add patch for 29ef6cf

.gitignore

@@ -0,0 +1,2 @@
+/target
+Cargo.lock

Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "core_io"
+version = "0.1.0"
+edition = "2018"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[features]
+alloc = []
+collections = ["alloc"]

src/buffered/bufreader.rs

@@ -0,0 +1,438 @@
+use core::cmp;
+use core::fmt;
+use crate::{
+    self as io, BufRead, Initializer, IoSliceMut, Read, Seek, SeekFrom, SizeHint, DEFAULT_BUF_SIZE,
+};
+use crate::prelude::*;
+
+/// The `BufReader<R>` struct adds buffering to any reader.
+///
+/// It can be excessively inefficient to work directly with a [`Read`] instance.
+/// For example, every call to [`read`][`TcpStream::read`] on [`TcpStream`]
+/// results in a system call. A `BufReader<R>` performs large, infrequent reads on
+/// the underlying [`Read`] and maintains an in-memory buffer of the results.
+///
+/// `BufReader<R>` can improve the speed of programs that make *small* and
+/// *repeated* read calls to the same file or network socket. It does not
+/// help when reading very large amounts at once, or reading just one or a few
+/// times. It also provides no advantage when reading from a source that is
+/// already in memory, like a [`Vec`]`<u8>`.
+///
+/// When the `BufReader<R>` is dropped, the contents of its buffer will be
+/// discarded. Creating multiple instances of a `BufReader<R>` on the same
+/// stream can cause data loss. Reading from the underlying reader after
+/// unwrapping the `BufReader<R>` with [`BufReader::into_inner`] can also cause
+/// data loss.
+///
+// HACK(#78696): can't use `crate` for associated items
+/// [`TcpStream::read`]: super::super::super::net::TcpStream::read
+/// [`TcpStream`]: crate::net::TcpStream
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::io::prelude::*;
+/// use std::io::BufReader;
+/// use std::fs::File;
+///
+/// fn main() -> std::io::Result<()> {
+///     let f = File::open("log.txt")?;
+///     let mut reader = BufReader::new(f);
+///
+///     let mut line = String::new();
+///     let len = reader.read_line(&mut line)?;
+///     println!("First line is {} bytes long", len);
+///     Ok(())
+/// }
+/// ```
+pub struct BufReader<R> {
+    inner: R,
+    buf: Box<[u8]>,
+    pos: usize,
+    cap: usize,
+}
+
+impl<R: Read> BufReader<R> {
+    /// Creates a new `BufReader<R>` with a default buffer capacity. The default is currently 8 KB,
+    /// but may change in the future.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufReader;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f = File::open("log.txt")?;
+    ///     let reader = BufReader::new(f);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn new(inner: R) -> BufReader<R> {
+        BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
+    }
+
+    /// Creates a new `BufReader<R>` with the specified buffer capacity.
+    ///
+    /// # Examples
+    ///
+    /// Creating a buffer with ten bytes of capacity:
+    ///
+    /// ```no_run
+    /// use std::io::BufReader;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f = File::open("log.txt")?;
+    ///     let reader = BufReader::with_capacity(10, f);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
+        unsafe {
+            let mut buf = Box::new_uninit_slice(capacity).assume_init();
+            inner.initializer().initialize(&mut buf);
+            BufReader { inner, buf, pos: 0, cap: 0 }
+        }
+    }
+}
+
+impl<R> BufReader<R> {
+    /// Gets a reference to the underlying reader.
+    ///
+    /// It is inadvisable to directly read from the underlying reader.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufReader;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f1 = File::open("log.txt")?;
+    ///     let reader = BufReader::new(f1);
+    ///
+    ///     let f2 = reader.get_ref();
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn get_ref(&self) -> &R {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying reader.
+    ///
+    /// It is inadvisable to directly read from the underlying reader.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufReader;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f1 = File::open("log.txt")?;
+    ///     let mut reader = BufReader::new(f1);
+    ///
+    ///     let f2 = reader.get_mut();
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn get_mut(&mut self) -> &mut R {
+        &mut self.inner
+    }
+
+    /// Returns a reference to the internally buffered data.
+    ///
+    /// Unlike [`fill_buf`], this will not attempt to fill the buffer if it is empty.
+    ///
+    /// [`fill_buf`]: BufRead::fill_buf
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::{BufReader, BufRead};
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f = File::open("log.txt")?;
+    ///     let mut reader = BufReader::new(f);
+    ///     assert!(reader.buffer().is_empty());
+    ///
+    ///     if reader.fill_buf()?.len() > 0 {
+    ///         assert!(!reader.buffer().is_empty());
+    ///     }
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn buffer(&self) -> &[u8] {
+        &self.buf[self.pos..self.cap]
+    }
+
+    /// Returns the number of bytes the internal buffer can hold at once.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::{BufReader, BufRead};
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f = File::open("log.txt")?;
+    ///     let mut reader = BufReader::new(f);
+    ///
+    ///     let capacity = reader.capacity();
+    ///     let buffer = reader.fill_buf()?;
+    ///     assert!(buffer.len() <= capacity);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.buf.len()
+    }
+
+    /// Unwraps this `BufReader<R>`, returning the underlying reader.
+    ///
+    /// Note that any leftover data in the internal buffer is lost. Therefore,
+    /// a following read from the underlying reader may lead to data loss.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufReader;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let f1 = File::open("log.txt")?;
+    ///     let reader = BufReader::new(f1);
+    ///
+    ///     let f2 = reader.into_inner();
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn into_inner(self) -> R {
+        self.inner
+    }
+
+    /// Invalidates all data in the internal buffer.
+    #[inline]
+    fn discard_buffer(&mut self) {
+        self.pos = 0;
+        self.cap = 0;
+    }
+}
+
+impl<R: Seek> BufReader<R> {
+    /// Seeks relative to the current position. If the new position lies within the buffer,
+    /// the buffer will not be flushed, allowing for more efficient seeks.
+    /// This method does not return the location of the underlying reader, so the caller
+    /// must track this information themselves if it is required.
+    pub fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
+        let pos = self.pos as u64;
+        if offset < 0 {
+            if let Some(new_pos) = pos.checked_sub((-offset) as u64) {
+                self.pos = new_pos as usize;
+                return Ok(());
+            }
+        } else {
+            if let Some(new_pos) = pos.checked_add(offset as u64) {
+                if new_pos <= self.cap as u64 {
+                    self.pos = new_pos as usize;
+                    return Ok(());
+                }
+            }
+        }
+        self.seek(SeekFrom::Current(offset)).map(drop)
+    }
+}
+
+impl<R: Read> Read for BufReader<R> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        // If we don't have any buffered data and we're doing a massive read
+        // (larger than our internal buffer), bypass our internal buffer
+        // entirely.
+        if self.pos == self.cap && buf.len() >= self.buf.len() {
+            self.discard_buffer();
+            return self.inner.read(buf);
+        }
+        let nread = {
+            let mut rem = self.fill_buf()?;
+            rem.read(buf)?
+        };
+        self.consume(nread);
+        Ok(nread)
+    }
+
+    // Small read_exacts from a BufReader are extremely common when used with a deserializer.
+    // The default implementation calls read in a loop, which results in surprisingly poor code
+    // generation for the common path where the buffer has enough bytes to fill the passed-in
+    // buffer.
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        if self.buffer().len() >= buf.len() {
+            buf.copy_from_slice(&self.buffer()[..buf.len()]);
+            self.consume(buf.len());
+            return Ok(());
+        }
+
+        crate::default_read_exact(self, buf)
+    }
+
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
+        if self.pos == self.cap && total_len >= self.buf.len() {
+            self.discard_buffer();
+            return self.inner.read_vectored(bufs);
+        }
+        let nread = {
+            let mut rem = self.fill_buf()?;
+            rem.read_vectored(bufs)?
+        };
+        self.consume(nread);
+        Ok(nread)
+    }
+
+    fn is_read_vectored(&self) -> bool {
+        self.inner.is_read_vectored()
+    }
+
+    // we can't skip unconditionally because of the large buffer case in read.
+    unsafe fn initializer(&self) -> Initializer {
+        self.inner.initializer()
+    }
+}
+
+impl<R: Read> BufRead for BufReader<R> {
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        // If we've reached the end of our internal buffer then we need to fetch
+        // some more data from the underlying reader.
+        // Branch using `>=` instead of the more correct `==`
+        // to tell the compiler that the pos..cap slice is always valid.
+        if self.pos >= self.cap {
+            debug_assert!(self.pos == self.cap);
+            self.cap = self.inner.read(&mut self.buf)?;
+            self.pos = 0;
+        }
+        Ok(&self.buf[self.pos..self.cap])
+    }
+
+    fn consume(&mut self, amt: usize) {
+        self.pos = cmp::min(self.pos + amt, self.cap);
+    }
+}
+
+impl<R> fmt::Debug for BufReader<R>
+where
+    R: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("BufReader")
+            .field("reader", &self.inner)
+            .field("buffer", &format_args!("{}/{}", self.cap - self.pos, self.buf.len()))
+            .finish()
+    }
+}
+
+impl<R: Seek> Seek for BufReader<R> {
+    /// Seek to an offset, in bytes, in the underlying reader.
+    ///
+    /// The position used for seeking with [`SeekFrom::Current`]`(_)` is the
+    /// position the underlying reader would be at if the `BufReader<R>` had no
+    /// internal buffer.
+    ///
+    /// Seeking always discards the internal buffer, even if the seek position
+    /// would otherwise fall within it. This guarantees that calling
+    /// [`BufReader::into_inner()`] immediately after a seek yields the underlying reader
+    /// at the same position.
+    ///
+    /// To seek without discarding the internal buffer, use [`BufReader::seek_relative`].
+    ///
+    /// See [`std::io::Seek`] for more details.
+    ///
+    /// Note: In the edge case where you're seeking with [`SeekFrom::Current`]`(n)`
+    /// where `n` minus the internal buffer length overflows an `i64`, two
+    /// seeks will be performed instead of one. If the second seek returns
+    /// [`Err`], the underlying reader will be left at the same position it would
+    /// have if you called `seek` with [`SeekFrom::Current`]`(0)`.
+    ///
+    /// [`std::io::Seek`]: Seek
+    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+        let result: u64;
+        if let SeekFrom::Current(n) = pos {
+            let remainder = (self.cap - self.pos) as i64;
+            // it should be safe to assume that remainder fits within an i64 as the alternative
+            // means we managed to allocate 8 exbibytes and that's absurd.
+            // But it's not out of the realm of possibility for some weird underlying reader to
+            // support seeking by i64::MIN so we need to handle underflow when subtracting
+            // remainder.
+            if let Some(offset) = n.checked_sub(remainder) {
+                result = self.inner.seek(SeekFrom::Current(offset))?;
+            } else {
+                // seek backwards by our remainder, and then by the offset
+                self.inner.seek(SeekFrom::Current(-remainder))?;
+                self.discard_buffer();
+                result = self.inner.seek(SeekFrom::Current(n))?;
+            }
+        } else {
+            // Seeking with Start/End doesn't care about our buffer length.
+            result = self.inner.seek(pos)?;
+        }
+        self.discard_buffer();
+        Ok(result)
+    }
+
+    /// Returns the current seek position from the start of the stream.
+    ///
+    /// The value returned is equivalent to `self.seek(SeekFrom::Current(0))`
+    /// but does not flush the internal buffer. Due to this optimization the
+    /// function does not guarantee that calling `.into_inner()` immediately
+    /// afterwards will yield the underlying reader at the same position. Use
+    /// [`BufReader::seek`] instead if you require that guarantee.
+    ///
+    /// # Panics
+    ///
+    /// This function will panic if the position of the inner reader is smaller
+    /// than the amount of buffered data. That can happen if the inner reader
+    /// has an incorrect implementation of [`Seek::stream_position`], or if the
+    /// position has gone out of sync due to calling [`Seek::seek`] directly on
+    /// the underlying reader.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// use std::{
+    ///     io::{self, BufRead, BufReader, Seek},
+    ///     fs::File,
+    /// };
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = BufReader::new(File::open("foo.txt")?);
+    ///
+    ///     let before = f.stream_position()?;
+    ///     f.read_line(&mut String::new())?;
+    ///     let after = f.stream_position()?;
+    ///
+    ///     println!("The first line was {} bytes long", after - before);
+    ///     Ok(())
+    /// }
+    /// ```
+    fn stream_position(&mut self) -> io::Result<u64> {
+        let remainder = (self.cap - self.pos) as u64;
+        self.inner.stream_position().map(|pos| {
+            pos.checked_sub(remainder).expect(
+                "overflow when subtracting remaining buffer size from inner stream position",
+            )
+        })
+    }
+}
+
+impl<T> SizeHint for BufReader<T> {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        SizeHint::lower_bound(self.get_ref()) + self.buffer().len()
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        SizeHint::upper_bound(self.get_ref()).and_then(|up| self.buffer().len().checked_add(up))
+    }
+}

