit_u16(&mut self, v: u16) {
	self.write_array(v.to_le_bytes());
	}

	#[inline]
	fn emit_u8(&mut self, v: u8) {
	self.write_array([v]);
	}

	write_leb128!(emit_isize, isize, write_isize_leb128);
	write_leb128!(emit_i128, i128, write_i128_leb128);
	write_leb128!(emit_i64, i64, write_i64_leb128);
	write_leb128!(emit_i32, i32, write_i32_leb128);

	#[inline]
	fn emit_i16(&mut self, v: i16) {
	self.write_array(v.to_le_bytes());
	}

	#[inline]
	fn emit_raw_bytes(&mut self, s: &[u8]) {
	self.write_all(s);
	}
	}

	// -----------------------------------------------------------------------------
	// Decoder
	// -----------------------------------------------------------------------------

	// Conceptually, `MemDecoder` wraps a `&[u8]` with a cursor into it that is always valid.
	// This is implemented with three pointers, two which represent the original slice and a
	// third that is our cursor.
	// It is an invariant of this type that start <= current <= end.
	// Additionally, the implementation of this type never modifies start and end.
	pub struct MemDecoder<'a> {
	start: *const u8,
	current: *const u8,
	end: *const u8,
	_marker: PhantomData<&'a u8>,
	}

	impl<'a> MemDecoder<'a> {
	#[inline]
	pub fn new(data: &'a [u8], position: usize) -> Result<MemDecoder<'a>, ()> {
	let data = data.strip_suffix(MAGIC_END_BYTES).ok_or(())?;
	let Range { start, end } = data.as_ptr_range();
	Ok(MemDecoder { start, current: data[position..].as_ptr(), end, _marker: PhantomData })
	}

	#[inline]
	pub fn split_at(&self, position: usize) -> MemDecoder<'a> {
	assert!(position <= self.len());
	// SAFETY: We checked above that this offset is within the original slice
	let current = unsafe { self.start.add(position) };
	MemDecoder { start: self.start, current, end: self.end, _marker: PhantomData }
	}

	#[inline]
	pub fn len(&self) -> usize {
	// SAFETY: This recovers the length of the original slice, only using members we never modify.
	unsafe { self.end.sub_ptr(self.start) }
	}

	#[inline]
	pub fn remaining(&self) -> usize {
	// SAFETY: This type guarantees current <= end.
	unsafe { self.end.sub_ptr(self.current) }
	}

	#[cold]
	#[inline(never)]
	fn decoder_exhausted() -> ! {
	panic!("MemDecoder exhausted")
	}

	#[inline]
	pub fn read_array<const N: usize>(&mut self) -> [u8; N] {
	self.read_raw_bytes(N).try_into().unwrap()
	}

	/// While we could manually expose manipulation of the decoder position,
	/// all current users of that method would need to reset the position later,
	/// incurring the bounds check of set_position twice.
	#[inline]
	pub fn with_position<F, T>(&mut self, pos: usize, func: F) -> T
	where
	F: Fn(&mut MemDecoder<'a>) -> T,
	{
	struct SetOnDrop<'a, 'guarded> {
	decoder: &'guarded mut MemDecoder<'a>,
	current: *const u8,
	}
	impl Drop for SetOnDrop<'_, '_> {
	fn drop(&mut self) {
	self.decoder.current = self.current;
	}
	}

	if pos >= self.len() {
	Self::decoder_exhausted();
	}
	let previous = self.current;
	// SAFETY: We just checked if this add is in-bounds above.
	unsafe {
	self.current = self.start.add(pos);
	}
	let guard = SetOnDrop { current: previous, decoder: self };
	func(guard.decoder)
	}
	}

	macro_rules! read_leb128 {
	($this_fn:ident, $int_ty:ty, $read_leb_fn:ident) => {
	#[inline]
	fn $this_fn(&mut self) -> $int_ty {
	leb128::$read_leb_fn(self)
	}
	};
	}

	impl<'a> Decoder for MemDecoder<'a> {
	read_leb128!(read_usize, usize, read_usize_leb128);
	read_leb128!(read_u128, u128, read_u128_leb128);
	read_leb128!(read_u64, u64, read_u64_leb128);
	read_leb128!(read_u32, u32, read_u32_leb128);