src/buffered/bufwriter.rs

@@ -0,0 +1,648 @@
+use core::fmt;
+use crate::{
+    self as io, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
+};
+use core::ptr;
+use core::mem;
+use crate::Vec;
+
+/// Wraps a writer and buffers its output.
+///
+/// It can be excessively inefficient to work directly with something that
+/// implements [`Write`]. For example, every call to
+/// [`write`][`TcpStream::write`] on [`TcpStream`] results in a system call. A
+/// `BufWriter<W>` keeps an in-memory buffer of data and writes it to an underlying
+/// writer in large, infrequent batches.
+///
+/// `BufWriter<W>` can improve the speed of programs that make *small* and
+/// *repeated* write calls to the same file or network socket. It does not
+/// help when writing very large amounts at once, or writing just one or a few
+/// times. It also provides no advantage when writing to a destination that is
+/// in memory, like a [`Vec`]`<u8>`.
+///
+/// It is critical to call [`flush`] before `BufWriter<W>` is dropped. Though
+/// dropping will attempt to flush the contents of the buffer, any errors
+/// that happen in the process of dropping will be ignored. Calling [`flush`]
+/// ensures that the buffer is empty and thus dropping will not even attempt
+/// file operations.
+///
+/// # Examples
+///
+/// Let's write the numbers one through ten to a [`TcpStream`]:
+///
+/// ```no_run
+/// use std::io::prelude::*;
+/// use std::net::TcpStream;
+///
+/// let mut stream = TcpStream::connect("127.0.0.1:34254").unwrap();
+///
+/// for i in 0..10 {
+///     stream.write(&[i+1]).unwrap();
+/// }
+/// ```
+///
+/// Because we're not buffering, we write each one in turn, incurring the
+/// overhead of a system call per byte written. We can fix this with a
+/// `BufWriter<W>`:
+///
+/// ```no_run
+/// use std::io::prelude::*;
+/// use std::io::BufWriter;
+/// use std::net::TcpStream;
+///
+/// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+///
+/// for i in 0..10 {
+///     stream.write(&[i+1]).unwrap();
+/// }
+/// stream.flush().unwrap();
+/// ```
+///
+/// By wrapping the stream with a `BufWriter<W>`, these ten writes are all grouped
+/// together by the buffer and will all be written out in one system call when
+/// the `stream` is flushed.
+///
+// HACK(#78696): can't use `crate` for associated items
+/// [`TcpStream::write`]: super::super::super::net::TcpStream::write
+/// [`TcpStream`]: crate::net::TcpStream
+/// [`flush`]: BufWriter::flush
+pub struct BufWriter<W: Write> {
+    inner: W,
+    // The buffer. Avoid using this like a normal `Vec` in common code paths.
+    // That is, don't use `buf.push`, `buf.extend_from_slice`, or any other
+    // methods that require bounds checking or the like. This makes an enormous
+    // difference to performance (we may want to stop using a `Vec` entirely).
+    buf: Vec<u8>,
+    // #30888: If the inner writer panics in a call to write, we don't want to
+    // write the buffered data a second time in BufWriter's destructor. This
+    // flag tells the Drop impl if it should skip the flush.
+    panicked: bool,
+}
+
+impl<W: Write> BufWriter<W> {
+    /// Creates a new `BufWriter<W>` with a default buffer capacity. The default is currently 8 KB,
+    /// but may change in the future.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    /// ```
+    pub fn new(inner: W) -> BufWriter<W> {
+        BufWriter::with_capacity(DEFAULT_BUF_SIZE, inner)
+    }
+
+    /// Creates a new `BufWriter<W>` with the specified buffer capacity.
+    ///
+    /// # Examples
+    ///
+    /// Creating a buffer with a buffer of a hundred bytes.
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let stream = TcpStream::connect("127.0.0.1:34254").unwrap();
+    /// let mut buffer = BufWriter::with_capacity(100, stream);
+    /// ```
+    pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> {
+        BufWriter { inner, buf: Vec::with_capacity(capacity), panicked: false }
+    }
+
+    /// Send data in our local buffer into the inner writer, looping as
+    /// necessary until either it's all been sent or an error occurs.
+    ///
+    /// Because all the data in the buffer has been reported to our owner as
+    /// "successfully written" (by returning nonzero success values from
+    /// `write`), any 0-length writes from `inner` must be reported as i/o
+    /// errors from this method.
+    pub(crate) fn flush_buf(&mut self) -> io::Result<()> {
+        /// Helper struct to ensure the buffer is updated after all the writes
+        /// are complete. It tracks the number of written bytes and drains them
+        /// all from the front of the buffer when dropped.
+        struct BufGuard<'a> {
+            buffer: &'a mut Vec<u8>,
+            written: usize,
+        }
+
+        impl<'a> BufGuard<'a> {
+            fn new(buffer: &'a mut Vec<u8>) -> Self {
+                Self { buffer, written: 0 }
+            }
+
+            /// The unwritten part of the buffer
+            fn remaining(&self) -> &[u8] {
+                &self.buffer[self.written..]
+            }
+
+            /// Flag some bytes as removed from the front of the buffer
+            fn consume(&mut self, amt: usize) {
+                self.written += amt;
+            }
+
+            /// true if all of the bytes have been written
+            fn done(&self) -> bool {
+                self.written >= self.buffer.len()
+            }
+        }
+
+        impl Drop for BufGuard<'_> {
+            fn drop(&mut self) {
+                if self.written > 0 {
+                    self.buffer.drain(..self.written);
+                }
+            }
+        }
+
+        let mut guard = BufGuard::new(&mut self.buf);
+        while !guard.done() {
+            self.panicked = true;
+            let r = self.inner.write(guard.remaining());
+            self.panicked = false;
+
+            match r {
+                Ok(0) => {
+                    return Err(Error::new_const(
+                        ErrorKind::WriteZero,
+                        &"failed to write the buffered data",
+                    ));
+                }
+                Ok(n) => guard.consume(n),
+                Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(())
+    }
+
+    /// Buffer some data without flushing it, regardless of the size of the
+    /// data. Writes as much as possible without exceeding capacity. Returns
+    /// the number of bytes written.
+    pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize {
+        let available = self.spare_capacity();
+        let amt_to_buffer = available.min(buf.len());
+
+        // SAFETY: `amt_to_buffer` is <= buffer's spare capacity by construction.
+        unsafe {
+            self.write_to_buffer_unchecked(&buf[..amt_to_buffer]);
+        }
+
+        amt_to_buffer
+    }
+
+    /// Gets a reference to the underlying writer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // we can use reference just like buffer
+    /// let reference = buffer.get_ref();
+    /// ```
+    pub fn get_ref(&self) -> &W {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying writer.
+    ///
+    /// It is inadvisable to directly write to the underlying writer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // we can use reference just like buffer
+    /// let reference = buffer.get_mut();
+    /// ```
+    pub fn get_mut(&mut self) -> &mut W {
+        &mut self.inner
+    }
+
+    /// Returns a reference to the internally buffered data.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // See how many bytes are currently buffered
+    /// let bytes_buffered = buf_writer.buffer().len();
+    /// ```
+    pub fn buffer(&self) -> &[u8] {
+        &self.buf
+    }
+
+    /// Returns a mutable reference to the internal buffer.
+    ///
+    /// This can be used to write data directly into the buffer without triggering writers
+    /// to the underlying writer.
+    ///
+    /// That the buffer is a `Vec` is an implementation detail.
+    /// Callers should not modify the capacity as there currently is no public API to do so
+    /// and thus any capacity changes would be unexpected by the user.
+    pub(crate) fn buffer_mut(&mut self) -> &mut Vec<u8> {
+        &mut self.buf
+    }
+
+    /// Returns the number of bytes the internal buffer can hold without flushing.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // Check the capacity of the inner buffer
+    /// let capacity = buf_writer.capacity();
+    /// // Calculate how many bytes can be written without flushing
+    /// let without_flush = capacity - buf_writer.buffer().len();
+    /// ```
+    pub fn capacity(&self) -> usize {
+        self.buf.capacity()
+    }
+
+    /// Unwraps this `BufWriter<W>`, returning the underlying writer.
+    ///
+    /// The buffer is written out before returning the writer.
+    ///
+    /// # Errors
+    ///
+    /// An [`Err`] will be returned if an error occurs while flushing the buffer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // unwrap the TcpStream and flush the buffer
+    /// let stream = buffer.into_inner().unwrap();
+    /// ```
+    pub fn into_inner(mut self) -> Result<W, IntoInnerError<BufWriter<W>>> {
+        match self.flush_buf() {
+            Err(e) => Err(IntoInnerError::new(self, e)),
+            Ok(()) => Ok(self.into_parts().0),
+        }
+    }
+
+    /// Disassembles this `BufWriter<W>`, returning the underlying writer, and any buffered but
+    /// unwritten data.
+    ///
+    /// If the underlying writer panicked, it is not known what portion of the data was written.
+    /// In this case, we return `WriterPanicked` for the buffered data (from which the buffer
+    /// contents can still be recovered).
+    ///
+    /// `into_parts` makes no attempt to flush data and cannot fail.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{BufWriter, Write};
+    ///
+    /// let mut buffer = [0u8; 10];
+    /// let mut stream = BufWriter::new(buffer.as_mut());
+    /// write!(stream, "too much data").unwrap();
+    /// stream.flush().expect_err("it doesn't fit");
+    /// let (recovered_writer, buffered_data) = stream.into_parts();
+    /// assert_eq!(recovered_writer.len(), 0);
+    /// assert_eq!(&buffered_data.unwrap(), b"ata");
+    /// ```
+    pub fn into_parts(mut self) -> (W, Result<Vec<u8>, WriterPanicked>) {
+        let buf = mem::take(&mut self.buf);
+        let buf = if !self.panicked { Ok(buf) } else { Err(WriterPanicked { buf }) };
+
+        // SAFETY: forget(self) prevents double dropping inner
+        let inner = unsafe { ptr::read(&mut self.inner) };
+        mem::forget(self);
+
+        (inner, buf)
+    }
+
+    // Ensure this function does not get inlined into `write`, so that it
+    // remains inlineable and its common path remains as short as possible.
+    // If this function ends up being called frequently relative to `write`,
+    // it's likely a sign that the client is using an improperly sized buffer
+    // or their write patterns are somewhat pathological.
+    #[cold]
+    #[inline(never)]
+    fn write_cold(&mut self, buf: &[u8]) -> io::Result<usize> {
+        if buf.len() > self.spare_capacity() {
+            self.flush_buf()?;
+        }
+
+        // Why not len > capacity? To avoid a needless trip through the buffer when the input
+        // exactly fills it. We'd just need to flush it to the underlying writer anyway.
+        if buf.len() >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write(buf);
+            self.panicked = false;
+            r
+        } else {
+            // Write to the buffer. In this case, we write to the buffer even if it fills it
+            // exactly. Doing otherwise would mean flushing the buffer, then writing this
+            // input to the inner writer, which in many cases would be a worse strategy.
+
+            // SAFETY: There was either enough spare capacity already, or there wasn't and we
+            // flushed the buffer to ensure that there is. In the latter case, we know that there
+            // is because flushing ensured that our entire buffer is spare capacity, and we entered
+            // this block because the input buffer length is less than that capacity. In either
+            // case, it's safe to write the input buffer to our buffer.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
+            Ok(buf.len())
+        }
+    }
+
+    // Ensure this function does not get inlined into `write_all`, so that it
+    // remains inlineable and its common path remains as short as possible.
+    // If this function ends up being called frequently relative to `write_all`,
+    // it's likely a sign that the client is using an improperly sized buffer
+    // or their write patterns are somewhat pathological.
+    #[cold]
+    #[inline(never)]
+    fn write_all_cold(&mut self, buf: &[u8]) -> io::Result<()> {
+        // Normally, `write_all` just calls `write` in a loop. We can do better
+        // by calling `self.get_mut().write_all()` directly, which avoids
+        // round trips through the buffer in the event of a series of partial
+        // writes in some circumstances.
+
+        if buf.len() > self.spare_capacity() {
+            self.flush_buf()?;
+        }
+
+        // Why not len > capacity? To avoid a needless trip through the buffer when the input
+        // exactly fills it. We'd just need to flush it to the underlying writer anyway.
+        if buf.len() >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write_all(buf);
+            self.panicked = false;
+            r
+        } else {
+            // Write to the buffer. In this case, we write to the buffer even if it fills it
+            // exactly. Doing otherwise would mean flushing the buffer, then writing this
+            // input to the inner writer, which in many cases would be a worse strategy.
+
+            // SAFETY: There was either enough spare capacity already, or there wasn't and we
+            // flushed the buffer to ensure that there is. In the latter case, we know that there
+            // is because flushing ensured that our entire buffer is spare capacity, and we entered
+            // this block because the input buffer length is less than that capacity. In either
+            // case, it's safe to write the input buffer to our buffer.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
+            Ok(())
+        }
+    }
+
+    // SAFETY: Requires `buf.len() <= self.buf.capacity() - self.buf.len()`,
+    // i.e., that input buffer length is less than or equal to spare capacity.
+    #[inline]
+    unsafe fn write_to_buffer_unchecked(&mut self, buf: &[u8]) {
+        debug_assert!(buf.len() <= self.spare_capacity());
+        let old_len = self.buf.len();
+        let buf_len = buf.len();
+        let src = buf.as_ptr();
+        let dst = self.buf.as_mut_ptr().add(old_len);
+        ptr::copy_nonoverlapping(src, dst, buf_len);
+        self.buf.set_len(old_len + buf_len);
+    }
+
+    #[inline]
+    fn spare_capacity(&self) -> usize {
+        self.buf.capacity() - self.buf.len()
+    }
+}
+
+/// Error returned for the buffered data from `BufWriter::into_parts`, when the underlying
+/// writer has previously panicked.  Contains the (possibly partly written) buffered data.
+///
+/// # Example
+///
+/// ```
+/// use std::io::{self, BufWriter, Write};
+/// use std::panic::{catch_unwind, AssertUnwindSafe};
+///
+/// struct PanickingWriter;
+/// impl Write for PanickingWriter {
+///   fn write(&mut self, buf: &[u8]) -> io::Result<usize> { panic!() }
+///   fn flush(&mut self) -> io::Result<()> { panic!() }
+/// }
+///
+/// let mut stream = BufWriter::new(PanickingWriter);
+/// write!(stream, "some data").unwrap();
+/// let result = catch_unwind(AssertUnwindSafe(|| {
+///     stream.flush().unwrap()
+/// }));
+/// assert!(result.is_err());
+/// let (recovered_writer, buffered_data) = stream.into_parts();
+/// assert!(matches!(recovered_writer, PanickingWriter));
+/// assert_eq!(buffered_data.unwrap_err().into_inner(), b"some data");
+/// ```
+pub struct WriterPanicked {
+    buf: Vec<u8>,
+}
+
+impl WriterPanicked {
+    /// Returns the perhaps-unwritten data.  Some of this data may have been written by the
+    /// panicking call(s) to the underlying writer, so simply writing it again is not a good idea.
+    pub fn into_inner(self) -> Vec<u8> {
+        self.buf
+    }
+
+    const DESCRIPTION: &'static str =
+        "BufWriter inner writer panicked, what data remains unwritten is not known";
+}
+
+impl fmt::Display for WriterPanicked {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", Self::DESCRIPTION)
+    }
+}
+
+impl fmt::Debug for WriterPanicked {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("WriterPanicked")
+            .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
+            .finish()
+    }
+}
+
+impl<W: Write> Write for BufWriter<W> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        // Use < instead of <= to avoid a needless trip through the buffer in some cases.
+        // See `write_cold` for details.
+        if buf.len() < self.spare_capacity() {
+            // SAFETY: safe by above conditional.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
+            Ok(buf.len())
+        } else {
+            self.write_cold(buf)
+        }
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        // Use < instead of <= to avoid a needless trip through the buffer in some cases.
+        // See `write_all_cold` for details.
+        if buf.len() < self.spare_capacity() {
+            // SAFETY: safe by above conditional.
+            unsafe {
+                self.write_to_buffer_unchecked(buf);
+            }
+
+            Ok(())
+        } else {
+            self.write_all_cold(buf)
+        }
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        // FIXME: Consider applying `#[inline]` / `#[inline(never)]` optimizations already applied
+        // to `write` and `write_all`. The performance benefits can be significant. See #79930.
+        if self.get_ref().is_write_vectored() {
+            // We have to handle the possibility that the total length of the buffers overflows
+            // `usize` (even though this can only happen if multiple `IoSlice`s reference the
+            // same underlying buffer, as otherwise the buffers wouldn't fit in memory). If the
+            // computation overflows, then surely the input cannot fit in our buffer, so we forward
+            // to the inner writer's `write_vectored` method to let it handle it appropriately.
+            let saturated_total_len =
+                bufs.iter().fold(0usize, |acc, b| acc.saturating_add(b.len()));
+
+            if saturated_total_len > self.spare_capacity() {
+                // Flush if the total length of the input exceeds our buffer's spare capacity.
+                // If we would have overflowed, this condition also holds, and we need to flush.
+                self.flush_buf()?;
+            }
+
+            if saturated_total_len >= self.buf.capacity() {
+                // Forward to our inner writer if the total length of the input is greater than or
+                // equal to our buffer capacity. If we would have overflowed, this condition also
+                // holds, and we punt to the inner writer.
+                self.panicked = true;
+                let r = self.get_mut().write_vectored(bufs);
+                self.panicked = false;
+                r
+            } else {
+                // `saturated_total_len < self.buf.capacity()` implies that we did not saturate.
+
+                // SAFETY: We checked whether or not the spare capacity was large enough above. If
+                // it was, then we're safe already. If it wasn't, we flushed, making sufficient
+                // room for any input <= the buffer size, which includes this input.
+                unsafe {
+                    bufs.iter().for_each(|b| self.write_to_buffer_unchecked(b));
+                };
+
+                Ok(saturated_total_len)
+            }
+        } else {
+            let mut iter = bufs.iter();
+            let mut total_written = if let Some(buf) = iter.by_ref().find(|&buf| !buf.is_empty()) {
+                // This is the first non-empty slice to write, so if it does
+                // not fit in the buffer, we still get to flush and proceed.
+                if buf.len() > self.spare_capacity() {
+                    self.flush_buf()?;
+                }
+                if buf.len() >= self.buf.capacity() {
+                    // The slice is at least as large as the buffering capacity,
+                    // so it's better to write it directly, bypassing the buffer.
+                    self.panicked = true;
+                    let r = self.get_mut().write(buf);
+                    self.panicked = false;
+                    return r;
+                } else {
+                    // SAFETY: We checked whether or not the spare capacity was large enough above.
+                    // If it was, then we're safe already. If it wasn't, we flushed, making
+                    // sufficient room for any input <= the buffer size, which includes this input.
+                    unsafe {
+                        self.write_to_buffer_unchecked(buf);
+                    }
+
+                    buf.len()
+                }
+            } else {
+                return Ok(0);
+            };
+            debug_assert!(total_written != 0);
+            for buf in iter {
+                if buf.len() <= self.spare_capacity() {
+                    // SAFETY: safe by above conditional.
+                    unsafe {
+                        self.write_to_buffer_unchecked(buf);
+                    }
+
+                    // This cannot overflow `usize`. If we are here, we've written all of the bytes
+                    // so far to our buffer, and we've ensured that we never exceed the buffer's
+                    // capacity. Therefore, `total_written` <= `self.buf.capacity()` <= `usize::MAX`.
+                    total_written += buf.len();
+                } else {
+                    break;
+                }
+            }
+            Ok(total_written)
+        }
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.flush_buf().and_then(|()| self.get_mut().flush())
+    }
+}
+
+impl<W: Write> fmt::Debug for BufWriter<W>
+where
+    W: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("BufWriter")
+            .field("writer", &self.inner)
+            .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
+            .finish()
+    }
+}
+
+impl<W: Write + Seek> Seek for BufWriter<W> {
+    /// Seek to the offset, in bytes, in the underlying writer.
+    ///
+    /// Seeking always writes out the internal buffer before seeking.
+    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+        self.flush_buf()?;
+        self.get_mut().seek(pos)
+    }
+}
+
+impl<W: Write> Drop for BufWriter<W> {
+    fn drop(&mut self) {
+        if !self.panicked {
+            // dtors should not panic, so we ignore a failed flush
+            let _r = self.flush_buf();
+        }
+    }
+}

src/buffered/linewriter.rs

@@ -0,0 +1,224 @@
+use core::fmt;
+use crate::{self as io, buffered::LineWriterShim, BufWriter, IntoInnerError, IoSlice, Write};
+
+/// Wraps a writer and buffers output to it, flushing whenever a newline
+/// (`0x0a`, `'\n'`) is detected.
+///
+/// The [`BufWriter`] struct wraps a writer and buffers its output.
+/// But it only does this batched write when it goes out of scope, or when the
+/// internal buffer is full. Sometimes, you'd prefer to write each line as it's
+/// completed, rather than the entire buffer at once. Enter `LineWriter`. It
+/// does exactly that.
+///
+/// Like [`BufWriter`], a `LineWriter`’s buffer will also be flushed when the
+/// `LineWriter` goes out of scope or when its internal buffer is full.
+///
+/// If there's still a partial line in the buffer when the `LineWriter` is
+/// dropped, it will flush those contents.
+///
+/// # Examples
+///
+/// We can use `LineWriter` to write one line at a time, significantly
+/// reducing the number of actual writes to the file.
+///
+/// ```no_run
+/// use std::fs::{self, File};
+/// use std::io::prelude::*;
+/// use std::io::LineWriter;
+///
+/// fn main() -> std::io::Result<()> {
+///     let road_not_taken = b"I shall be telling this with a sigh
+/// Somewhere ages and ages hence:
+/// Two roads diverged in a wood, and I -
+/// I took the one less traveled by,
+/// And that has made all the difference.";
+///
+///     let file = File::create("poem.txt")?;
+///     let mut file = LineWriter::new(file);
+///
+///     file.write_all(b"I shall be telling this with a sigh")?;
+///
+///     // No bytes are written until a newline is encountered (or
+///     // the internal buffer is filled).
+///     assert_eq!(fs::read_to_string("poem.txt")?, "");
+///     file.write_all(b"\n")?;
+///     assert_eq!(
+///         fs::read_to_string("poem.txt")?,
+///         "I shall be telling this with a sigh\n",
+///     );
+///
+///     // Write the rest of the poem.
+///     file.write_all(b"Somewhere ages and ages hence:
+/// Two roads diverged in a wood, and I -
+/// I took the one less traveled by,
+/// And that has made all the difference.")?;
+///
+///     // The last line of the poem doesn't end in a newline, so
+///     // we have to flush or drop the `LineWriter` to finish
+///     // writing.
+///     file.flush()?;
+///
+///     // Confirm the whole poem was written.
+///     assert_eq!(fs::read("poem.txt")?, &road_not_taken[..]);
+///     Ok(())
+/// }
+/// ```
+pub struct LineWriter<W: Write> {
+    inner: BufWriter<W>,
+}
+
+impl<W: Write> LineWriter<W> {
+    /// Creates a new `LineWriter`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::fs::File;
+    /// use std::io::LineWriter;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let file = File::create("poem.txt")?;
+    ///     let file = LineWriter::new(file);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn new(inner: W) -> LineWriter<W> {
+        // Lines typically aren't that long, don't use a giant buffer
+        LineWriter::with_capacity(1024, inner)
+    }
+
+    /// Creates a new `LineWriter` with a specified capacity for the internal
+    /// buffer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::fs::File;
+    /// use std::io::LineWriter;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let file = File::create("poem.txt")?;
+    ///     let file = LineWriter::with_capacity(100, file);
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn with_capacity(capacity: usize, inner: W) -> LineWriter<W> {
+        LineWriter { inner: BufWriter::with_capacity(capacity, inner) }
+    }
+
+    /// Gets a reference to the underlying writer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::fs::File;
+    /// use std::io::LineWriter;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let file = File::create("poem.txt")?;
+    ///     let file = LineWriter::new(file);
+    ///
+    ///     let reference = file.get_ref();
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn get_ref(&self) -> &W {
+        self.inner.get_ref()
+    }
+
+    /// Gets a mutable reference to the underlying writer.
+    ///
+    /// Caution must be taken when calling methods on the mutable reference
+    /// returned as extra writes could corrupt the output stream.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::fs::File;
+    /// use std::io::LineWriter;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let file = File::create("poem.txt")?;
+    ///     let mut file = LineWriter::new(file);
+    ///
+    ///     // we can use reference just like file
+    ///     let reference = file.get_mut();
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn get_mut(&mut self) -> &mut W {
+        self.inner.get_mut()
+    }
+
+    /// Unwraps this `LineWriter`, returning the underlying writer.
+    ///
+    /// The internal buffer is written out before returning the writer.
+    ///
+    /// # Errors
+    ///
+    /// An [`Err`] will be returned if an error occurs while flushing the buffer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::fs::File;
+    /// use std::io::LineWriter;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let file = File::create("poem.txt")?;
+    ///
+    ///     let writer: LineWriter<File> = LineWriter::new(file);
+    ///
+    ///     let file: File = writer.into_inner()?;
+    ///     Ok(())
+    /// }
+    /// ```
+    pub fn into_inner(self) -> Result<W, IntoInnerError<LineWriter<W>>> {
+        self.inner.into_inner().map_err(|err| err.new_wrapped(|inner| LineWriter { inner }))
+    }
+}
+
+impl<W: Write> Write for LineWriter<W> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        LineWriterShim::new(&mut self.inner).write(buf)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.flush()
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        LineWriterShim::new(&mut self.inner).write_vectored(bufs)
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        self.inner.is_write_vectored()
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        LineWriterShim::new(&mut self.inner).write_all(buf)
+    }
+
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        LineWriterShim::new(&mut self.inner).write_all_vectored(bufs)
+    }
+
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
+        LineWriterShim::new(&mut self.inner).write_fmt(fmt)
+    }
+}
+
+impl<W: Write> fmt::Debug for LineWriter<W>
+where
+    W: fmt::Debug,
+{
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.debug_struct("LineWriter")
+            .field("writer", &self.get_ref())
+            .field(
+                "buffer",
+                &format_args!("{}/{}", self.inner.buffer().len(), self.inner.capacity()),
+            )
+            .finish_non_exhaustive()
+    }
+}

src/buffered/linewritershim.rs

@@ -0,0 +1,276 @@
+use crate::{self as io, BufWriter, IoSlice, Write};
+use core::slice::memchr;
+
+/// Private helper struct for implementing the line-buffered writing logic.
+/// This shim temporarily wraps a BufWriter, and uses its internals to
+/// implement a line-buffered writer (specifically by using the internal
+/// methods like write_to_buf and flush_buf). In this way, a more
+/// efficient abstraction can be created than one that only had access to
+/// `write` and `flush`, without needlessly duplicating a lot of the
+/// implementation details of BufWriter. This also allows existing
+/// `BufWriters` to be temporarily given line-buffering logic; this is what
+/// enables Stdout to be alternately in line-buffered or block-buffered mode.
+#[derive(Debug)]
+pub struct LineWriterShim<'a, W: Write> {
+    buffer: &'a mut BufWriter<W>,
+}
+
+impl<'a, W: Write> LineWriterShim<'a, W> {
+    pub fn new(buffer: &'a mut BufWriter<W>) -> Self {
+        Self { buffer }
+    }
+
+    /// Get a reference to the inner writer (that is, the writer
+    /// wrapped by the BufWriter).
+    fn inner(&self) -> &W {
+        self.buffer.get_ref()
+    }
+
+    /// Get a mutable reference to the inner writer (that is, the writer
+    /// wrapped by the BufWriter). Be careful with this writer, as writes to
+    /// it will bypass the buffer.
+    fn inner_mut(&mut self) -> &mut W {
+        self.buffer.get_mut()
+    }
+
+    /// Get the content currently buffered in self.buffer
+    fn buffered(&self) -> &[u8] {
+        self.buffer.buffer()
+    }
+
+    /// Flush the buffer iff the last byte is a newline (indicating that an
+    /// earlier write only succeeded partially, and we want to retry flushing
+    /// the buffered line before continuing with a subsequent write)
+    fn flush_if_completed_line(&mut self) -> io::Result<()> {
+        match self.buffered().last().copied() {
+            Some(b'\n') => self.buffer.flush_buf(),
+            _ => Ok(()),
+        }
+    }
+}
+
+impl<'a, W: Write> Write for LineWriterShim<'a, W> {
+    /// Write some data into this BufReader with line buffering. This means
+    /// that, if any newlines are present in the data, the data up to the last
+    /// newline is sent directly to the underlying writer, and data after it
+    /// is buffered. Returns the number of bytes written.
+    ///
+    /// This function operates on a "best effort basis"; in keeping with the
+    /// convention of `Write::write`, it makes at most one attempt to write
+    /// new data to the underlying writer. If that write only reports a partial
+    /// success, the remaining data will be buffered.
+    ///
+    /// Because this function attempts to send completed lines to the underlying
+    /// writer, it will also flush the existing buffer if it ends with a
+    /// newline, even if the incoming data does not contain any newlines.
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        let newline_idx = match memchr::memrchr(b'\n', buf) {
+            // If there are no new newlines (that is, if this write is less than
+            // one line), just do a regular buffered write (which may flush if
+            // we exceed the inner buffer's size)
+            None => {
+                self.flush_if_completed_line()?;
+                return self.buffer.write(buf);
+            }
+            // Otherwise, arrange for the lines to be written directly to the
+            // inner writer.
+            Some(newline_idx) => newline_idx + 1,
+        };
+
+        // Flush existing content to prepare for our write. We have to do this
+        // before attempting to write `buf` in order to maintain consistency;
+        // if we add `buf` to the buffer then try to flush it all at once,
+        // we're obligated to return Ok(), which would mean suppressing any
+        // errors that occur during flush.
+        self.buffer.flush_buf()?;
+
+        // This is what we're going to try to write directly to the inner
+        // writer. The rest will be buffered, if nothing goes wrong.
+        let lines = &buf[..newline_idx];
+
+        // Write `lines` directly to the inner writer. In keeping with the
+        // `write` convention, make at most one attempt to add new (unbuffered)
+        // data. Because this write doesn't touch the BufWriter state directly,
+        // and the buffer is known to be empty, we don't need to worry about
+        // self.buffer.panicked here.
+        let flushed = self.inner_mut().write(lines)?;
+
+        // If buffer returns Ok(0), propagate that to the caller without
+        // doing additional buffering; otherwise we're just guaranteeing
+        // an "ErrorKind::WriteZero" later.
+        if flushed == 0 {
+            return Ok(0);
+        }
+
+        // Now that the write has succeeded, buffer the rest (or as much of
+        // the rest as possible). If there were any unwritten newlines, we
+        // only buffer out to the last unwritten newline that fits in the
+        // buffer; this helps prevent flushing partial lines on subsequent
+        // calls to LineWriterShim::write.
+
+        // Handle the cases in order of most-common to least-common, under
+        // the presumption that most writes succeed in totality, and that most
+        // writes are smaller than the buffer.
+        // - Is this a partial line (ie, no newlines left in the unwritten tail)
+        // - If not, does the data out to the last unwritten newline fit in
+        //   the buffer?
+        // - If not, scan for the last newline that *does* fit in the buffer
+        let tail = if flushed >= newline_idx {
+            &buf[flushed..]
+        } else if newline_idx - flushed <= self.buffer.capacity() {
+            &buf[flushed..newline_idx]
+        } else {
+            let scan_area = &buf[flushed..];
+            let scan_area = &scan_area[..self.buffer.capacity()];
+            match memchr::memrchr(b'\n', scan_area) {
+                Some(newline_idx) => &scan_area[..newline_idx + 1],
+                None => scan_area,
+            }
+        };
+
+        let buffered = self.buffer.write_to_buf(tail);
+        Ok(flushed + buffered)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.buffer.flush()
+    }
+
+    /// Write some vectored data into this BufReader with line buffering. This
+    /// means that, if any newlines are present in the data, the data up to
+    /// and including the buffer containing the last newline is sent directly
+    /// to the inner writer, and the data after it is buffered. Returns the
+    /// number of bytes written.
+    ///
+    /// This function operates on a "best effort basis"; in keeping with the
+    /// convention of `Write::write`, it makes at most one attempt to write
+    /// new data to the underlying writer.
+    ///
+    /// Because this function attempts to send completed lines to the underlying
+    /// writer, it will also flush the existing buffer if it contains any
+    /// newlines.
+    ///
+    /// Because sorting through an array of `IoSlice` can be a bit convoluted,
+    /// This method differs from write in the following ways:
+    ///
+    /// - It attempts to write the full content of all the buffers up to and
+    ///   including the one containing the last newline. This means that it
+    ///   may attempt to write a partial line, that buffer has data past the
+    ///   newline.
+    /// - If the write only reports partial success, it does not attempt to
+    ///   find the precise location of the written bytes and buffer the rest.
+    ///
+    /// If the underlying vector doesn't support vectored writing, we instead
+    /// simply write the first non-empty buffer with `write`. This way, we
+    /// get the benefits of more granular partial-line handling without losing
+    /// anything in efficiency
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        // If there's no specialized behavior for write_vectored, just use
+        // write. This has the benefit of more granular partial-line handling.
+        if !self.is_write_vectored() {
+            return match bufs.iter().find(|buf| !buf.is_empty()) {
+                Some(buf) => self.write(buf),
+                None => Ok(0),
+            };
+        }
+
+        // Find the buffer containing the last newline
+        let last_newline_buf_idx = bufs
+            .iter()
+            .enumerate()
+            .rev()
+            .find_map(|(i, buf)| memchr::memchr(b'\n', buf).map(|_| i));
+
+        // If there are no new newlines (that is, if this write is less than
+        // one line), just do a regular buffered write
+        let last_newline_buf_idx = match last_newline_buf_idx {
+            // No newlines; just do a normal buffered write
+            None => {
+                self.flush_if_completed_line()?;
+                return self.buffer.write_vectored(bufs);
+            }
+            Some(i) => i,
+        };
+
+        // Flush existing content to prepare for our write
+        self.buffer.flush_buf()?;
+
+        // This is what we're going to try to write directly to the inner
+        // writer. The rest will be buffered, if nothing goes wrong.
+        let (lines, tail) = bufs.split_at(last_newline_buf_idx + 1);
+
+        // Write `lines` directly to the inner writer. In keeping with the
+        // `write` convention, make at most one attempt to add new (unbuffered)
+        // data. Because this write doesn't touch the BufWriter state directly,
+        // and the buffer is known to be empty, we don't need to worry about
+        // self.panicked here.
+        let flushed = self.inner_mut().write_vectored(lines)?;
+
+        // If inner returns Ok(0), propagate that to the caller without
+        // doing additional buffering; otherwise we're just guaranteeing
+        // an "ErrorKind::WriteZero" later.
+        if flushed == 0 {
+            return Ok(0);
+        }
+
+        // Don't try to reconstruct the exact amount written; just bail
+        // in the event of a partial write
+        let lines_len = lines.iter().map(|buf| buf.len()).sum();
+        if flushed < lines_len {
+            return Ok(flushed);
+        }
+
+        // Now that the write has succeeded, buffer the rest (or as much of the
+        // rest as possible)
+        let buffered: usize = tail
+            .iter()
+            .filter(|buf| !buf.is_empty())
+            .map(|buf| self.buffer.write_to_buf(buf))
+            .take_while(|&n| n > 0)
+            .sum();
+
+        Ok(flushed + buffered)
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        self.inner().is_write_vectored()
+    }
+
+    /// Write some data into this BufReader with line buffering. This means
+    /// that, if any newlines are present in the data, the data up to the last
+    /// newline is sent directly to the underlying writer, and data after it
+    /// is buffered.
+    ///
+    /// Because this function attempts to send completed lines to the underlying
+    /// writer, it will also flush the existing buffer if it contains any
+    /// newlines, even if the incoming data does not contain any newlines.
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        match memchr::memrchr(b'\n', buf) {
+            // If there are no new newlines (that is, if this write is less than
+            // one line), just do a regular buffered write (which may flush if
+            // we exceed the inner buffer's size)
+            None => {
+                self.flush_if_completed_line()?;
+                self.buffer.write_all(buf)
+            }
+            Some(newline_idx) => {
+                let (lines, tail) = buf.split_at(newline_idx + 1);
+
+                if self.buffered().is_empty() {
+                    self.inner_mut().write_all(lines)?;
+                } else {
+                    // If there is any buffered data, we add the incoming lines
+                    // to that buffer before flushing, which saves us at least
+                    // one write call. We can't really do this with `write`,
+                    // since we can't do this *and* not suppress errors *and*
+                    // report a consistent state to the caller in a return
+                    // value, but here in write_all it's fine.
+                    self.buffer.write_all(lines)?;
+                    self.buffer.flush_buf()?;
+                }
+
+                self.buffer.write_all(tail)
+            }
+        }
+    }
+}

src/buffered/mod.rs

@@ -0,0 +1,179 @@
+//! Buffering wrappers for I/O traits
+
+mod bufreader;
+mod bufwriter;
+mod linewriter;
+mod linewritershim;
+
+#[cfg(test)]
+mod tests;
+
+use core::fmt;
+use crate::Error;
+
+pub use self::bufreader::BufReader;
+pub use self::bufwriter::BufWriter;
+pub use self::bufwriter::WriterPanicked;
+pub use self::linewriter::LineWriter;
+use self::linewritershim::LineWriterShim;
+
+/// An error returned by [`BufWriter::into_inner`] which combines an error that
+/// happened while writing out the buffer, and the buffered writer object
+/// which may be used to recover from the condition.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::io::BufWriter;
+/// use std::net::TcpStream;
+///
+/// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+///
+/// // do stuff with the stream
+///
+/// // we want to get our `TcpStream` back, so let's try:
+///
+/// let stream = match stream.into_inner() {
+///     Ok(s) => s,
+///     Err(e) => {
+///         // Here, e is an IntoInnerError
+///         panic!("An error occurred");
+///     }
+/// };
+/// ```
+#[derive(Debug)]
+pub struct IntoInnerError<W>(W, Error);
+
+impl<W> IntoInnerError<W> {
+    /// Construct a new IntoInnerError
+    fn new(writer: W, error: Error) -> Self {
+        Self(writer, error)
+    }
+
+    /// Helper to construct a new IntoInnerError; intended to help with
+    /// adapters that wrap other adapters
+    fn new_wrapped<W2>(self, f: impl FnOnce(W) -> W2) -> IntoInnerError<W2> {
+        let Self(writer, error) = self;
+        IntoInnerError::new(f(writer), error)
+    }
+
+    /// Returns the error which caused the call to [`BufWriter::into_inner()`]
+    /// to fail.
+    ///
+    /// This error was returned when attempting to write the internal buffer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // do stuff with the stream
+    ///
+    /// // we want to get our `TcpStream` back, so let's try:
+    ///
+    /// let stream = match stream.into_inner() {
+    ///     Ok(s) => s,
+    ///     Err(e) => {
+    ///         // Here, e is an IntoInnerError, let's log the inner error.
+    ///         //
+    ///         // We'll just 'log' to stdout for this example.
+    ///         println!("{}", e.error());
+    ///
+    ///         panic!("An unexpected error occurred.");
+    ///     }
+    /// };
+    /// ```
+    pub fn error(&self) -> &Error {
+        &self.1
+    }
+
+    /// Returns the buffered writer instance which generated the error.
+    ///
+    /// The returned object can be used for error recovery, such as
+    /// re-inspecting the buffer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::BufWriter;
+    /// use std::net::TcpStream;
+    ///
+    /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
+    ///
+    /// // do stuff with the stream
+    ///
+    /// // we want to get our `TcpStream` back, so let's try:
+    ///
+    /// let stream = match stream.into_inner() {
+    ///     Ok(s) => s,
+    ///     Err(e) => {
+    ///         // Here, e is an IntoInnerError, let's re-examine the buffer:
+    ///         let buffer = e.into_inner();
+    ///
+    ///         // do stuff to try to recover
+    ///
+    ///         // afterwards, let's just return the stream
+    ///         buffer.into_inner().unwrap()
+    ///     }
+    /// };
+    /// ```
+    pub fn into_inner(self) -> W {
+        self.0
+    }
+
+    /// Consumes the [`IntoInnerError`] and returns the error which caused the call to
+    /// [`BufWriter::into_inner()`] to fail.  Unlike `error`, this can be used to
+    /// obtain ownership of the underlying error.
+    ///
+    /// # Example
+    /// ```
+    /// use std::io::{BufWriter, ErrorKind, Write};
+    ///
+    /// let mut not_enough_space = [0u8; 10];
+    /// let mut stream = BufWriter::new(not_enough_space.as_mut());
+    /// write!(stream, "this cannot be actually written").unwrap();
+    /// let into_inner_err = stream.into_inner().expect_err("now we discover it's too small");
+    /// let err = into_inner_err.into_error();
+    /// assert_eq!(err.kind(), ErrorKind::WriteZero);
+    /// ```
+    pub fn into_error(self) -> Error {
+        self.1
+    }
+
+    /// Consumes the [`IntoInnerError`] and returns the error which caused the call to
+    /// [`BufWriter::into_inner()`] to fail, and the underlying writer.
+    ///
+    /// This can be used to simply obtain ownership of the underlying error; it can also be used for
+    /// advanced error recovery.
+    ///
+    /// # Example
+    /// ```
+    /// use std::io::{BufWriter, ErrorKind, Write};
+    ///
+    /// let mut not_enough_space = [0u8; 10];
+    /// let mut stream = BufWriter::new(not_enough_space.as_mut());
+    /// write!(stream, "this cannot be actually written").unwrap();
+    /// let into_inner_err = stream.into_inner().expect_err("now we discover it's too small");
+    /// let (err, recovered_writer) = into_inner_err.into_parts();
+    /// assert_eq!(err.kind(), ErrorKind::WriteZero);
+    /// assert_eq!(recovered_writer.buffer(), b"t be actually written");
+    /// ```
+    pub fn into_parts(self) -> (Error, W) {
+        (self.1, self.0)
+    }
+}
+
+impl<W> From<IntoInnerError<W>> for Error {
+    fn from(iie: IntoInnerError<W>) -> Error {
+        iie.1
+    }
+}
+
+impl<W> fmt::Display for IntoInnerError<W> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.error().fmt(f)
+    }
+}