	#[inline]
	fn read_u16(&mut self) -> u16 {
	u16::from_le_bytes(self.read_array())
	}

	#[inline]
	fn read_u8(&mut self) -> u8 {
	if self.current == self.end {
	Self::decoder_exhausted();
	}
	// SAFETY: This type guarantees current <= end, and we just checked current == end.
	unsafe {
	let byte = *self.current;
	self.current = self.current.add(1);
	byte
	}
	}

	read_leb128!(read_isize, isize, read_isize_leb128);
	read_leb128!(read_i128, i128, read_i128_leb128);
	read_leb128!(read_i64, i64, read_i64_leb128);
	read_leb128!(read_i32, i32, read_i32_leb128);

	#[inline]
	fn read_i16(&mut self) -> i16 {
	i16::from_le_bytes(self.read_array())
	}

	#[inline]
	fn read_raw_bytes(&mut self, bytes: usize) -> &'a [u8] {
	if bytes > self.remaining() {
	Self::decoder_exhausted();
	}
	// SAFETY: We just checked if this range is in-bounds above.
	unsafe {
	let slice = std::slice::from_raw_parts(self.current, bytes);
	self.current = self.current.add(bytes);
	slice
	}
	}

	#[inline]
	fn peek_byte(&self) -> u8 {
	if self.current == self.end {
	Self::decoder_exhausted();
	}
	// SAFETY: This type guarantees current is inbounds or one-past-the-end, which is end.
	// Since we just checked current == end, the current pointer must be inbounds.
	unsafe { *self.current }
	}

	#[inline]
	fn position(&self) -> usize {
	// SAFETY: This type guarantees start <= current
	unsafe { self.current.sub_ptr(self.start) }
	}
	}

	// Specializations for contiguous byte sequences follow. The default implementations for slices
	// encode and decode each element individually. This isn't necessary for `u8` slices when using
	// opaque encoders and decoders, because each `u8` is unchanged by encoding and decoding.
	// Therefore, we can use more efficient implementations that process the entire sequence at once.

	// Specialize encoding byte slices. This specialization also applies to encoding `Vec<u8>`s, etc.,
	// since the default implementations call `encode` on their slices internally.
	impl Encodable<FileEncoder> for [u8] {
	fn encode(&self, e: &mut FileEncoder) {
	Encoder::emit_usize(e, self.len());
	e.emit_raw_bytes(self);
	}
	}

	// Specialize decoding `Vec<u8>`. This specialization also applies to decoding `Box<[u8]>`s, etc.,
	// since the default implementations call `decode` to produce a `Vec<u8>` internally.
	impl<'a> Decodable<MemDecoder<'a>> for Vec<u8> {
	fn decode(d: &mut MemDecoder<'a>) -> Self {
	let len = Decoder::read_usize(d);
	d.read_raw_bytes(len).to_owned()
	}
	}

	/// An integer that will always encode to 8 bytes.
	pub struct IntEncodedWithFixedSize(pub u64);

	impl IntEncodedWithFixedSize {
	pub const ENCODED_SIZE: usize = 8;
	}

	impl Encodable<FileEncoder> for IntEncodedWithFixedSize {
	#[inline]
	fn encode(&self, e: &mut FileEncoder) {
	let _start_pos = e.position();
	e.write_array(self.0.to_le_bytes());
	let _end_pos = e.position();
	debug_assert_eq!((_end_pos - _start_pos), IntEncodedWithFixedSize::ENCODED_SIZE);
	}
	}

	impl<'a> Decodable<MemDecoder<'a>> for IntEncodedWithFixedSize {
	#[inline]
	fn decode(decoder: &mut MemDecoder<'a>) -> IntEncodedWithFixedSize {
	let bytes = decoder.read_array::<{ IntEncodedWithFixedSize::ENCODED_SIZE }>();
	IntEncodedWithFixedSize(u64::from_le_bytes(bytes))
	}
	}