src/buffered/tests.rs

@@ -0,0 +1,970 @@
+use crate::io::prelude::*;
+use crate::io::{self, BufReader, BufWriter, ErrorKind, IoSlice, LineWriter, SeekFrom};
+use crate::panic;
+use crate::sync::atomic::{AtomicUsize, Ordering};
+use crate::thread;
+
+/// A dummy reader intended at testing short-reads propagation.
+pub struct ShortReader {
+    lengths: Vec<usize>,
+}
+
+// FIXME: rustfmt and tidy disagree about the correct formatting of this
+// function. This leads to issues for users with editors configured to
+// rustfmt-on-save.
+impl Read for ShortReader {
+    fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
+        if self.lengths.is_empty() { Ok(0) } else { Ok(self.lengths.remove(0)) }
+    }
+}
+
+#[test]
+fn test_buffered_reader() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(2, inner);
+
+    let mut buf = [0, 0, 0];
+    let nread = reader.read(&mut buf);
+    assert_eq!(nread.unwrap(), 3);
+    assert_eq!(buf, [5, 6, 7]);
+    assert_eq!(reader.buffer(), []);
+
+    let mut buf = [0, 0];
+    let nread = reader.read(&mut buf);
+    assert_eq!(nread.unwrap(), 2);
+    assert_eq!(buf, [0, 1]);
+    assert_eq!(reader.buffer(), []);
+
+    let mut buf = [0];
+    let nread = reader.read(&mut buf);
+    assert_eq!(nread.unwrap(), 1);
+    assert_eq!(buf, [2]);
+    assert_eq!(reader.buffer(), [3]);
+
+    let mut buf = [0, 0, 0];
+    let nread = reader.read(&mut buf);
+    assert_eq!(nread.unwrap(), 1);
+    assert_eq!(buf, [3, 0, 0]);
+    assert_eq!(reader.buffer(), []);
+
+    let nread = reader.read(&mut buf);
+    assert_eq!(nread.unwrap(), 1);
+    assert_eq!(buf, [4, 0, 0]);
+    assert_eq!(reader.buffer(), []);
+
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn test_buffered_reader_seek() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(2, io::Cursor::new(inner));
+
+    assert_eq!(reader.seek(SeekFrom::Start(3)).ok(), Some(3));
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert_eq!(reader.seek(SeekFrom::Current(0)).ok(), Some(3));
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert_eq!(reader.seek(SeekFrom::Current(1)).ok(), Some(4));
+    assert_eq!(reader.fill_buf().ok(), Some(&[1, 2][..]));
+    reader.consume(1);
+    assert_eq!(reader.seek(SeekFrom::Current(-2)).ok(), Some(3));
+}
+
+#[test]
+fn test_buffered_reader_seek_relative() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(2, io::Cursor::new(inner));
+
+    assert!(reader.seek_relative(3).is_ok());
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert!(reader.seek_relative(0).is_ok());
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert!(reader.seek_relative(1).is_ok());
+    assert_eq!(reader.fill_buf().ok(), Some(&[1][..]));
+    assert!(reader.seek_relative(-1).is_ok());
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert!(reader.seek_relative(2).is_ok());
+    assert_eq!(reader.fill_buf().ok(), Some(&[2, 3][..]));
+}
+
+#[test]
+fn test_buffered_reader_stream_position() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(2, io::Cursor::new(inner));
+
+    assert_eq!(reader.stream_position().ok(), Some(0));
+    assert_eq!(reader.seek(SeekFrom::Start(3)).ok(), Some(3));
+    assert_eq!(reader.stream_position().ok(), Some(3));
+    // relative seeking within the buffer and reading position should keep the buffer
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
+    assert!(reader.seek_relative(0).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(3));
+    assert_eq!(reader.buffer(), &[0, 1][..]);
+    assert!(reader.seek_relative(1).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(4));
+    assert_eq!(reader.buffer(), &[1][..]);
+    assert!(reader.seek_relative(-1).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(3));
+    assert_eq!(reader.buffer(), &[0, 1][..]);
+    // relative seeking outside the buffer will discard it
+    assert!(reader.seek_relative(2).is_ok());
+    assert_eq!(reader.stream_position().ok(), Some(5));
+    assert_eq!(reader.buffer(), &[][..]);
+}
+
+#[test]
+fn test_buffered_reader_stream_position_panic() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(4, io::Cursor::new(inner));
+
+    // cause internal buffer to be filled but read only partially
+    let mut buffer = [0, 0];
+    assert!(reader.read_exact(&mut buffer).is_ok());
+    // rewinding the internal reader will cause buffer to loose sync
+    let inner = reader.get_mut();
+    assert!(inner.seek(SeekFrom::Start(0)).is_ok());
+    // overflow when subtracting the remaining buffer size from current position
+    let result = panic::catch_unwind(panic::AssertUnwindSafe(|| reader.stream_position().ok()));
+    assert!(result.is_err());
+}
+
+#[test]
+fn test_buffered_reader_invalidated_after_read() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(3, io::Cursor::new(inner));
+
+    assert_eq!(reader.fill_buf().ok(), Some(&[5, 6, 7][..]));
+    reader.consume(3);
+
+    let mut buffer = [0, 0, 0, 0, 0];
+    assert_eq!(reader.read(&mut buffer).ok(), Some(5));
+    assert_eq!(buffer, [0, 1, 2, 3, 4]);
+
+    assert!(reader.seek_relative(-2).is_ok());
+    let mut buffer = [0, 0];
+    assert_eq!(reader.read(&mut buffer).ok(), Some(2));
+    assert_eq!(buffer, [3, 4]);
+}
+
+#[test]
+fn test_buffered_reader_invalidated_after_seek() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(3, io::Cursor::new(inner));
+
+    assert_eq!(reader.fill_buf().ok(), Some(&[5, 6, 7][..]));
+    reader.consume(3);
+
+    assert!(reader.seek(SeekFrom::Current(5)).is_ok());
+
+    assert!(reader.seek_relative(-2).is_ok());
+    let mut buffer = [0, 0];
+    assert_eq!(reader.read(&mut buffer).ok(), Some(2));
+    assert_eq!(buffer, [3, 4]);
+}
+
+#[test]
+fn test_buffered_reader_seek_underflow() {
+    // gimmick reader that yields its position modulo 256 for each byte
+    struct PositionReader {
+        pos: u64,
+    }
+    impl Read for PositionReader {
+        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+            let len = buf.len();
+            for x in buf {
+                *x = self.pos as u8;
+                self.pos = self.pos.wrapping_add(1);
+            }
+            Ok(len)
+        }
+    }
+    impl Seek for PositionReader {
+        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+            match pos {
+                SeekFrom::Start(n) => {
+                    self.pos = n;
+                }
+                SeekFrom::Current(n) => {
+                    self.pos = self.pos.wrapping_add(n as u64);
+                }
+                SeekFrom::End(n) => {
+                    self.pos = u64::MAX.wrapping_add(n as u64);
+                }
+            }
+            Ok(self.pos)
+        }
+    }
+
+    let mut reader = BufReader::with_capacity(5, PositionReader { pos: 0 });
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1, 2, 3, 4][..]));
+    assert_eq!(reader.seek(SeekFrom::End(-5)).ok(), Some(u64::MAX - 5));
+    assert_eq!(reader.fill_buf().ok().map(|s| s.len()), Some(5));
+    // the following seek will require two underlying seeks
+    let expected = 9223372036854775802;
+    assert_eq!(reader.seek(SeekFrom::Current(i64::MIN)).ok(), Some(expected));
+    assert_eq!(reader.fill_buf().ok().map(|s| s.len()), Some(5));
+    // seeking to 0 should empty the buffer.
+    assert_eq!(reader.seek(SeekFrom::Current(0)).ok(), Some(expected));
+    assert_eq!(reader.get_ref().pos, expected);
+}
+
+#[test]
+fn test_buffered_reader_seek_underflow_discard_buffer_between_seeks() {
+    // gimmick reader that returns Err after first seek
+    struct ErrAfterFirstSeekReader {
+        first_seek: bool,
+    }
+    impl Read for ErrAfterFirstSeekReader {
+        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+            for x in &mut *buf {
+                *x = 0;
+            }
+            Ok(buf.len())
+        }
+    }
+    impl Seek for ErrAfterFirstSeekReader {
+        fn seek(&mut self, _: SeekFrom) -> io::Result<u64> {
+            if self.first_seek {
+                self.first_seek = false;
+                Ok(0)
+            } else {
+                Err(io::Error::new(io::ErrorKind::Other, "oh no!"))
+            }
+        }
+    }
+
+    let mut reader = BufReader::with_capacity(5, ErrAfterFirstSeekReader { first_seek: true });
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 0, 0, 0, 0][..]));
+
+    // The following seek will require two underlying seeks.  The first will
+    // succeed but the second will fail.  This should still invalidate the
+    // buffer.
+    assert!(reader.seek(SeekFrom::Current(i64::MIN)).is_err());
+    assert_eq!(reader.buffer().len(), 0);
+}
+
+#[test]
+fn test_buffered_writer() {
+    let inner = Vec::new();
+    let mut writer = BufWriter::with_capacity(2, inner);
+
+    writer.write(&[0, 1]).unwrap();
+    assert_eq!(writer.buffer(), []);
+    assert_eq!(*writer.get_ref(), [0, 1]);
+
+    writer.write(&[2]).unwrap();
+    assert_eq!(writer.buffer(), [2]);
+    assert_eq!(*writer.get_ref(), [0, 1]);
+
+    writer.write(&[3]).unwrap();
+    assert_eq!(writer.buffer(), [2, 3]);
+    assert_eq!(*writer.get_ref(), [0, 1]);
+
+    writer.flush().unwrap();
+    assert_eq!(writer.buffer(), []);
+    assert_eq!(*writer.get_ref(), [0, 1, 2, 3]);
+
+    writer.write(&[4]).unwrap();
+    writer.write(&[5]).unwrap();
+    assert_eq!(writer.buffer(), [4, 5]);
+    assert_eq!(*writer.get_ref(), [0, 1, 2, 3]);
+
+    writer.write(&[6]).unwrap();
+    assert_eq!(writer.buffer(), [6]);
+    assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5]);
+
+    writer.write(&[7, 8]).unwrap();
+    assert_eq!(writer.buffer(), []);
+    assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5, 6, 7, 8]);
+
+    writer.write(&[9, 10, 11]).unwrap();
+    assert_eq!(writer.buffer(), []);
+    assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]);
+
+    writer.flush().unwrap();
+    assert_eq!(writer.buffer(), []);
+    assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]);
+}
+
+#[test]
+fn test_buffered_writer_inner_flushes() {
+    let mut w = BufWriter::with_capacity(3, Vec::new());
+    w.write(&[0, 1]).unwrap();
+    assert_eq!(*w.get_ref(), []);
+    let w = w.into_inner().unwrap();
+    assert_eq!(w, [0, 1]);
+}
+
+#[test]
+fn test_buffered_writer_seek() {
+    let mut w = BufWriter::with_capacity(3, io::Cursor::new(Vec::new()));
+    w.write_all(&[0, 1, 2, 3, 4, 5]).unwrap();
+    w.write_all(&[6, 7]).unwrap();
+    assert_eq!(w.seek(SeekFrom::Current(0)).ok(), Some(8));
+    assert_eq!(&w.get_ref().get_ref()[..], &[0, 1, 2, 3, 4, 5, 6, 7][..]);
+    assert_eq!(w.seek(SeekFrom::Start(2)).ok(), Some(2));
+    w.write_all(&[8, 9]).unwrap();
+    assert_eq!(&w.into_inner().unwrap().into_inner()[..], &[0, 1, 8, 9, 4, 5, 6, 7]);
+}
+
+#[test]
+fn test_read_until() {
+    let inner: &[u8] = &[0, 1, 2, 1, 0];
+    let mut reader = BufReader::with_capacity(2, inner);
+    let mut v = Vec::new();
+    reader.read_until(0, &mut v).unwrap();
+    assert_eq!(v, [0]);
+    v.truncate(0);
+    reader.read_until(2, &mut v).unwrap();
+    assert_eq!(v, [1, 2]);
+    v.truncate(0);
+    reader.read_until(1, &mut v).unwrap();
+    assert_eq!(v, [1]);
+    v.truncate(0);
+    reader.read_until(8, &mut v).unwrap();
+    assert_eq!(v, [0]);
+    v.truncate(0);
+    reader.read_until(9, &mut v).unwrap();
+    assert_eq!(v, []);
+}
+
+#[test]
+fn test_line_buffer() {
+    let mut writer = LineWriter::new(Vec::new());
+    writer.write(&[0]).unwrap();
+    assert_eq!(*writer.get_ref(), []);
+    writer.write(&[1]).unwrap();
+    assert_eq!(*writer.get_ref(), []);
+    writer.flush().unwrap();
+    assert_eq!(*writer.get_ref(), [0, 1]);
+    writer.write(&[0, b'\n', 1, b'\n', 2]).unwrap();
+    assert_eq!(*writer.get_ref(), [0, 1, 0, b'\n', 1, b'\n']);
+    writer.flush().unwrap();
+    assert_eq!(*writer.get_ref(), [0, 1, 0, b'\n', 1, b'\n', 2]);
+    writer.write(&[3, b'\n']).unwrap();
+    assert_eq!(*writer.get_ref(), [0, 1, 0, b'\n', 1, b'\n', 2, 3, b'\n']);
+}
+
+#[test]
+fn test_read_line() {
+    let in_buf: &[u8] = b"a\nb\nc";
+    let mut reader = BufReader::with_capacity(2, in_buf);
+    let mut s = String::new();
+    reader.read_line(&mut s).unwrap();
+    assert_eq!(s, "a\n");
+    s.truncate(0);
+    reader.read_line(&mut s).unwrap();
+    assert_eq!(s, "b\n");
+    s.truncate(0);
+    reader.read_line(&mut s).unwrap();
+    assert_eq!(s, "c");
+    s.truncate(0);
+    reader.read_line(&mut s).unwrap();
+    assert_eq!(s, "");
+}
+
+#[test]
+fn test_lines() {
+    let in_buf: &[u8] = b"a\nb\nc";
+    let reader = BufReader::with_capacity(2, in_buf);
+    let mut it = reader.lines();
+    assert_eq!(it.next().unwrap().unwrap(), "a".to_string());
+    assert_eq!(it.next().unwrap().unwrap(), "b".to_string());
+    assert_eq!(it.next().unwrap().unwrap(), "c".to_string());
+    assert!(it.next().is_none());
+}
+
+#[test]
+fn test_short_reads() {
+    let inner = ShortReader { lengths: vec![0, 1, 2, 0, 1, 0] };
+    let mut reader = BufReader::new(inner);
+    let mut buf = [0, 0];
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    assert_eq!(reader.read(&mut buf).unwrap(), 1);
+    assert_eq!(reader.read(&mut buf).unwrap(), 2);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    assert_eq!(reader.read(&mut buf).unwrap(), 1);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+#[should_panic]
+fn dont_panic_in_drop_on_panicked_flush() {
+    struct FailFlushWriter;
+
+    impl Write for FailFlushWriter {
+        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+            Ok(buf.len())
+        }
+        fn flush(&mut self) -> io::Result<()> {
+            Err(io::Error::last_os_error())
+        }
+    }
+
+    let writer = FailFlushWriter;
+    let _writer = BufWriter::new(writer);
+
+    // If writer panics *again* due to the flush error then the process will
+    // abort.
+    panic!();
+}
+
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn panic_in_write_doesnt_flush_in_drop() {
+    static WRITES: AtomicUsize = AtomicUsize::new(0);
+
+    struct PanicWriter;
+
+    impl Write for PanicWriter {
+        fn write(&mut self, _: &[u8]) -> io::Result<usize> {
+            WRITES.fetch_add(1, Ordering::SeqCst);
+            panic!();
+        }
+        fn flush(&mut self) -> io::Result<()> {
+            Ok(())
+        }
+    }
+
+    thread::spawn(|| {
+        let mut writer = BufWriter::new(PanicWriter);
+        let _ = writer.write(b"hello world");
+        let _ = writer.flush();
+    })
+    .join()
+    .unwrap_err();
+
+    assert_eq!(WRITES.load(Ordering::SeqCst), 1);
+}
+
+#[bench]
+fn bench_buffered_reader(b: &mut test::Bencher) {
+    b.iter(|| BufReader::new(io::empty()));
+}
+
+#[bench]
+fn bench_buffered_reader_small_reads(b: &mut test::Bencher) {
+    let data = (0..u8::MAX).cycle().take(1024 * 4).collect::<Vec<_>>();
+    b.iter(|| {
+        let mut reader = BufReader::new(&data[..]);
+        let mut buf = [0u8; 4];
+        for _ in 0..1024 {
+            reader.read_exact(&mut buf).unwrap();
+        }
+    });
+}
+
+#[bench]
+fn bench_buffered_writer(b: &mut test::Bencher) {
+    b.iter(|| BufWriter::new(io::sink()));
+}
+
+/// A simple `Write` target, designed to be wrapped by `LineWriter` /
+/// `BufWriter` / etc, that can have its `write` & `flush` behavior
+/// configured
+#[derive(Default, Clone)]
+struct ProgrammableSink {
+    // Writes append to this slice
+    pub buffer: Vec<u8>,
+
+    // Flush sets this flag
+    pub flushed: bool,
+
+    // If true, writes will always be an error
+    pub always_write_error: bool,
+
+    // If true, flushes will always be an error
+    pub always_flush_error: bool,
+
+    // If set, only up to this number of bytes will be written in a single
+    // call to `write`
+    pub accept_prefix: Option<usize>,
+
+    // If set, counts down with each write, and writes return an error
+    // when it hits 0
+    pub max_writes: Option<usize>,
+
+    // If set, attempting to write when max_writes == Some(0) will be an
+    // error; otherwise, it will return Ok(0).
+    pub error_after_max_writes: bool,
+}
+
+impl Write for ProgrammableSink {
+    fn write(&mut self, data: &[u8]) -> io::Result<usize> {
+        if self.always_write_error {
+            return Err(io::Error::new(io::ErrorKind::Other, "test - always_write_error"));
+        }
+
+        match self.max_writes {
+            Some(0) if self.error_after_max_writes => {
+                return Err(io::Error::new(io::ErrorKind::Other, "test - max_writes"));
+            }
+            Some(0) => return Ok(0),
+            Some(ref mut count) => *count -= 1,
+            None => {}
+        }
+
+        let len = match self.accept_prefix {
+            None => data.len(),
+            Some(prefix) => data.len().min(prefix),
+        };
+
+        let data = &data[..len];
+        self.buffer.extend_from_slice(data);
+
+        Ok(len)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        if self.always_flush_error {
+            Err(io::Error::new(io::ErrorKind::Other, "test - always_flush_error"))
+        } else {
+            self.flushed = true;
+            Ok(())
+        }
+    }
+}
+
+/// Previously the `LineWriter` could successfully write some bytes but
+/// then fail to report that it has done so. Additionally, an erroneous
+/// flush after a successful write was permanently ignored.
+///
+/// Test that a line writer correctly reports the number of written bytes,
+/// and that it attempts to flush buffered lines from previous writes
+/// before processing new data
+///
+/// Regression test for #37807
+#[test]
+fn erroneous_flush_retried() {
+    let writer = ProgrammableSink {
+        // Only write up to 4 bytes at a time
+        accept_prefix: Some(4),
+
+        // Accept the first two writes, then error the others
+        max_writes: Some(2),
+        error_after_max_writes: true,
+
+        ..Default::default()
+    };
+
+    // This should write the first 4 bytes. The rest will be buffered, out
+    // to the last newline.
+    let mut writer = LineWriter::new(writer);
+    assert_eq!(writer.write(b"a\nb\nc\nd\ne").unwrap(), 8);
+
+    // This write should attempt to flush "c\nd\n", then buffer "e". No
+    // errors should happen here because no further writes should be
+    // attempted against `writer`.
+    assert_eq!(writer.write(b"e").unwrap(), 1);
+    assert_eq!(&writer.get_ref().buffer, b"a\nb\nc\nd\n");
+}
+
+#[test]
+fn line_vectored() {
+    let mut a = LineWriter::new(Vec::new());
+    assert_eq!(
+        a.write_vectored(&[
+            IoSlice::new(&[]),
+            IoSlice::new(b"\n"),
+            IoSlice::new(&[]),
+            IoSlice::new(b"a"),
+        ])
+        .unwrap(),
+        2,
+    );
+    assert_eq!(a.get_ref(), b"\n");
+
+    assert_eq!(
+        a.write_vectored(&[
+            IoSlice::new(&[]),
+            IoSlice::new(b"b"),
+            IoSlice::new(&[]),
+            IoSlice::new(b"a"),
+            IoSlice::new(&[]),
+            IoSlice::new(b"c"),
+        ])
+        .unwrap(),
+        3,
+    );
+    assert_eq!(a.get_ref(), b"\n");
+    a.flush().unwrap();
+    assert_eq!(a.get_ref(), b"\nabac");
+    assert_eq!(a.write_vectored(&[]).unwrap(), 0);
+    assert_eq!(
+        a.write_vectored(&[
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+            IoSlice::new(&[]),
+        ])
+        .unwrap(),
+        0,
+    );
+    assert_eq!(a.write_vectored(&[IoSlice::new(b"a\nb"),]).unwrap(), 3);
+    assert_eq!(a.get_ref(), b"\nabaca\nb");
+}
+
+#[test]
+fn line_vectored_partial_and_errors() {
+    use crate::collections::VecDeque;
+
+    enum Call {
+        Write { inputs: Vec<&'static [u8]>, output: io::Result<usize> },
+        Flush { output: io::Result<()> },
+    }
+
+    #[derive(Default)]
+    struct Writer {
+        calls: VecDeque<Call>,
+    }
+
+    impl Write for Writer {
+        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+            self.write_vectored(&[IoSlice::new(buf)])
+        }
+
+        fn write_vectored(&mut self, buf: &[IoSlice<'_>]) -> io::Result<usize> {
+            match self.calls.pop_front().expect("unexpected call to write") {
+                Call::Write { inputs, output } => {
+                    assert_eq!(inputs, buf.iter().map(|b| &**b).collect::<Vec<_>>());
+                    output
+                }
+                Call::Flush { .. } => panic!("unexpected call to write; expected a flush"),
+            }
+        }
+
+        fn is_write_vectored(&self) -> bool {
+            true
+        }
+
+        fn flush(&mut self) -> io::Result<()> {
+            match self.calls.pop_front().expect("Unexpected call to flush") {
+                Call::Flush { output } => output,
+                Call::Write { .. } => panic!("unexpected call to flush; expected a write"),
+            }
+        }
+    }
+
+    impl Drop for Writer {
+        fn drop(&mut self) {
+            if !thread::panicking() {
+                assert_eq!(self.calls.len(), 0);
+            }
+        }
+    }
+
+    // partial writes keep going
+    let mut a = LineWriter::new(Writer::default());
+    a.write_vectored(&[IoSlice::new(&[]), IoSlice::new(b"abc")]).unwrap();
+
+    a.get_mut().calls.push_back(Call::Write { inputs: vec![b"abc"], output: Ok(1) });
+    a.get_mut().calls.push_back(Call::Write { inputs: vec![b"bc"], output: Ok(2) });
+    a.get_mut().calls.push_back(Call::Write { inputs: vec![b"x", b"\n"], output: Ok(2) });
+
+    a.write_vectored(&[IoSlice::new(b"x"), IoSlice::new(b"\n")]).unwrap();
+
+    a.get_mut().calls.push_back(Call::Flush { output: Ok(()) });
+    a.flush().unwrap();
+
+    // erroneous writes stop and don't write more
+    a.get_mut().calls.push_back(Call::Write { inputs: vec![b"x", b"\na"], output: Err(err()) });
+    a.get_mut().calls.push_back(Call::Flush { output: Ok(()) });
+    assert!(a.write_vectored(&[IoSlice::new(b"x"), IoSlice::new(b"\na")]).is_err());
+    a.flush().unwrap();
+
+    fn err() -> io::Error {
+        io::Error::new(io::ErrorKind::Other, "x")
+    }
+}
+
+/// Test that, in cases where vectored writing is not enabled, the
+/// LineWriter uses the normal `write` call, which more-correctly handles
+/// partial lines
+#[test]
+fn line_vectored_ignored() {
+    let writer = ProgrammableSink::default();
+    let mut writer = LineWriter::new(writer);
+
+    let content = [
+        IoSlice::new(&[]),
+        IoSlice::new(b"Line 1\nLine"),
+        IoSlice::new(b" 2\nLine 3\nL"),
+        IoSlice::new(&[]),
+        IoSlice::new(&[]),
+        IoSlice::new(b"ine 4"),
+        IoSlice::new(b"\nLine 5\n"),
+    ];
+
+    let count = writer.write_vectored(&content).unwrap();
+    assert_eq!(count, 11);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\n");
+
+    let count = writer.write_vectored(&content[2..]).unwrap();
+    assert_eq!(count, 11);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2\nLine 3\n");
+
+    let count = writer.write_vectored(&content[5..]).unwrap();
+    assert_eq!(count, 5);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2\nLine 3\n");
+
+    let count = writer.write_vectored(&content[6..]).unwrap();
+    assert_eq!(count, 8);
+    assert_eq!(
+        writer.get_ref().buffer.as_slice(),
+        b"Line 1\nLine 2\nLine 3\nLine 4\nLine 5\n".as_ref()
+    );
+}
+
+/// Test that, given this input:
+///
+/// Line 1\n
+/// Line 2\n
+/// Line 3\n
+/// Line 4
+///
+/// And given a result that only writes to midway through Line 2
+///
+/// That only up to the end of Line 3 is buffered
+///
+/// This behavior is desirable because it prevents flushing partial lines
+#[test]
+fn partial_write_buffers_line() {
+    let writer = ProgrammableSink { accept_prefix: Some(13), ..Default::default() };
+    let mut writer = LineWriter::new(writer);
+
+    assert_eq!(writer.write(b"Line 1\nLine 2\nLine 3\nLine4").unwrap(), 21);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2");
+
+    assert_eq!(writer.write(b"Line 4").unwrap(), 6);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2\nLine 3\n");
+}
+
+/// Test that, given this input:
+///
+/// Line 1\n
+/// Line 2\n
+/// Line 3
+///
+/// And given that the full write of lines 1 and 2 was successful
+/// That data up to Line 3 is buffered
+#[test]
+fn partial_line_buffered_after_line_write() {
+    let writer = ProgrammableSink::default();
+    let mut writer = LineWriter::new(writer);
+
+    assert_eq!(writer.write(b"Line 1\nLine 2\nLine 3").unwrap(), 20);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2\n");
+
+    assert!(writer.flush().is_ok());
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2\nLine 3");
+}
+
+/// Test that, given a partial line that exceeds the length of
+/// LineBuffer's buffer (that is, without a trailing newline), that that
+/// line is written to the inner writer
+#[test]
+fn long_line_flushed() {
+    let writer = ProgrammableSink::default();
+    let mut writer = LineWriter::with_capacity(5, writer);
+
+    assert_eq!(writer.write(b"0123456789").unwrap(), 10);
+    assert_eq!(&writer.get_ref().buffer, b"0123456789");
+}
+
+/// Test that, given a very long partial line *after* successfully
+/// flushing a complete line, that that line is buffered unconditionally,
+/// and no additional writes take place. This assures the property that
+/// `write` should make at-most-one attempt to write new data.
+#[test]
+fn line_long_tail_not_flushed() {
+    let writer = ProgrammableSink::default();
+    let mut writer = LineWriter::with_capacity(5, writer);
+
+    // Assert that Line 1\n is flushed, and 01234 is buffered
+    assert_eq!(writer.write(b"Line 1\n0123456789").unwrap(), 12);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\n");
+
+    // Because the buffer is full, this subsequent write will flush it
+    assert_eq!(writer.write(b"5").unwrap(), 1);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\n01234");
+}
+
+/// Test that, if an attempt to pre-flush buffered data returns Ok(0),
+/// this is propagated as an error.
+#[test]
+fn line_buffer_write0_error() {
+    let writer = ProgrammableSink {
+        // Accept one write, then return Ok(0) on subsequent ones
+        max_writes: Some(1),
+
+        ..Default::default()
+    };
+    let mut writer = LineWriter::new(writer);
+
+    // This should write "Line 1\n" and buffer "Partial"
+    assert_eq!(writer.write(b"Line 1\nPartial").unwrap(), 14);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\n");
+
+    // This will attempt to flush "partial", which will return Ok(0), which
+    // needs to be an error, because we've already informed the client
+    // that we accepted the write.
+    let err = writer.write(b" Line End\n").unwrap_err();
+    assert_eq!(err.kind(), ErrorKind::WriteZero);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\n");
+}
+
+/// Test that, if a write returns Ok(0) after a successful pre-flush, this
+/// is propagated as Ok(0)
+#[test]
+fn line_buffer_write0_normal() {
+    let writer = ProgrammableSink {
+        // Accept two writes, then return Ok(0) on subsequent ones
+        max_writes: Some(2),
+
+        ..Default::default()
+    };
+    let mut writer = LineWriter::new(writer);
+
+    // This should write "Line 1\n" and buffer "Partial"
+    assert_eq!(writer.write(b"Line 1\nPartial").unwrap(), 14);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\n");
+
+    // This will flush partial, which will succeed, but then return Ok(0)
+    // when flushing " Line End\n"
+    assert_eq!(writer.write(b" Line End\n").unwrap(), 0);
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nPartial");
+}
+
+/// LineWriter has a custom `write_all`; make sure it works correctly
+#[test]
+fn line_write_all() {
+    let writer = ProgrammableSink {
+        // Only write 5 bytes at a time
+        accept_prefix: Some(5),
+        ..Default::default()
+    };
+    let mut writer = LineWriter::new(writer);
+
+    writer.write_all(b"Line 1\nLine 2\nLine 3\nLine 4\nPartial").unwrap();
+    assert_eq!(&writer.get_ref().buffer, b"Line 1\nLine 2\nLine 3\nLine 4\n");
+    writer.write_all(b" Line 5\n").unwrap();
+    assert_eq!(
+        writer.get_ref().buffer.as_slice(),
+        b"Line 1\nLine 2\nLine 3\nLine 4\nPartial Line 5\n".as_ref(),
+    );
+}
+
+#[test]
+fn line_write_all_error() {
+    let writer = ProgrammableSink {
+        // Only accept up to 3 writes of up to 5 bytes each
+        accept_prefix: Some(5),
+        max_writes: Some(3),
+        ..Default::default()
+    };
+
+    let mut writer = LineWriter::new(writer);
+    let res = writer.write_all(b"Line 1\nLine 2\nLine 3\nLine 4\nPartial");
+    assert!(res.is_err());
+    // An error from write_all leaves everything in an indeterminate state,
+    // so there's nothing else to test here
+}
+
+/// Under certain circumstances, the old implementation of LineWriter
+/// would try to buffer "to the last newline" but be forced to buffer
+/// less than that, leading to inappropriate partial line writes.
+/// Regression test for that issue.
+#[test]
+fn partial_multiline_buffering() {
+    let writer = ProgrammableSink {
+        // Write only up to 5 bytes at a time
+        accept_prefix: Some(5),
+        ..Default::default()
+    };
+
+    let mut writer = LineWriter::with_capacity(10, writer);
+
+    let content = b"AAAAABBBBB\nCCCCDDDDDD\nEEE";
+
+    // When content is written, LineWriter will try to write blocks A, B,
+    // C, and D. Only block A will succeed. Under the old behavior, LineWriter
+    // would then try to buffer B, C and D, but because its capacity is 10,
+    // it will only be able to buffer B and C. We don't want to buffer
+    // partial lines concurrent with whole lines, so the correct behavior
+    // is to buffer only block B (out to the newline)
+    assert_eq!(writer.write(content).unwrap(), 11);
+    assert_eq!(writer.get_ref().buffer, *b"AAAAA");
+
+    writer.flush().unwrap();
+    assert_eq!(writer.get_ref().buffer, *b"AAAAABBBBB\n");
+}
+
+/// Same as test_partial_multiline_buffering, but in the event NO full lines
+/// fit in the buffer, just buffer as much as possible
+#[test]
+fn partial_multiline_buffering_without_full_line() {
+    let writer = ProgrammableSink {
+        // Write only up to 5 bytes at a time
+        accept_prefix: Some(5),
+        ..Default::default()
+    };
+
+    let mut writer = LineWriter::with_capacity(5, writer);
+
+    let content = b"AAAAABBBBBBBBBB\nCCCCC\nDDDDD";
+
+    // When content is written, LineWriter will try to write blocks A, B,
+    // and C. Only block A will succeed. Under the old behavior, LineWriter
+    // would then try to buffer B and C, but because its capacity is 5,
+    // it will only be able to buffer part of B. Because it's not possible
+    // for it to buffer any complete lines, it should buffer as much of B as
+    // possible
+    assert_eq!(writer.write(content).unwrap(), 10);
+    assert_eq!(writer.get_ref().buffer, *b"AAAAA");
+
+    writer.flush().unwrap();
+    assert_eq!(writer.get_ref().buffer, *b"AAAAABBBBB");
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+enum RecordedEvent {
+    Write(String),
+    Flush,
+}
+
+#[derive(Debug, Clone, Default)]
+struct WriteRecorder {
+    pub events: Vec<RecordedEvent>,
+}
+
+impl Write for WriteRecorder {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        use crate::str::from_utf8;
+
+        self.events.push(RecordedEvent::Write(from_utf8(buf).unwrap().to_string()));
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.events.push(RecordedEvent::Flush);
+        Ok(())
+    }
+}
+
+/// Test that a normal, formatted writeln only results in a single write
+/// call to the underlying writer. A naive implementation of
+/// LineWriter::write_all results in two writes: one of the buffered data,
+/// and another of the final substring in the formatted set
+#[test]
+fn single_formatted_write() {
+    let writer = WriteRecorder::default();
+    let mut writer = LineWriter::new(writer);
+
+    // Under a naive implementation of LineWriter, this will result in two
+    // writes: "hello, world" and "!\n", because write() has to flush the
+    // buffer before attempting to write the last "!\n". write_all shouldn't
+    // have this limitation.
+    writeln!(&mut writer, "{}, {}!", "hello", "world").unwrap();
+    assert_eq!(writer.get_ref().events, [RecordedEvent::Write("hello, world!\n".to_string())]);
+}

src/copy.rs

@@ -0,0 +1,151 @@
+use super::{ErrorKind, Read, Result, Write, DEFAULT_BUF_SIZE};
+#[cfg(feature = "collections")] use super::BufWriter;
+use core::mem::MaybeUninit;
+
+/// Copies the entire contents of a reader into a writer.
+///
+/// This function will continuously read data from `reader` and then
+/// write it into `writer` in a streaming fashion until `reader`
+/// returns EOF.
+///
+/// On success, the total number of bytes that were copied from
+/// `reader` to `writer` is returned.
+///
+/// If you’re wanting to copy the contents of one file to another and you’re
+/// working with filesystem paths, see the [`fs::copy`] function.
+///
+/// [`fs::copy`]: crate::fs::copy
+///
+/// # Errors
+///
+/// This function will return an error immediately if any call to [`read`] or
+/// [`write`] returns an error. All instances of [`ErrorKind::Interrupted`] are
+/// handled by this function and the underlying operation is retried.
+///
+/// [`read`]: Read::read
+/// [`write`]: Write::write
+///
+/// # Examples
+///
+/// ```
+/// use std::io;
+///
+/// fn main() -> io::Result<()> {
+///     let mut reader: &[u8] = b"hello";
+///     let mut writer: Vec<u8> = vec![];
+///
+///     io::copy(&mut reader, &mut writer)?;
+///
+///     assert_eq!(&b"hello"[..], &writer[..]);
+///     Ok(())
+/// }
+/// ```
+pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    generic_copy(reader, writer)
+}
+
+/// The userspace read-write-loop implementation of `io::copy` that is used when
+/// OS-specific specializations for copy offloading are not available or not applicable.
+pub(crate) fn generic_copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    BufferedCopySpec::copy_to(reader, writer)
+}
+
+/// Specialization of the read-write loop that either uses a stack buffer
+/// or reuses the internal buffer of a BufWriter
+trait BufferedCopySpec: Write {
+    fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64>;
+}
+
+impl<W: Write + ?Sized> BufferedCopySpec for W {
+    default fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
+        stack_buffer_copy(reader, writer)
+    }
+}
+
+#[cfg(feature = "collections")]
+impl<I: Write> BufferedCopySpec for BufWriter<I> {
+    fn copy_to<R: Read + ?Sized>(reader: &mut R, writer: &mut Self) -> Result<u64> {
+        if writer.capacity() < DEFAULT_BUF_SIZE {
+            return stack_buffer_copy(reader, writer);
+        }
+
+        // FIXME: #42788
+        //
+        //   - This creates a (mut) reference to a slice of
+        //     _uninitialized_ integers, which is **undefined behavior**
+        //
+        //   - Only the standard library gets to soundly "ignore" this,
+        //     based on its privileged knowledge of unstable rustc
+        //     internals;
+        unsafe {
+            let spare_cap = writer.buffer_mut().spare_capacity_mut();
+            reader.initializer().initialize(MaybeUninit::slice_assume_init_mut(spare_cap));
+        }
+
+        let mut len = 0;
+
+        loop {
+            let buf = writer.buffer_mut();
+            let spare_cap = buf.spare_capacity_mut();
+
+            if spare_cap.len() >= DEFAULT_BUF_SIZE {
+                match reader.read(unsafe { MaybeUninit::slice_assume_init_mut(spare_cap) }) {
+                    Ok(0) => return Ok(len), // EOF reached
+                    Ok(bytes_read) => {
+                        assert!(bytes_read <= spare_cap.len());
+                        // SAFETY: The initializer contract guarantees that either it or `read`
+                        // will have initialized these bytes. And we just checked that the number
+                        // of bytes is within the buffer capacity.
+                        unsafe { buf.set_len(buf.len() + bytes_read) };
+                        len += bytes_read as u64;
+                        // Read again if the buffer still has enough capacity, as BufWriter itself would do
+                        // This will occur if the reader returns short reads
+                        continue;
+                    }
+                    Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+                    Err(e) => return Err(e),
+                }
+            }
+
+            writer.flush_buf()?;
+        }
+    }
+}
+
+fn stack_buffer_copy<R: Read + ?Sized, W: Write + ?Sized>(
+    reader: &mut R,
+    writer: &mut W,
+) -> Result<u64> {
+    let mut buf = MaybeUninit::<[u8; DEFAULT_BUF_SIZE]>::uninit();
+    // FIXME: #42788
+    //
+    //   - This creates a (mut) reference to a slice of
+    //     _uninitialized_ integers, which is **undefined behavior**
+    //
+    //   - Only the standard library gets to soundly "ignore" this,
+    //     based on its privileged knowledge of unstable rustc
+    //     internals;
+    unsafe {
+        reader.initializer().initialize(buf.assume_init_mut());
+    }
+
+    let mut written = 0;
+    loop {
+        let len = match reader.read(unsafe { buf.assume_init_mut() }) {
+            Ok(0) => return Ok(written),
+            Ok(len) => len,
+            Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+            Err(e) => return Err(e),
+        };
+        writer.write_all(unsafe { &buf.assume_init_ref()[..len] })?;
+        written += len as u64;
+    }
+}

src/cursor.rs

@@ -0,0 +1,515 @@
+#[cfg(test)]
+mod tests;
+
+use crate::prelude::*;
+
+use core::cmp;
+use crate::{self as io, Error, ErrorKind, Initializer, IoSlice, IoSliceMut, SeekFrom};
+
+#[cfg(feature="collections")] use core::convert::TryInto;
+#[cfg(feature="collections")] use crate::Vec;
+
+
+/// A `Cursor` wraps an in-memory buffer and provides it with a
+/// [`Seek`] implementation.
+///
+/// `Cursor`s are used with in-memory buffers, anything implementing
+/// [`AsRef`]`<[u8]>`, to allow them to implement [`Read`] and/or [`Write`],
+/// allowing these buffers to be used anywhere you might use a reader or writer
+/// that does actual I/O.
+///
+/// The standard library implements some I/O traits on various types which
+/// are commonly used as a buffer, like `Cursor<`[`Vec`]`<u8>>` and
+/// `Cursor<`[`&[u8]`][bytes]`>`.
+///
+/// # Examples
+///
+/// We may want to write bytes to a [`File`] in our production
+/// code, but use an in-memory buffer in our tests. We can do this with
+/// `Cursor`:
+///
+/// [bytes]: crate::slice
+/// [`File`]: crate::fs::File
+///
+/// ```no_run
+/// use std::io::prelude::*;
+/// use std::io::{self, SeekFrom};
+/// use std::fs::File;
+///
+/// // a library function we've written
+/// fn write_ten_bytes_at_end<W: Write + Seek>(writer: &mut W) -> io::Result<()> {
+///     writer.seek(SeekFrom::End(-10))?;
+///
+///     for i in 0..10 {
+///         writer.write(&[i])?;
+///     }
+///
+///     // all went well
+///     Ok(())
+/// }
+///
+/// # fn foo() -> io::Result<()> {
+/// // Here's some code that uses this library function.
+/// //
+/// // We might want to use a BufReader here for efficiency, but let's
+/// // keep this example focused.
+/// let mut file = File::create("foo.txt")?;
+///
+/// write_ten_bytes_at_end(&mut file)?;
+/// # Ok(())
+/// # }
+///
+/// // now let's write a test
+/// #[test]
+/// fn test_writes_bytes() {
+///     // setting up a real File is much slower than an in-memory buffer,
+///     // let's use a cursor instead
+///     use std::io::Cursor;
+///     let mut buff = Cursor::new(vec![0; 15]);
+///
+///     write_ten_bytes_at_end(&mut buff).unwrap();
+///
+///     assert_eq!(&buff.get_ref()[5..15], &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+/// }
+/// ```
+#[derive(Debug, Default, Eq, PartialEq)]
+pub struct Cursor<T> {
+    inner: T,
+    pos: u64,
+}
+
+impl<T> Cursor<T> {
+    /// Creates a new cursor wrapping the provided underlying in-memory buffer.
+    ///
+    /// Cursor initial position is `0` even if underlying buffer (e.g., [`Vec`])
+    /// is not empty. So writing to cursor starts with overwriting [`Vec`]
+    /// content, not with appending to it.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    /// ```
+    pub const fn new(inner: T) -> Cursor<T> {
+        Cursor { pos: 0, inner }
+    }
+
+    /// Consumes this cursor, returning the underlying value.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    ///
+    /// let vec = buff.into_inner();
+    /// ```
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+
+    /// Gets a reference to the underlying value in this cursor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    ///
+    /// let reference = buff.get_ref();
+    /// ```
+    pub const fn get_ref(&self) -> &T {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying value in this cursor.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying value as it may corrupt this cursor's position.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let mut buff = Cursor::new(Vec::new());
+    /// # fn force_inference(_: &Cursor<Vec<u8>>) {}
+    /// # force_inference(&buff);
+    ///
+    /// let reference = buff.get_mut();
+    /// ```
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+
+    /// Returns the current position of this cursor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    /// use std::io::prelude::*;
+    /// use std::io::SeekFrom;
+    ///
+    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
+    ///
+    /// assert_eq!(buff.position(), 0);
+    ///
+    /// buff.seek(SeekFrom::Current(2)).unwrap();
+    /// assert_eq!(buff.position(), 2);
+    ///
+    /// buff.seek(SeekFrom::Current(-1)).unwrap();
+    /// assert_eq!(buff.position(), 1);
+    /// ```
+    pub const fn position(&self) -> u64 {
+        self.pos
+    }
+
+    /// Sets the position of this cursor.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Cursor;
+    ///
+    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
+    ///
+    /// assert_eq!(buff.position(), 0);
+    ///
+    /// buff.set_position(2);
+    /// assert_eq!(buff.position(), 2);
+    ///
+    /// buff.set_position(4);
+    /// assert_eq!(buff.position(), 4);
+    /// ```
+    pub fn set_position(&mut self, pos: u64) {
+        self.pos = pos;
+    }
+}
+
+impl<T> Cursor<T>
+where
+    T: AsRef<[u8]>,
+{
+    /// Returns the remaining slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cursor_remaining)]
+    /// use std::io::Cursor;
+    ///
+    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
+    ///
+    /// assert_eq!(buff.remaining_slice(), &[1, 2, 3, 4, 5]);
+    ///
+    /// buff.set_position(2);
+    /// assert_eq!(buff.remaining_slice(), &[3, 4, 5]);
+    ///
+    /// buff.set_position(4);
+    /// assert_eq!(buff.remaining_slice(), &[5]);
+    ///
+    /// buff.set_position(6);
+    /// assert_eq!(buff.remaining_slice(), &[]);
+    /// ```
+    pub fn remaining_slice(&self) -> &[u8] {
+        let len = self.pos.min(self.inner.as_ref().len() as u64);
+        &self.inner.as_ref()[(len as usize)..]
+    }
+
+    /// Returns `true` if the remaining slice is empty.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(cursor_remaining)]
+    /// use std::io::Cursor;
+    ///
+    /// let mut buff = Cursor::new(vec![1, 2, 3, 4, 5]);
+    ///
+    /// buff.set_position(2);
+    /// assert!(!buff.is_empty());
+    ///
+    /// buff.set_position(5);
+    /// assert!(buff.is_empty());
+    ///
+    /// buff.set_position(10);
+    /// assert!(buff.is_empty());
+    /// ```
+    pub fn is_empty(&self) -> bool {
+        self.pos >= self.inner.as_ref().len() as u64
+    }
+}
+
+impl<T> Clone for Cursor<T>
+where
+    T: Clone,
+{
+    #[inline]
+    fn clone(&self) -> Self {
+        Cursor { inner: self.inner.clone(), pos: self.pos }
+    }
+
+    #[inline]
+    fn clone_from(&mut self, other: &Self) {
+        self.inner.clone_from(&other.inner);
+        self.pos = other.pos;
+    }
+}
+
+impl<T> io::Seek for Cursor<T>
+where
+    T: AsRef<[u8]>,
+{
+    fn seek(&mut self, style: SeekFrom) -> io::Result<u64> {
+        let (base_pos, offset) = match style {
+            SeekFrom::Start(n) => {
+                self.pos = n;
+                return Ok(n);
+            }
+            SeekFrom::End(n) => (self.inner.as_ref().len() as u64, n),
+            SeekFrom::Current(n) => (self.pos, n),
+        };
+        let new_pos = if offset >= 0 {
+            base_pos.checked_add(offset as u64)
+        } else {
+            base_pos.checked_sub((offset.wrapping_neg()) as u64)
+        };
+        match new_pos {
+            Some(n) => {
+                self.pos = n;
+                Ok(self.pos)
+            }
+            None => Err(Error::new_const(
+                ErrorKind::InvalidInput,
+                &"invalid seek to a negative or overflowing position",
+            )),
+        }
+    }
+
+    fn stream_len(&mut self) -> io::Result<u64> {
+        Ok(self.inner.as_ref().len() as u64)
+    }
+
+    fn stream_position(&mut self) -> io::Result<u64> {
+        Ok(self.pos)
+    }
+}
+
+impl<T> Read for Cursor<T>
+where
+    T: AsRef<[u8]>,
+{
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let n = Read::read(&mut self.remaining_slice(), buf)?;
+        self.pos += n as u64;
+        Ok(n)
+    }
+
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        let mut nread = 0;
+        for buf in bufs {
+            let n = self.read(buf)?;
+            nread += n;
+            if n < buf.len() {
+                break;
+            }
+        }
+        Ok(nread)
+    }
+
+    fn is_read_vectored(&self) -> bool {
+        true
+    }
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        let n = buf.len();
+        Read::read_exact(&mut self.remaining_slice(), buf)?;
+        self.pos += n as u64;
+        Ok(())
+    }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
+    }
+}
+
+#[cfg(feature="collections")]
+impl<T> BufRead for Cursor<T>
+where
+    T: AsRef<[u8]>,
+{
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        Ok(self.remaining_slice())
+    }
+    fn consume(&mut self, amt: usize) {
+        self.pos += amt as u64;
+    }
+}
+
+// Non-resizing write implementation
+#[inline]
+fn slice_write(pos_mut: &mut u64, slice: &mut [u8], buf: &[u8]) -> io::Result<usize> {
+    let pos = cmp::min(*pos_mut, slice.len() as u64);
+    let amt = (&mut slice[(pos as usize)..]).write(buf)?;
+    *pos_mut += amt as u64;
+    Ok(amt)
+}
+
+#[inline]
+fn slice_write_vectored(
+    pos_mut: &mut u64,
+    slice: &mut [u8],
+    bufs: &[IoSlice<'_>],
+) -> io::Result<usize> {
+    let mut nwritten = 0;
+    for buf in bufs {
+        let n = slice_write(pos_mut, slice, buf)?;
+        nwritten += n;
+        if n < buf.len() {
+            break;
+        }
+    }
+    Ok(nwritten)
+}
+
+// Resizing write implementation
+#[cfg(feature="collections")]
+fn vec_write(pos_mut: &mut u64, vec: &mut Vec<u8>, buf: &[u8]) -> io::Result<usize> {
+    let pos: usize = (*pos_mut).try_into().map_err(|_| {
+        Error::new_const(
+            ErrorKind::InvalidInput,
+            &"cursor position exceeds maximum possible vector length",
+        )
+    })?;
+    // Make sure the internal buffer is as least as big as where we
+    // currently are
+    let len = vec.len();
+    if len < pos {
+        // use `resize` so that the zero filling is as efficient as possible
+        vec.resize(pos, 0);
+    }
+    // Figure out what bytes will be used to overwrite what's currently
+    // there (left), and what will be appended on the end (right)
+    {
+        let space = vec.len() - pos;
+        let (left, right) = buf.split_at(cmp::min(space, buf.len()));
+        vec[pos..pos + left.len()].copy_from_slice(left);
+        vec.extend_from_slice(right);
+    }
+
+    // Bump us forward
+    *pos_mut = (pos + buf.len()) as u64;
+    Ok(buf.len())
+}
+
+#[cfg(feature="collections")]
+fn vec_write_vectored(
+    pos_mut: &mut u64,
+    vec: &mut Vec<u8>,
+    bufs: &[IoSlice<'_>],
+) -> io::Result<usize> {
+    let mut nwritten = 0;
+    for buf in bufs {
+        nwritten += vec_write(pos_mut, vec, buf)?;
+    }
+    Ok(nwritten)
+}
+
+impl Write for Cursor<&mut [u8]> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        slice_write(&mut self.pos, self.inner, buf)
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        slice_write_vectored(&mut self.pos, self.inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[cfg(feature="collections")]
+impl Write for Cursor<&mut Vec<u8>> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        vec_write(&mut self.pos, self.inner, buf)
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        vec_write_vectored(&mut self.pos, self.inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[cfg(feature = "collections")]
+impl Write for Cursor<Vec<u8>> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        vec_write(&mut self.pos, &mut self.inner, buf)
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        vec_write_vectored(&mut self.pos, &mut self.inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[cfg(feature = "alloc")]
+impl Write for Cursor<::alloc::boxed::Box<[u8]>> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        slice_write(&mut self.pos, &mut self.inner, buf)
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        slice_write_vectored(&mut self.pos, &mut self.inner, bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}

src/cursor/tests.rs

@@ -0,0 +1,523 @@
+use crate::io::prelude::*;
+use crate::io::{Cursor, IoSlice, IoSliceMut, SeekFrom};
+
+#[test]
+fn test_vec_writer() {
+    let mut writer = Vec::new();
+    assert_eq!(writer.write(&[0]).unwrap(), 1);
+    assert_eq!(writer.write(&[1, 2, 3]).unwrap(), 3);
+    assert_eq!(writer.write(&[4, 5, 6, 7]).unwrap(), 4);
+    assert_eq!(
+        writer
+            .write_vectored(&[IoSlice::new(&[]), IoSlice::new(&[8, 9]), IoSlice::new(&[10])],)
+            .unwrap(),
+        3
+    );
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+    assert_eq!(writer, b);
+}
+
+#[test]
+fn test_mem_writer() {
+    let mut writer = Cursor::new(Vec::new());
+    assert_eq!(writer.write(&[0]).unwrap(), 1);
+    assert_eq!(writer.write(&[1, 2, 3]).unwrap(), 3);
+    assert_eq!(writer.write(&[4, 5, 6, 7]).unwrap(), 4);
+    assert_eq!(
+        writer
+            .write_vectored(&[IoSlice::new(&[]), IoSlice::new(&[8, 9]), IoSlice::new(&[10])],)
+            .unwrap(),
+        3
+    );
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+    assert_eq!(&writer.get_ref()[..], b);
+}
+
+#[test]
+fn test_mem_mut_writer() {
+    let mut vec = Vec::new();
+    let mut writer = Cursor::new(&mut vec);
+    assert_eq!(writer.write(&[0]).unwrap(), 1);
+    assert_eq!(writer.write(&[1, 2, 3]).unwrap(), 3);
+    assert_eq!(writer.write(&[4, 5, 6, 7]).unwrap(), 4);
+    assert_eq!(
+        writer
+            .write_vectored(&[IoSlice::new(&[]), IoSlice::new(&[8, 9]), IoSlice::new(&[10])],)
+            .unwrap(),
+        3
+    );
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
+    assert_eq!(&writer.get_ref()[..], b);
+}
+
+#[test]
+fn test_box_slice_writer() {
+    let mut writer = Cursor::new(vec![0u8; 9].into_boxed_slice());
+    assert_eq!(writer.position(), 0);
+    assert_eq!(writer.write(&[0]).unwrap(), 1);
+    assert_eq!(writer.position(), 1);
+    assert_eq!(writer.write(&[1, 2, 3]).unwrap(), 3);
+    assert_eq!(writer.write(&[4, 5, 6, 7]).unwrap(), 4);
+    assert_eq!(writer.position(), 8);
+    assert_eq!(writer.write(&[]).unwrap(), 0);
+    assert_eq!(writer.position(), 8);
+
+    assert_eq!(writer.write(&[8, 9]).unwrap(), 1);
+    assert_eq!(writer.write(&[10]).unwrap(), 0);
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8];
+    assert_eq!(&**writer.get_ref(), b);
+}
+
+#[test]
+fn test_box_slice_writer_vectored() {
+    let mut writer = Cursor::new(vec![0u8; 9].into_boxed_slice());
+    assert_eq!(writer.position(), 0);
+    assert_eq!(writer.write_vectored(&[IoSlice::new(&[0])]).unwrap(), 1);
+    assert_eq!(writer.position(), 1);
+    assert_eq!(
+        writer.write_vectored(&[IoSlice::new(&[1, 2, 3]), IoSlice::new(&[4, 5, 6, 7]),]).unwrap(),
+        7,
+    );
+    assert_eq!(writer.position(), 8);
+    assert_eq!(writer.write_vectored(&[]).unwrap(), 0);
+    assert_eq!(writer.position(), 8);
+
+    assert_eq!(writer.write_vectored(&[IoSlice::new(&[8, 9])]).unwrap(), 1);
+    assert_eq!(writer.write_vectored(&[IoSlice::new(&[10])]).unwrap(), 0);
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8];
+    assert_eq!(&**writer.get_ref(), b);
+}
+
+#[test]
+fn test_buf_writer() {
+    let mut buf = [0 as u8; 9];
+    {
+        let mut writer = Cursor::new(&mut buf[..]);
+        assert_eq!(writer.position(), 0);
+        assert_eq!(writer.write(&[0]).unwrap(), 1);
+        assert_eq!(writer.position(), 1);
+        assert_eq!(writer.write(&[1, 2, 3]).unwrap(), 3);
+        assert_eq!(writer.write(&[4, 5, 6, 7]).unwrap(), 4);
+        assert_eq!(writer.position(), 8);
+        assert_eq!(writer.write(&[]).unwrap(), 0);
+        assert_eq!(writer.position(), 8);
+
+        assert_eq!(writer.write(&[8, 9]).unwrap(), 1);
+        assert_eq!(writer.write(&[10]).unwrap(), 0);
+    }
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8];
+    assert_eq!(buf, b);
+}
+
+#[test]
+fn test_buf_writer_vectored() {
+    let mut buf = [0 as u8; 9];
+    {
+        let mut writer = Cursor::new(&mut buf[..]);
+        assert_eq!(writer.position(), 0);
+        assert_eq!(writer.write_vectored(&[IoSlice::new(&[0])]).unwrap(), 1);
+        assert_eq!(writer.position(), 1);
+        assert_eq!(
+            writer
+                .write_vectored(&[IoSlice::new(&[1, 2, 3]), IoSlice::new(&[4, 5, 6, 7])],)
+                .unwrap(),
+            7,
+        );
+        assert_eq!(writer.position(), 8);
+        assert_eq!(writer.write_vectored(&[]).unwrap(), 0);
+        assert_eq!(writer.position(), 8);
+
+        assert_eq!(writer.write_vectored(&[IoSlice::new(&[8, 9])]).unwrap(), 1);
+        assert_eq!(writer.write_vectored(&[IoSlice::new(&[10])]).unwrap(), 0);
+    }
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7, 8];
+    assert_eq!(buf, b);
+}
+
+#[test]
+fn test_buf_writer_seek() {
+    let mut buf = [0 as u8; 8];
+    {
+        let mut writer = Cursor::new(&mut buf[..]);
+        assert_eq!(writer.position(), 0);
+        assert_eq!(writer.write(&[1]).unwrap(), 1);
+        assert_eq!(writer.position(), 1);
+
+        assert_eq!(writer.seek(SeekFrom::Start(2)).unwrap(), 2);
+        assert_eq!(writer.position(), 2);
+        assert_eq!(writer.write(&[2]).unwrap(), 1);
+        assert_eq!(writer.position(), 3);
+
+        assert_eq!(writer.seek(SeekFrom::Current(-2)).unwrap(), 1);
+        assert_eq!(writer.position(), 1);
+        assert_eq!(writer.write(&[3]).unwrap(), 1);
+        assert_eq!(writer.position(), 2);
+
+        assert_eq!(writer.seek(SeekFrom::End(-1)).unwrap(), 7);
+        assert_eq!(writer.position(), 7);
+        assert_eq!(writer.write(&[4]).unwrap(), 1);
+        assert_eq!(writer.position(), 8);
+    }
+    let b: &[_] = &[1, 3, 2, 0, 0, 0, 0, 4];
+    assert_eq!(buf, b);
+}
+
+#[test]
+fn test_buf_writer_error() {
+    let mut buf = [0 as u8; 2];
+    let mut writer = Cursor::new(&mut buf[..]);
+    assert_eq!(writer.write(&[0]).unwrap(), 1);
+    assert_eq!(writer.write(&[0, 0]).unwrap(), 1);
+    assert_eq!(writer.write(&[0, 0]).unwrap(), 0);
+}
+
+#[test]
+fn test_mem_reader() {
+    let mut reader = Cursor::new(vec![0, 1, 2, 3, 4, 5, 6, 7]);
+    let mut buf = [];
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    assert_eq!(reader.position(), 0);
+    let mut buf = [0];
+    assert_eq!(reader.read(&mut buf).unwrap(), 1);
+    assert_eq!(reader.position(), 1);
+    let b: &[_] = &[0];
+    assert_eq!(buf, b);
+    let mut buf = [0; 4];
+    assert_eq!(reader.read(&mut buf).unwrap(), 4);
+    assert_eq!(reader.position(), 5);
+    let b: &[_] = &[1, 2, 3, 4];
+    assert_eq!(buf, b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 3);
+    let b: &[_] = &[5, 6, 7];
+    assert_eq!(&buf[..3], b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn test_mem_reader_vectored() {
+    let mut reader = Cursor::new(vec![0, 1, 2, 3, 4, 5, 6, 7]);
+    let mut buf = [];
+    assert_eq!(reader.read_vectored(&mut [IoSliceMut::new(&mut buf)]).unwrap(), 0);
+    assert_eq!(reader.position(), 0);
+    let mut buf = [0];
+    assert_eq!(
+        reader.read_vectored(&mut [IoSliceMut::new(&mut []), IoSliceMut::new(&mut buf),]).unwrap(),
+        1,
+    );
+    assert_eq!(reader.position(), 1);
+    let b: &[_] = &[0];
+    assert_eq!(buf, b);
+    let mut buf1 = [0; 4];
+    let mut buf2 = [0; 4];
+    assert_eq!(
+        reader
+            .read_vectored(&mut [IoSliceMut::new(&mut buf1), IoSliceMut::new(&mut buf2),])
+            .unwrap(),
+        7,
+    );
+    let b1: &[_] = &[1, 2, 3, 4];
+    let b2: &[_] = &[5, 6, 7];
+    assert_eq!(buf1, b1);
+    assert_eq!(&buf2[..3], b2);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn test_boxed_slice_reader() {
+    let mut reader = Cursor::new(vec![0, 1, 2, 3, 4, 5, 6, 7].into_boxed_slice());
+    let mut buf = [];
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    assert_eq!(reader.position(), 0);
+    let mut buf = [0];
+    assert_eq!(reader.read(&mut buf).unwrap(), 1);
+    assert_eq!(reader.position(), 1);
+    let b: &[_] = &[0];
+    assert_eq!(buf, b);
+    let mut buf = [0; 4];
+    assert_eq!(reader.read(&mut buf).unwrap(), 4);
+    assert_eq!(reader.position(), 5);
+    let b: &[_] = &[1, 2, 3, 4];
+    assert_eq!(buf, b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 3);
+    let b: &[_] = &[5, 6, 7];
+    assert_eq!(&buf[..3], b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn test_boxed_slice_reader_vectored() {
+    let mut reader = Cursor::new(vec![0, 1, 2, 3, 4, 5, 6, 7].into_boxed_slice());
+    let mut buf = [];
+    assert_eq!(reader.read_vectored(&mut [IoSliceMut::new(&mut buf)]).unwrap(), 0);
+    assert_eq!(reader.position(), 0);
+    let mut buf = [0];
+    assert_eq!(
+        reader.read_vectored(&mut [IoSliceMut::new(&mut []), IoSliceMut::new(&mut buf),]).unwrap(),
+        1,
+    );
+    assert_eq!(reader.position(), 1);
+    let b: &[_] = &[0];
+    assert_eq!(buf, b);
+    let mut buf1 = [0; 4];
+    let mut buf2 = [0; 4];
+    assert_eq!(
+        reader
+            .read_vectored(&mut [IoSliceMut::new(&mut buf1), IoSliceMut::new(&mut buf2)],)
+            .unwrap(),
+        7,
+    );
+    let b1: &[_] = &[1, 2, 3, 4];
+    let b2: &[_] = &[5, 6, 7];
+    assert_eq!(buf1, b1);
+    assert_eq!(&buf2[..3], b2);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn read_to_end() {
+    let mut reader = Cursor::new(vec![0, 1, 2, 3, 4, 5, 6, 7]);
+    let mut v = Vec::new();
+    reader.read_to_end(&mut v).unwrap();
+    assert_eq!(v, [0, 1, 2, 3, 4, 5, 6, 7]);
+}
+
+#[test]
+fn test_slice_reader() {
+    let in_buf = vec![0, 1, 2, 3, 4, 5, 6, 7];
+    let reader = &mut &in_buf[..];
+    let mut buf = [];
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    let mut buf = [0];
+    assert_eq!(reader.read(&mut buf).unwrap(), 1);
+    assert_eq!(reader.len(), 7);
+    let b: &[_] = &[0];
+    assert_eq!(&buf[..], b);
+    let mut buf = [0; 4];
+    assert_eq!(reader.read(&mut buf).unwrap(), 4);
+    assert_eq!(reader.len(), 3);
+    let b: &[_] = &[1, 2, 3, 4];
+    assert_eq!(&buf[..], b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 3);
+    let b: &[_] = &[5, 6, 7];
+    assert_eq!(&buf[..3], b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn test_slice_reader_vectored() {
+    let in_buf = vec![0, 1, 2, 3, 4, 5, 6, 7];
+    let reader = &mut &in_buf[..];
+    let mut buf = [];
+    assert_eq!(reader.read_vectored(&mut [IoSliceMut::new(&mut buf)]).unwrap(), 0);
+    let mut buf = [0];
+    assert_eq!(
+        reader.read_vectored(&mut [IoSliceMut::new(&mut []), IoSliceMut::new(&mut buf),]).unwrap(),
+        1,
+    );
+    assert_eq!(reader.len(), 7);
+    let b: &[_] = &[0];
+    assert_eq!(buf, b);
+    let mut buf1 = [0; 4];
+    let mut buf2 = [0; 4];
+    assert_eq!(
+        reader
+            .read_vectored(&mut [IoSliceMut::new(&mut buf1), IoSliceMut::new(&mut buf2)],)
+            .unwrap(),
+        7,
+    );
+    let b1: &[_] = &[1, 2, 3, 4];
+    let b2: &[_] = &[5, 6, 7];
+    assert_eq!(buf1, b1);
+    assert_eq!(&buf2[..3], b2);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn test_read_exact() {
+    let in_buf = vec![0, 1, 2, 3, 4, 5, 6, 7];
+    let reader = &mut &in_buf[..];
+    let mut buf = [];
+    assert!(reader.read_exact(&mut buf).is_ok());
+    let mut buf = [8];
+    assert!(reader.read_exact(&mut buf).is_ok());
+    assert_eq!(buf[0], 0);
+    assert_eq!(reader.len(), 7);
+    let mut buf = [0, 0, 0, 0, 0, 0, 0];
+    assert!(reader.read_exact(&mut buf).is_ok());
+    assert_eq!(buf, [1, 2, 3, 4, 5, 6, 7]);
+    assert_eq!(reader.len(), 0);
+    let mut buf = [0];
+    assert!(reader.read_exact(&mut buf).is_err());
+}
+
+#[test]
+fn test_buf_reader() {
+    let in_buf = vec![0, 1, 2, 3, 4, 5, 6, 7];
+    let mut reader = Cursor::new(&in_buf[..]);
+    let mut buf = [];
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+    assert_eq!(reader.position(), 0);
+    let mut buf = [0];
+    assert_eq!(reader.read(&mut buf).unwrap(), 1);
+    assert_eq!(reader.position(), 1);
+    let b: &[_] = &[0];
+    assert_eq!(buf, b);
+    let mut buf = [0; 4];
+    assert_eq!(reader.read(&mut buf).unwrap(), 4);
+    assert_eq!(reader.position(), 5);
+    let b: &[_] = &[1, 2, 3, 4];
+    assert_eq!(buf, b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 3);
+    let b: &[_] = &[5, 6, 7];
+    assert_eq!(&buf[..3], b);
+    assert_eq!(reader.read(&mut buf).unwrap(), 0);
+}
+
+#[test]
+fn seek_past_end() {
+    let buf = [0xff];
+    let mut r = Cursor::new(&buf[..]);
+    assert_eq!(r.seek(SeekFrom::Start(10)).unwrap(), 10);
+    assert_eq!(r.read(&mut [0]).unwrap(), 0);
+
+    let mut r = Cursor::new(vec![10]);
+    assert_eq!(r.seek(SeekFrom::Start(10)).unwrap(), 10);
+    assert_eq!(r.read(&mut [0]).unwrap(), 0);
+
+    let mut buf = [0];
+    let mut r = Cursor::new(&mut buf[..]);
+    assert_eq!(r.seek(SeekFrom::Start(10)).unwrap(), 10);
+    assert_eq!(r.write(&[3]).unwrap(), 0);
+
+    let mut r = Cursor::new(vec![10].into_boxed_slice());
+    assert_eq!(r.seek(SeekFrom::Start(10)).unwrap(), 10);
+    assert_eq!(r.write(&[3]).unwrap(), 0);
+}
+
+#[test]
+fn seek_past_i64() {
+    let buf = [0xff];
+    let mut r = Cursor::new(&buf[..]);
+    assert_eq!(r.seek(SeekFrom::Start(6)).unwrap(), 6);
+    assert_eq!(r.seek(SeekFrom::Current(0x7ffffffffffffff0)).unwrap(), 0x7ffffffffffffff6);
+    assert_eq!(r.seek(SeekFrom::Current(0x10)).unwrap(), 0x8000000000000006);
+    assert_eq!(r.seek(SeekFrom::Current(0)).unwrap(), 0x8000000000000006);
+    assert!(r.seek(SeekFrom::Current(0x7ffffffffffffffd)).is_err());
+    assert_eq!(r.seek(SeekFrom::Current(-0x8000000000000000)).unwrap(), 6);
+
+    let mut r = Cursor::new(vec![10]);
+    assert_eq!(r.seek(SeekFrom::Start(6)).unwrap(), 6);
+    assert_eq!(r.seek(SeekFrom::Current(0x7ffffffffffffff0)).unwrap(), 0x7ffffffffffffff6);
+    assert_eq!(r.seek(SeekFrom::Current(0x10)).unwrap(), 0x8000000000000006);
+    assert_eq!(r.seek(SeekFrom::Current(0)).unwrap(), 0x8000000000000006);
+    assert!(r.seek(SeekFrom::Current(0x7ffffffffffffffd)).is_err());
+    assert_eq!(r.seek(SeekFrom::Current(-0x8000000000000000)).unwrap(), 6);
+
+    let mut buf = [0];
+    let mut r = Cursor::new(&mut buf[..]);
+    assert_eq!(r.seek(SeekFrom::Start(6)).unwrap(), 6);
+    assert_eq!(r.seek(SeekFrom::Current(0x7ffffffffffffff0)).unwrap(), 0x7ffffffffffffff6);
+    assert_eq!(r.seek(SeekFrom::Current(0x10)).unwrap(), 0x8000000000000006);
+    assert_eq!(r.seek(SeekFrom::Current(0)).unwrap(), 0x8000000000000006);
+    assert!(r.seek(SeekFrom::Current(0x7ffffffffffffffd)).is_err());
+    assert_eq!(r.seek(SeekFrom::Current(-0x8000000000000000)).unwrap(), 6);
+
+    let mut r = Cursor::new(vec![10].into_boxed_slice());
+    assert_eq!(r.seek(SeekFrom::Start(6)).unwrap(), 6);
+    assert_eq!(r.seek(SeekFrom::Current(0x7ffffffffffffff0)).unwrap(), 0x7ffffffffffffff6);
+    assert_eq!(r.seek(SeekFrom::Current(0x10)).unwrap(), 0x8000000000000006);
+    assert_eq!(r.seek(SeekFrom::Current(0)).unwrap(), 0x8000000000000006);
+    assert!(r.seek(SeekFrom::Current(0x7ffffffffffffffd)).is_err());
+    assert_eq!(r.seek(SeekFrom::Current(-0x8000000000000000)).unwrap(), 6);
+}
+
+#[test]
+fn seek_before_0() {
+    let buf = [0xff];
+    let mut r = Cursor::new(&buf[..]);
+    assert!(r.seek(SeekFrom::End(-2)).is_err());
+
+    let mut r = Cursor::new(vec![10]);
+    assert!(r.seek(SeekFrom::End(-2)).is_err());
+
+    let mut buf = [0];
+    let mut r = Cursor::new(&mut buf[..]);
+    assert!(r.seek(SeekFrom::End(-2)).is_err());
+
+    let mut r = Cursor::new(vec![10].into_boxed_slice());
+    assert!(r.seek(SeekFrom::End(-2)).is_err());
+}
+
+#[test]
+fn test_seekable_mem_writer() {
+    let mut writer = Cursor::new(Vec::<u8>::new());
+    assert_eq!(writer.position(), 0);
+    assert_eq!(writer.write(&[0]).unwrap(), 1);
+    assert_eq!(writer.position(), 1);
+    assert_eq!(writer.write(&[1, 2, 3]).unwrap(), 3);
+    assert_eq!(writer.write(&[4, 5, 6, 7]).unwrap(), 4);
+    assert_eq!(writer.position(), 8);
+    let b: &[_] = &[0, 1, 2, 3, 4, 5, 6, 7];
+    assert_eq!(&writer.get_ref()[..], b);
+
+    assert_eq!(writer.seek(SeekFrom::Start(0)).unwrap(), 0);
+    assert_eq!(writer.position(), 0);
+    assert_eq!(writer.write(&[3, 4]).unwrap(), 2);
+    let b: &[_] = &[3, 4, 2, 3, 4, 5, 6, 7];
+    assert_eq!(&writer.get_ref()[..], b);
+
+    assert_eq!(writer.seek(SeekFrom::Current(1)).unwrap(), 3);
+    assert_eq!(writer.write(&[0, 1]).unwrap(), 2);
+    let b: &[_] = &[3, 4, 2, 0, 1, 5, 6, 7];
+    assert_eq!(&writer.get_ref()[..], b);
+
+    assert_eq!(writer.seek(SeekFrom::End(-1)).unwrap(), 7);
+    assert_eq!(writer.write(&[1, 2]).unwrap(), 2);
+    let b: &[_] = &[3, 4, 2, 0, 1, 5, 6, 1, 2];
+    assert_eq!(&writer.get_ref()[..], b);
+
+    assert_eq!(writer.seek(SeekFrom::End(1)).unwrap(), 10);
+    assert_eq!(writer.write(&[1]).unwrap(), 1);
+    let b: &[_] = &[3, 4, 2, 0, 1, 5, 6, 1, 2, 0, 1];
+    assert_eq!(&writer.get_ref()[..], b);
+}
+
+#[test]
+fn vec_seek_past_end() {
+    let mut r = Cursor::new(Vec::new());
+    assert_eq!(r.seek(SeekFrom::Start(10)).unwrap(), 10);
+    assert_eq!(r.write(&[3]).unwrap(), 1);
+}
+
+#[test]
+fn vec_seek_before_0() {
+    let mut r = Cursor::new(Vec::new());
+    assert!(r.seek(SeekFrom::End(-2)).is_err());
+}
+
+#[test]
+#[cfg(target_pointer_width = "32")]
+fn vec_seek_and_write_past_usize_max() {
+    let mut c = Cursor::new(Vec::new());
+    c.set_position(usize::MAX as u64 + 1);
+    assert!(c.write_all(&[1, 2, 3]).is_err());
+}
+
+#[test]
+fn test_partial_eq() {
+    assert_eq!(Cursor::new(Vec::<u8>::new()), Cursor::new(Vec::<u8>::new()));
+}
+
+#[test]
+fn test_eq() {
+    struct AssertEq<T: Eq>(pub T);
+
+    let _: AssertEq<Cursor<Vec<u8>>> = AssertEq(Cursor::new(Vec::new()));
+}
+
+#[allow(dead_code)]
+fn const_cursor() {
+    const CURSOR: Cursor<&[u8]> = Cursor::new(&[0]);
+    const _: &&[u8] = CURSOR.get_ref();
+    const _: u64 = CURSOR.position();
+}

src/error.rs

@@ -0,0 +1,662 @@
+#[cfg(test)]
+mod tests;
+
+use core::convert::From;
+use core::fmt;
+use core::result;
+
+use core::convert::Into;
+use core::marker::{Send, Sync};
+use core::option::Option::{self, Some, None};
+#[cfg(feature="alloc")] use alloc::boxed::Box;
+#[cfg(not(feature="alloc"))] use crate::FakeBox as Box;
+#[cfg(feature="collections")] use alloc::string::String;
+#[cfg(not(feature="collections"))] use crate::ErrorString as String;
+
+/// A specialized [`Result`] type for I/O operations.
+///
+/// This type is broadly used across [`std::io`] for any operation which may
+/// produce an error.
+///
+/// This typedef is generally used to avoid writing out [`io::Error`] directly and
+/// is otherwise a direct mapping to [`Result`].
+///
+/// While usual Rust style is to import types directly, aliases of [`Result`]
+/// often are not, to make it easier to distinguish between them. [`Result`] is
+/// generally assumed to be [`std::result::Result`][`Result`], and so users of this alias
+/// will generally use `io::Result` instead of shadowing the [prelude]'s import
+/// of [`std::result::Result`][`Result`].
+///
+/// [`std::io`]: crate::io
+/// [`io::Error`]: Error
+/// [`Result`]: crate::result::Result
+/// [prelude]: crate::prelude
+///
+/// # Examples
+///
+/// A convenience function that bubbles an `io::Result` to its caller:
+///
+/// ```
+/// use std::io;
+///
+/// fn get_string() -> io::Result<String> {
+///     let mut buffer = String::new();
+///
+///     io::stdin().read_line(&mut buffer)?;
+///
+///     Ok(buffer)
+/// }
+/// ```
+pub type Result<T> = result::Result<T, Error>;
+
+/// The error type for I/O operations of the [`Read`], [`Write`], [`Seek`], and
+/// associated traits.
+///
+/// Errors mostly originate from the underlying OS, but custom instances of
+/// `Error` can be created with crafted error messages and a particular value of
+/// [`ErrorKind`].
+///
+/// [`Read`]: crate::io::Read
+/// [`Write`]: crate::io::Write
+/// [`Seek`]: crate::io::Seek
+pub struct Error {
+    repr: Repr,
+}
+
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.repr, f)
+    }
+}
+
+enum Repr {
+    Os(i32),
+    Simple(ErrorKind),
+    // &str is a fat pointer, but &&str is a thin pointer.
+    SimpleMessage(ErrorKind, &'static &'static str),
+    #[cfg(feature="alloc")]
+    Custom(Box<Custom>),
+    #[cfg(not(feature="alloc"))]
+    Custom(Custom),
+}
+
+#[derive(Debug)]
+struct Custom {
+    kind: ErrorKind,
+    error: String,
+}
+
+/// A list specifying general categories of I/O error.
+///
+/// This list is intended to grow over time and it is not recommended to
+/// exhaustively match against it.
+///
+/// It is used with the [`io::Error`] type.
+///
+/// [`io::Error`]: Error
+#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[allow(deprecated)]
+#[non_exhaustive]
+pub enum ErrorKind {
+    /// An entity was not found, often a file.
+    NotFound,
+    /// The operation lacked the necessary privileges to complete.
+    PermissionDenied,
+    /// The connection was refused by the remote server.
+    ConnectionRefused,
+    /// The connection was reset by the remote server.
+    ConnectionReset,
+    /// The remote host is not reachable.
+    HostUnreachable,
+    /// The network containing the remote host is not reachable.
+    NetworkUnreachable,
+    /// The connection was aborted (terminated) by the remote server.
+    ConnectionAborted,
+    /// The network operation failed because it was not connected yet.
+    NotConnected,
+    /// A socket address could not be bound because the address is already in
+    /// use elsewhere.
+    AddrInUse,
+    /// A nonexistent interface was requested or the requested address was not
+    /// local.
+    AddrNotAvailable,
+    /// The system's networking is down.
+    NetworkDown,
+    /// The operation failed because a pipe was closed.
+    BrokenPipe,
+    /// An entity already exists, often a file.
+    AlreadyExists,
+    /// The operation needs to block to complete, but the blocking operation was
+    /// requested to not occur.
+    WouldBlock,
+    /// For example, a filesystem path was specified where one of the intermediate directory
+    /// components was, in fact, a plain file.
+    NotADirectory,
+    /// The filesystem object is, unexpectedly, a directory.
+    ///
+    /// A directory was specified when a non-directory was expected.
+    IsADirectory,
+    /// A non-empty directory was specified where an empty directory was expected.
+    DirectoryNotEmpty,
+    /// The filesystem or storage medium is read-only, but a write operation was attempted.
+    ReadOnlyFilesystem,
+    /// Loop in the filesystem or IO subsystem; often, too many levels of symbolic links.
+    ///
+    /// There was a loop (or excessively long chain) resolving a filesystem object
+    /// or file IO object.
+    ///
+    /// On Unix this is usually the result of a symbolic link loop; or, of exceeding the
+    /// system-specific limit on the depth of symlink traversal.
+    FilesystemLoop,
+    /// Stale network file handle.
+    ///
+    /// With some network filesystems, notably NFS, an open file (or directory) can be invalidated
+    /// by problems with the network or server.
+    StaleNetworkFileHandle,
+    /// A parameter was incorrect.
+    InvalidInput,
+    /// Data not valid for the operation were encountered.
+    ///
+    /// Unlike [`InvalidInput`], this typically means that the operation
+    /// parameters were valid, however the error was caused by malformed
+    /// input data.
+    ///
+    /// For example, a function that reads a file into a string will error with
+    /// `InvalidData` if the file's contents are not valid UTF-8.
+    ///
+    /// [`InvalidInput`]: ErrorKind::InvalidInput
+    InvalidData,
+    /// The I/O operation's timeout expired, causing it to be canceled.
+    TimedOut,
+    /// An error returned when an operation could not be completed because a
+    /// call to [`write`] returned [`Ok(0)`].
+    ///
+    /// This typically means that an operation could only succeed if it wrote a
+    /// particular number of bytes but only a smaller number of bytes could be
+    /// written.
+    ///
+    /// [`write`]: crate::io::Write::write
+    /// [`Ok(0)`]: Ok
+    WriteZero,
+    ///
+    /// This does not include out of quota errors.
+    StorageFull,
+    /// Seek on unseekable file.
+    ///
+    /// Seeking was attempted on an open file handle which is not suitable for seeking - for
+    /// example, on Unix, a named pipe opened with `File::open`.
+    NotSeekable,
+    /// Filesystem quota was exceeded.
+    FilesystemQuotaExceeded,
+    /// File larger than allowed or supported.
+    ///
+    /// This might arise from a hard limit of the underlying filesystem or file access API, or from
+    /// an administratively imposed resource limitation.  Simple disk full, and out of quota, have
+    /// their own errors.
+    FileTooLarge,
+    /// Resource is busy.
+    ResourceBusy,
+    /// Executable file is busy.
+    ///
+    /// An attempt was made to write to a file which is also in use as a running program.  (Not all
+    /// operating systems detect this situation.)
+    ExecutableFileBusy,
+    /// Deadlock (avoided).
+    ///
+    /// A file locking operation would result in deadlock.  This situation is typically detected, if
+    /// at all, on a best-effort basis.
+    Deadlock,
+    /// Cross-device or cross-filesystem (hard) link or rename.
+    CrossesDevices,
+    /// Too many (hard) links to the same filesystem object.
+    ///
+    /// The filesystem does not support making so many hardlinks to the same file.
+    TooManyLinks,
+    /// Filename too long.
+    ///
+    /// The limit might be from the underlying filesystem or API, or an administratively imposed
+    /// resource limit.
+    FilenameTooLong,
+    /// Program argument list too long.
+    ///
+    /// When trying to run an external program, a system or process limit on the size of the
+    /// arguments would have been exceeded.
+    ArgumentListTooLong,
+    /// This operation was interrupted.
+    ///
+    /// Interrupted operations can typically be retried.
+    Interrupted,
+
+    /// This operation is unsupported on this platform.
+    ///
+    /// This means that the operation can never succeed.
+    Unsupported,
+
+    // ErrorKinds which are primarily categorisations for OS error
+    // codes should be added above.
+    //
+    /// An error returned when an operation could not be completed because an
+    /// "end of file" was reached prematurely.
+    ///
+    /// This typically means that an operation could only succeed if it read a
+    /// particular number of bytes but only a smaller number of bytes could be
+    /// read.
+    UnexpectedEof,
+
+    /// An operation could not be completed, because it failed
+    /// to allocate enough memory.
+    OutOfMemory,
+
+    // "Unusual" error kinds which do not correspond simply to (sets
+    // of) OS error codes, should be added just above this comment.
+    // `Other` and `Uncategorised` should remain at the end:
+    //
+    /// A custom error that does not fall under any other I/O error kind.
+    ///
+    /// This can be used to construct your own [`Error`]s that do not match any
+    /// [`ErrorKind`].
+    ///
+    /// This [`ErrorKind`] is not used by the standard library.
+    Other,
+
+    /// Any I/O error from the standard library that's not part of this list.
+    ///
+    /// Errors that are `Uncategorized` now may move to a different or a new
+    /// [`ErrorKind`] variant in the future. It is not recommended to match
+    /// an error against `Uncategorized`; use a wildcard match (`_`) instead.
+    #[doc(hidden)]
+    Uncategorized,
+}
+
+impl ErrorKind {
+    pub(crate) fn as_str(&self) -> &'static str {
+        use ErrorKind::*;
+        // Strictly alphabetical, please.  (Sadly rustfmt cannot do this yet.)
+        match *self {
+            AddrInUse => "address in use",
+            AddrNotAvailable => "address not available",
+            AlreadyExists => "entity already exists",
+            ArgumentListTooLong => "argument list too long",
+            BrokenPipe => "broken pipe",
+            ConnectionAborted => "connection aborted",
+            ConnectionRefused => "connection refused",
+            ConnectionReset => "connection reset",
+            CrossesDevices => "cross-device link or rename",
+            Deadlock => "deadlock",
+            DirectoryNotEmpty => "directory not empty",
+            ExecutableFileBusy => "executable file busy",
+            FileTooLarge => "file too large",
+            FilenameTooLong => "filename too long",
+            FilesystemLoop => "filesystem loop or indirection limit (e.g. symlink loop)",
+            FilesystemQuotaExceeded => "filesystem quota exceeded",
+            HostUnreachable => "host unreachable",
+            Interrupted => "operation interrupted",
+            InvalidData => "invalid data",
+            InvalidInput => "invalid input parameter",
+            IsADirectory => "is a directory",
+            NetworkDown => "network down",
+            NetworkUnreachable => "network unreachable",
+            NotADirectory => "not a directory",
+            NotConnected => "not connected",
+            NotFound => "entity not found",
+            NotSeekable => "seek on unseekable file",
+            Other => "other error",
+            OutOfMemory => "out of memory",
+            PermissionDenied => "permission denied",
+            ReadOnlyFilesystem => "read-only filesystem or storage medium",
+            ResourceBusy => "resource busy",
+            StaleNetworkFileHandle => "stale network file handle",
+            StorageFull => "no storage space",
+            TimedOut => "timed out",
+            TooManyLinks => "too many links",
+            Uncategorized => "uncategorized error",
+            UnexpectedEof => "unexpected end of file",
+            Unsupported => "unsupported",
+            WouldBlock => "operation would block",
+            WriteZero => "write zero",
+        }
+    }
+}
+
+/// Intended for use for errors not exposed to the user, where allocating onto
+/// the heap (for normal construction via Error::new) is too costly.
+impl From<ErrorKind> for Error {
+    /// Converts an [`ErrorKind`] into an [`Error`].
+    ///
+    /// This conversion allocates a new error with a simple representation of error kind.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// let not_found = ErrorKind::NotFound;
+    /// let error = Error::from(not_found);
+    /// assert_eq!("entity not found", format!("{}", error));
+    /// ```
+    #[inline]
+    fn from(kind: ErrorKind) -> Error {
+        Error { repr: Repr::Simple(kind) }
+    }
+}
+
+impl Error {
+    /// Creates a new I/O error from a known kind of error as well as an
+    /// arbitrary error payload.
+    ///
+    /// This function is used to generically create I/O errors which do not
+    /// originate from the OS itself. The `error` argument is an arbitrary
+    /// payload which will be contained in this [`Error`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// // errors can be created from strings
+    /// let custom_error = Error::new(ErrorKind::Other, "oh no!");
+    ///
+    /// // errors can also be created from other errors
+    /// let custom_error2 = Error::new(ErrorKind::Interrupted, custom_error);
+    /// ```
+    pub fn new<E>(kind: ErrorKind, error: E) -> Error
+    where
+        E: Into<String>,
+    {
+        Self::_new(kind, error.into())
+    }
+
+    fn _new(kind: ErrorKind, error: String) -> Error {
+        Error { repr: Repr::Custom(Box::new(Custom { kind, error })) }
+    }
+
+    /// Creates a new I/O error from a known kind of error as well as a
+    /// constant message.
+    ///
+    /// This function does not allocate.
+    ///
+    /// This function should maybe change to
+    /// `new_const<const MSG: &'static str>(kind: ErrorKind)`
+    /// in the future, when const generics allow that.
+    #[inline]
+    pub(crate) const fn new_const(kind: ErrorKind, message: &'static &'static str) -> Error {
+        Self { repr: Repr::SimpleMessage(kind, message) }
+    }
+
+    /// Creates a new instance of an [`Error`] from a particular OS error code.
+    ///
+    /// # Examples
+    ///
+    /// On Linux:
+    ///
+    /// ```
+    /// # if cfg!(target_os = "linux") {
+    /// use std::io;
+    ///
+    /// let error = io::Error::from_raw_os_error(22);
+    /// assert_eq!(error.kind(), io::ErrorKind::InvalidInput);
+    /// # }
+    /// ```
+    ///
+    /// On Windows:
+    ///
+    /// ```
+    /// # if cfg!(windows) {
+    /// use std::io;
+    ///
+    /// let error = io::Error::from_raw_os_error(10022);
+    /// assert_eq!(error.kind(), io::ErrorKind::InvalidInput);
+    /// # }
+    /// ```
+    #[inline]
+    pub fn from_raw_os_error(code: i32) -> Error {
+        Error { repr: Repr::Os(code) }
+    }
+
+    /// Returns the OS error that this error represents (if any).
+    ///
+    /// If this [`Error`] was constructed via [`last_os_error`] or
+    /// [`from_raw_os_error`], then this function will return [`Some`], otherwise
+    /// it will return [`None`].
+    ///
+    /// [`last_os_error`]: Error::last_os_error
+    /// [`from_raw_os_error`]: Error::from_raw_os_error
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// fn print_os_error(err: &Error) {
+    ///     if let Some(raw_os_err) = err.raw_os_error() {
+    ///         println!("raw OS error: {:?}", raw_os_err);
+    ///     } else {
+    ///         println!("Not an OS error");
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     // Will print "raw OS error: ...".
+    ///     print_os_error(&Error::last_os_error());
+    ///     // Will print "Not an OS error".
+    ///     print_os_error(&Error::new(ErrorKind::Other, "oh no!"));
+    /// }
+    /// ```
+    #[inline]
+    pub fn raw_os_error(&self) -> Option<i32> {
+        match self.repr {
+            Repr::Os(i) => Some(i),
+            Repr::Custom(..) => None,
+            Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
+        }
+    }
+
+    /// Returns a reference to the inner error wrapped by this error (if any).
+    ///
+    /// If this [`Error`] was constructed via [`new`] then this function will
+    /// return [`Some`], otherwise it will return [`None`].
+    ///
+    /// [`new`]: Error::new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// fn print_error(err: &Error) {
+    ///     if let Some(inner_err) = err.get_ref() {
+    ///         println!("Inner error: {:?}", inner_err);
+    ///     } else {
+    ///         println!("No inner error");
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     // Will print "No inner error".
+    ///     print_error(&Error::last_os_error());
+    ///     // Will print "Inner error: ...".
+    ///     print_error(&Error::new(ErrorKind::Other, "oh no!"));
+    /// }
+    /// ```
+    #[inline]
+    pub fn get_ref(&self) -> Option<&String> {
+        match self.repr {
+            Repr::Os(..) => None,
+            Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
+            Repr::Custom(ref c) => Some(&c.error),
+        }
+    }
+
+    /// Returns a mutable reference to the inner error wrapped by this error
+    /// (if any).
+    ///
+    /// If this [`Error`] was constructed via [`new`] then this function will
+    /// return [`Some`], otherwise it will return [`None`].
+    ///
+    /// [`new`]: Error::new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    /// use std::{error, fmt};
+    /// use std::fmt::Display;
+    ///
+    /// #[derive(Debug)]
+    /// struct MyError {
+    ///     v: String,
+    /// }
+    ///
+    /// impl MyError {
+    ///     fn new() -> MyError {
+    ///         MyError {
+    ///             v: "oh no!".to_string()
+    ///         }
+    ///     }
+    ///
+    ///     fn change_message(&mut self, new_message: &str) {
+    ///         self.v = new_message.to_string();
+    ///     }
+    /// }
+    ///
+    /// impl error::Error for MyError {}
+    ///
+    /// impl Display for MyError {
+    ///     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    ///         write!(f, "MyError: {}", &self.v)
+    ///     }
+    /// }
+    ///
+    /// fn change_error(mut err: Error) -> Error {
+    ///     if let Some(inner_err) = err.get_mut() {
+    ///         inner_err.downcast_mut::<MyError>().unwrap().change_message("I've been changed!");
+    ///     }
+    ///     err
+    /// }
+    ///
+    /// fn print_error(err: &Error) {
+    ///     if let Some(inner_err) = err.get_ref() {
+    ///         println!("Inner error: {}", inner_err);
+    ///     } else {
+    ///         println!("No inner error");
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     // Will print "No inner error".
+    ///     print_error(&change_error(Error::last_os_error()));
+    ///     // Will print "Inner error: ...".
+    ///     print_error(&change_error(Error::new(ErrorKind::Other, MyError::new())));
+    /// }
+    /// ```
+    #[inline]
+    pub fn get_mut(&mut self) -> Option<&mut String> {
+        match self.repr {
+            Repr::Os(..) => None,
+            Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
+            Repr::Custom(ref mut c) => Some(&mut c.error),
+        }
+    }
+
+    /// Consumes the `Error`, returning its inner error (if any).
+    ///
+    /// If this [`Error`] was constructed via [`new`] then this function will
+    /// return [`Some`], otherwise it will return [`None`].
+    ///
+    /// [`new`]: Error::new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// fn print_error(err: Error) {
+    ///     if let Some(inner_err) = err.into_inner() {
+    ///         println!("Inner error: {}", inner_err);
+    ///     } else {
+    ///         println!("No inner error");
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     // Will print "No inner error".
+    ///     print_error(Error::last_os_error());
+    ///     // Will print "Inner error: ...".
+    ///     print_error(Error::new(ErrorKind::Other, "oh no!"));
+    /// }
+    /// ```
+    #[inline]
+    pub fn into_inner(self) -> Option<String> {
+        match self.repr {
+            Repr::Os(..) => None,
+            Repr::Simple(..) => None,
+            Repr::SimpleMessage(..) => None,
+            Repr::Custom(c) => Some(c.error),
+        }
+    }
+
+    /// Returns the corresponding [`ErrorKind`] for this error.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{Error, ErrorKind};
+    ///
+    /// fn print_error(err: Error) {
+    ///     println!("{:?}", err.kind());
+    /// }
+    ///
+    /// fn main() {
+    ///     // Will print "Uncategorized".
+    ///     print_error(Error::last_os_error());
+    ///     // Will print "AddrInUse".
+    ///     print_error(Error::new(ErrorKind::AddrInUse, "oh no!"));
+    /// }
+    /// ```
+    #[inline]
+    pub fn kind(&self) -> ErrorKind {
+        match self.repr {
+            Repr::Os(_code) => ErrorKind::Other,
+            Repr::Custom(ref c) => c.kind,
+            Repr::Simple(kind) => kind,
+            Repr::SimpleMessage(kind, _) => kind,
+        }
+    }
+}
+
+impl fmt::Debug for Repr {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match *self {
+            Repr::Os(code) => fmt
+                .debug_struct("Os")
+                .field("code", &code)
+                .finish(),
+            Repr::Custom(ref c) => fmt::Debug::fmt(&c, fmt),
+            Repr::Simple(kind) => fmt.debug_tuple("Kind").field(&kind).finish(),
+            Repr::SimpleMessage(kind, &message) => {
+                fmt.debug_struct("Error").field("kind", &kind).field("message", &message).finish()
+            }
+        }
+    }
+}
+
+impl fmt::Display for Error {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.repr {
+            Repr::Os(code) => {
+                write!(fmt, "os error {}", code)
+            }
+            Repr::Custom(ref c) => c.error.fmt(fmt),
+            Repr::Simple(kind) => write!(fmt, "{}", kind.as_str()),
+            Repr::SimpleMessage(_, &msg) => msg.fmt(fmt),
+        }
+    }
+}
+
+fn _assert_error_is_sync_send() {
+    fn _is_sync_send<T: Sync + Send>() {}
+    _is_sync_send::<Error>();
+}

src/error/tests.rs

@@ -0,0 +1,69 @@
+use super::{Custom, Error, ErrorKind, Repr};
+use crate::error;
+use crate::fmt;
+use crate::mem::size_of;
+use crate::sys::decode_error_kind;
+use crate::sys::os::error_string;
+
+#[test]
+fn test_size() {
+    assert!(size_of::<Error>() <= size_of::<[usize; 2]>());
+}
+
+#[test]
+fn test_debug_error() {
+    let code = 6;
+    let msg = error_string(code);
+    let kind = decode_error_kind(code);
+    let err = Error {
+        repr: Repr::Custom(box Custom {
+            kind: ErrorKind::InvalidInput,
+            error: box Error { repr: super::Repr::Os(code) },
+        }),
+    };
+    let expected = format!(
+        "Custom {{ \
+         kind: InvalidInput, \
+         error: Os {{ \
+         code: {:?}, \
+         kind: {:?}, \
+         message: {:?} \
+         }} \
+         }}",
+        code, kind, msg
+    );
+    assert_eq!(format!("{:?}", err), expected);
+}
+
+#[test]
+fn test_downcasting() {
+    #[derive(Debug)]
+    struct TestError;
+
+    impl fmt::Display for TestError {
+        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+            f.write_str("asdf")
+        }
+    }
+
+    impl error::Error for TestError {}
+
+    // we have to call all of these UFCS style right now since method
+    // resolution won't implicitly drop the Send+Sync bounds
+    let mut err = Error::new(ErrorKind::Other, TestError);
+    assert!(err.get_ref().unwrap().is::<TestError>());
+    assert_eq!("asdf", err.get_ref().unwrap().to_string());
+    assert!(err.get_mut().unwrap().is::<TestError>());
+    let extracted = err.into_inner().unwrap();
+    extracted.downcast::<TestError>().unwrap();
+}
+
+#[test]
+fn test_const() {
+    const E: Error = Error::new_const(ErrorKind::NotFound, &"hello");
+
+    assert_eq!(E.kind(), ErrorKind::NotFound);
+    assert_eq!(E.to_string(), "hello");
+    assert!(format!("{:?}", E).contains("\"hello\""));
+    assert!(format!("{:?}", E).contains("NotFound"));
+}

src/impls.rs

@@ -0,0 +1,408 @@
+#[cfg(test)]
+mod tests;
+
+#[cfg(feature="collections")] use core::alloc::Allocator;
+use core::cmp;
+use core::fmt;
+use crate::{
+    self as io, Error, ErrorKind, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, Write,
+};
+#[cfg(feature="collections")] use crate::BufRead;
+use core::mem;
+
+#[cfg(feature="alloc")] use alloc::boxed::Box;
+#[cfg(feature="collections")] use alloc::string::String;
+#[cfg(feature="collections")] use alloc::vec::Vec;
+
+// =============================================================================
+// Forwarding implementations
+
+impl<R: Read + ?Sized> Read for &mut R {
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        (**self).read(buf)
+    }
+
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        (**self).read_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        (**self).is_read_vectored()
+    }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        (**self).initializer()
+    }
+
+    #[cfg(feature="collections")]
+    #[inline]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (**self).read_to_end(buf)
+    }
+
+    #[cfg(feature="collections")]
+    #[inline]
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        (**self).read_to_string(buf)
+    }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        (**self).read_exact(buf)
+    }
+}
+impl<W: Write + ?Sized> Write for &mut W {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        (**self).write(buf)
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        (**self).write_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        (**self).is_write_vectored()
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        (**self).flush()
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        (**self).write_all(buf)
+    }
+
+    #[inline]
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
+        (**self).write_fmt(fmt)
+    }
+}
+impl<S: Seek + ?Sized> Seek for &mut S {
+    #[inline]
+    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+        (**self).seek(pos)
+    }
+
+    #[inline]
+    fn stream_position(&mut self) -> io::Result<u64> {
+        (**self).stream_position()
+    }
+}
+#[cfg(feature="collections")]
+impl<B: BufRead + ?Sized> BufRead for &mut B {
+    #[inline]
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        (**self).fill_buf()
+    }
+
+    #[inline]
+    fn consume(&mut self, amt: usize) {
+        (**self).consume(amt)
+    }
+
+    #[inline]
+    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (**self).read_until(byte, buf)
+    }
+
+    #[inline]
+    fn read_line(&mut self, buf: &mut String) -> io::Result<usize> {
+        (**self).read_line(buf)
+    }
+}
+
+#[cfg(feature="alloc")]
+impl<R: Read + ?Sized> Read for Box<R> {
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        (**self).read(buf)
+    }
+
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        (**self).read_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        (**self).is_read_vectored()
+    }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        (**self).initializer()
+    }
+
+    #[cfg(feature="collections")]
+    #[inline]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (**self).read_to_end(buf)
+    }
+
+    #[cfg(feature="collections")]
+    #[inline]
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        (**self).read_to_string(buf)
+    }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        (**self).read_exact(buf)
+    }
+}
+#[cfg(feature="alloc")]
+impl<W: Write + ?Sized> Write for Box<W> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        (**self).write(buf)
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        (**self).write_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        (**self).is_write_vectored()
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        (**self).flush()
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        (**self).write_all(buf)
+    }
+
+    #[inline]
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
+        (**self).write_fmt(fmt)
+    }
+}
+#[cfg(feature="alloc")]
+impl<S: Seek + ?Sized> Seek for Box<S> {
+    #[inline]
+    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
+        (**self).seek(pos)
+    }
+
+    #[inline]
+    fn stream_position(&mut self) -> io::Result<u64> {
+        (**self).stream_position()
+    }
+}
+#[cfg(feature="collections")]
+impl<B: BufRead + ?Sized> BufRead for Box<B> {
+    #[inline]
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        (**self).fill_buf()
+    }
+
+    #[inline]
+    fn consume(&mut self, amt: usize) {
+        (**self).consume(amt)
+    }
+
+    #[inline]
+    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (**self).read_until(byte, buf)
+    }
+
+    #[inline]
+    fn read_line(&mut self, buf: &mut String) -> io::Result<usize> {
+        (**self).read_line(buf)
+    }
+}
+
+// =============================================================================
+// In-memory buffer implementations
+
+/// Read is implemented for `&[u8]` by copying from the slice.
+///
+/// Note that reading updates the slice to point to the yet unread part.
+/// The slice will be empty when EOF is reached.
+impl Read for &[u8] {
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let amt = cmp::min(buf.len(), self.len());
+        let (a, b) = self.split_at(amt);
+
+        // First check if the amount of bytes we want to read is small:
+        // `copy_from_slice` will generally expand to a call to `memcpy`, and
+        // for a single byte the overhead is significant.
+        if amt == 1 {
+            buf[0] = a[0];
+        } else {
+            buf[..amt].copy_from_slice(a);
+        }
+
+        *self = b;
+        Ok(amt)
+    }
+
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        let mut nread = 0;
+        for buf in bufs {
+            nread += self.read(buf)?;
+            if self.is_empty() {
+                break;
+            }
+        }
+
+        Ok(nread)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
+    }
+
+    #[inline]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        if buf.len() > self.len() {
+            return Err(Error::new_const(ErrorKind::UnexpectedEof, &"failed to fill whole buffer"));
+        }
+        let (a, b) = self.split_at(buf.len());
+
+        // First check if the amount of bytes we want to read is small:
+        // `copy_from_slice` will generally expand to a call to `memcpy`, and
+        // for a single byte the overhead is significant.
+        if buf.len() == 1 {
+            buf[0] = a[0];
+        } else {
+            buf.copy_from_slice(a);
+        }
+
+        *self = b;
+        Ok(())
+    }
+
+    #[cfg(feature="collections")]
+    #[inline]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        buf.extend_from_slice(*self);
+        let len = self.len();
+        *self = &self[len..];
+        Ok(len)
+    }
+}
+
+#[cfg(feature="collections")]
+impl BufRead for &[u8] {
+    #[inline]
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        Ok(*self)
+    }
+
+    #[inline]
+    fn consume(&mut self, amt: usize) {
+        *self = &self[amt..];
+    }
+}
+
+/// Write is implemented for `&mut [u8]` by copying into the slice, overwriting
+/// its data.
+///
+/// Note that writing updates the slice to point to the yet unwritten part.
+/// The slice will be empty when it has been completely overwritten.
+///
+/// If the number of bytes to be written exceeds the size of the slice, write operations will
+/// return short writes: ultimately, `Ok(0)`; in this situation, `write_all` returns an error of
+/// kind `ErrorKind::WriteZero`.
+impl Write for &mut [u8] {
+    #[inline]
+    fn write(&mut self, data: &[u8]) -> io::Result<usize> {
+        let amt = cmp::min(data.len(), self.len());
+        let (a, b) = mem::replace(self, &mut []).split_at_mut(amt);
+        a.copy_from_slice(&data[..amt]);
+        *self = b;
+        Ok(amt)
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let mut nwritten = 0;
+        for buf in bufs {
+            nwritten += self.write(buf)?;
+            if self.is_empty() {
+                break;
+            }
+        }
+
+        Ok(nwritten)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn write_all(&mut self, data: &[u8]) -> io::Result<()> {
+        if self.write(data)? == data.len() {
+            Ok(())
+        } else {
+            Err(Error::new_const(ErrorKind::WriteZero, &"failed to write whole buffer"))
+        }
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+/// Write is implemented for `Vec<u8>` by appending to the vector.
+/// The vector will grow as needed.
+#[cfg(feature="collections")]
+impl<A: Allocator> Write for Vec<u8, A> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.extend_from_slice(buf);
+        Ok(buf.len())
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let len = bufs.iter().map(|b| b.len()).sum();
+        self.reserve(len);
+        for buf in bufs {
+            self.extend_from_slice(buf);
+        }
+        Ok(len)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.extend_from_slice(buf);
+        Ok(())
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}

src/impls/tests.rs

@@ -0,0 +1,57 @@
+use crate::io::prelude::*;
+
+#[bench]
+fn bench_read_slice(b: &mut test::Bencher) {
+    let buf = [5; 1024];
+    let mut dst = [0; 128];
+
+    b.iter(|| {
+        let mut rd = &buf[..];
+        for _ in 0..8 {
+            let _ = rd.read(&mut dst);
+            test::black_box(&dst);
+        }
+    })
+}
+
+#[bench]
+fn bench_write_slice(b: &mut test::Bencher) {
+    let mut buf = [0; 1024];
+    let src = [5; 128];
+
+    b.iter(|| {
+        let mut wr = &mut buf[..];
+        for _ in 0..8 {
+            let _ = wr.write_all(&src);
+            test::black_box(&wr);
+        }
+    })
+}
+
+#[bench]
+fn bench_read_vec(b: &mut test::Bencher) {
+    let buf = vec![5; 1024];
+    let mut dst = [0; 128];
+
+    b.iter(|| {
+        let mut rd = &buf[..];
+        for _ in 0..8 {
+            let _ = rd.read(&mut dst);
+            test::black_box(&dst);
+        }
+    })
+}
+
+#[bench]
+fn bench_write_vec(b: &mut test::Bencher) {
+    let mut buf = Vec::with_capacity(1024);
+    let src = [5; 128];
+
+    b.iter(|| {
+        let mut wr = &mut buf[..];
+        for _ in 0..8 {
+            let _ = wr.write_all(&src);
+            test::black_box(&wr);
+        }
+    })
+}

src/prelude.rs

@@ -0,0 +1,15 @@
+//! The I/O Prelude.
+//!
+//! The purpose of this module is to alleviate imports of many common I/O traits
+//! by adding a glob import to the top of I/O heavy modules:
+//!
+//! ```
+//! # #![allow(unused_imports)]
+//! use std::io::prelude::*;
+//! ```
+
+pub use super::{Read, Seek, Write};
+#[cfg(feature="collections")] pub use super::BufRead;
+
+#[cfg(feature="collections")] pub use alloc::boxed::Box;
+#[cfg(feature="collections")] pub use alloc::vec::Vec;

src/stdio/tests.rs

@@ -0,0 +1,166 @@
+use super::*;
+use crate::panic::{RefUnwindSafe, UnwindSafe};
+use crate::sync::mpsc::sync_channel;
+use crate::thread;
+
+#[test]
+fn stdout_unwind_safe() {
+    assert_unwind_safe::<Stdout>();
+}
+#[test]
+fn stdoutlock_unwind_safe() {
+    assert_unwind_safe::<StdoutLock<'_>>();
+    assert_unwind_safe::<StdoutLock<'static>>();
+}
+#[test]
+fn stderr_unwind_safe() {
+    assert_unwind_safe::<Stderr>();
+}
+#[test]
+fn stderrlock_unwind_safe() {
+    assert_unwind_safe::<StderrLock<'_>>();
+    assert_unwind_safe::<StderrLock<'static>>();
+}
+
+fn assert_unwind_safe<T: UnwindSafe + RefUnwindSafe>() {}
+
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn panic_doesnt_poison() {
+    thread::spawn(|| {
+        let _a = stdin();
+        let _a = _a.lock();
+        let _a = stdout();
+        let _a = _a.lock();
+        let _a = stderr();
+        let _a = _a.lock();
+        panic!();
+    })
+    .join()
+    .unwrap_err();
+
+    let _a = stdin();
+    let _a = _a.lock();
+    let _a = stdout();
+    let _a = _a.lock();
+    let _a = stderr();
+    let _a = _a.lock();
+}
+
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn test_lock_stderr() {
+    test_lock(stderr, stderr_locked);
+}
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn test_lock_stdin() {
+    test_lock(stdin, stdin_locked);
+}
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn test_lock_stdout() {
+    test_lock(stdout, stdout_locked);
+}
+
+// Helper trait to make lock testing function generic.
+trait Stdio<'a>: 'static
+where
+    Self::Lock: 'a,
+{
+    type Lock;
+    fn lock(&'a self) -> Self::Lock;
+}
+impl<'a> Stdio<'a> for Stderr {
+    type Lock = StderrLock<'a>;
+    fn lock(&'a self) -> StderrLock<'a> {
+        self.lock()
+    }
+}
+impl<'a> Stdio<'a> for Stdin {
+    type Lock = StdinLock<'a>;
+    fn lock(&'a self) -> StdinLock<'a> {
+        self.lock()
+    }
+}
+impl<'a> Stdio<'a> for Stdout {
+    type Lock = StdoutLock<'a>;
+    fn lock(&'a self) -> StdoutLock<'a> {
+        self.lock()
+    }
+}
+
+// Helper trait to make lock testing function generic.
+trait StdioOwnedLock: 'static {}
+impl StdioOwnedLock for StderrLock<'static> {}
+impl StdioOwnedLock for StdinLock<'static> {}
+impl StdioOwnedLock for StdoutLock<'static> {}
+
+// Tests locking on stdio handles by starting two threads and checking that
+// they block each other appropriately.
+fn test_lock<T, U>(get_handle: fn() -> T, get_locked: fn() -> U)
+where
+    T: for<'a> Stdio<'a>,
+    U: StdioOwnedLock,
+{
+    // State enum to track different phases of the test, primarily when
+    // each lock is acquired and released.
+    #[derive(Debug, PartialEq)]
+    enum State {
+        Start1,
+        Acquire1,
+        Start2,
+        Release1,
+        Acquire2,
+        Release2,
+    }
+    use State::*;
+    // Logging vector to be checked to make sure lock acquisitions and
+    // releases happened in the correct order.
+    let log = Arc::new(Mutex::new(Vec::new()));
+    let ((tx1, rx1), (tx2, rx2)) = (sync_channel(0), sync_channel(0));
+    let th1 = {
+        let (log, tx) = (Arc::clone(&log), tx1);
+        thread::spawn(move || {
+            log.lock().unwrap().push(Start1);
+            let handle = get_handle();
+            {
+                let locked = handle.lock();
+                log.lock().unwrap().push(Acquire1);
+                tx.send(Acquire1).unwrap(); // notify of acquisition
+                tx.send(Release1).unwrap(); // wait for release command
+                log.lock().unwrap().push(Release1);
+            }
+            tx.send(Acquire1).unwrap(); // wait for th2 acquire
+            {
+                let locked = handle.lock();
+                log.lock().unwrap().push(Acquire1);
+            }
+            log.lock().unwrap().push(Release1);
+        })
+    };
+    let th2 = {
+        let (log, tx) = (Arc::clone(&log), tx2);
+        thread::spawn(move || {
+            tx.send(Start2).unwrap(); // wait for start command
+            let locked = get_locked();
+            log.lock().unwrap().push(Acquire2);
+            tx.send(Acquire2).unwrap(); // notify of acquisition
+            tx.send(Release2).unwrap(); // wait for release command
+            log.lock().unwrap().push(Release2);
+        })
+    };
+    assert_eq!(rx1.recv().unwrap(), Acquire1); // wait for th1 acquire
+    log.lock().unwrap().push(Start2);
+    assert_eq!(rx2.recv().unwrap(), Start2); // block th2
+    assert_eq!(rx1.recv().unwrap(), Release1); // release th1
+    assert_eq!(rx2.recv().unwrap(), Acquire2); // wait for th2 acquire
+    assert_eq!(rx1.recv().unwrap(), Acquire1); // block th1
+    assert_eq!(rx2.recv().unwrap(), Release2); // release th2
+    th2.join().unwrap();
+    th1.join().unwrap();
+    assert_eq!(
+        *log.lock().unwrap(),
+        [Start1, Acquire1, Start2, Release1, Acquire2, Release2, Acquire1, Release1]
+    );
+}

src/tests.rs

@@ -0,0 +1,573 @@
+use super::{repeat, Cursor, SeekFrom};
+use crate::cmp::{self, min};
+use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{BufRead, BufReader, Read, Seek, Write};
+use crate::ops::Deref;
+
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn read_until() {
+    let mut buf = Cursor::new(&b"12"[..]);
+    let mut v = Vec::new();
+    assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 2);
+    assert_eq!(v, b"12");
+
+    let mut buf = Cursor::new(&b"1233"[..]);
+    let mut v = Vec::new();
+    assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 3);
+    assert_eq!(v, b"123");
+    v.truncate(0);
+    assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 1);
+    assert_eq!(v, b"3");
+    v.truncate(0);
+    assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 0);
+    assert_eq!(v, []);
+}
+
+#[test]
+fn split() {
+    let buf = Cursor::new(&b"12"[..]);
+    let mut s = buf.split(b'3');
+    assert_eq!(s.next().unwrap().unwrap(), vec![b'1', b'2']);
+    assert!(s.next().is_none());
+
+    let buf = Cursor::new(&b"1233"[..]);
+    let mut s = buf.split(b'3');
+    assert_eq!(s.next().unwrap().unwrap(), vec![b'1', b'2']);
+    assert_eq!(s.next().unwrap().unwrap(), vec![]);
+    assert!(s.next().is_none());
+}
+
+#[test]
+fn read_line() {
+    let mut buf = Cursor::new(&b"12"[..]);
+    let mut v = String::new();
+    assert_eq!(buf.read_line(&mut v).unwrap(), 2);
+    assert_eq!(v, "12");
+
+    let mut buf = Cursor::new(&b"12\n\n"[..]);
+    let mut v = String::new();
+    assert_eq!(buf.read_line(&mut v).unwrap(), 3);
+    assert_eq!(v, "12\n");
+    v.truncate(0);
+    assert_eq!(buf.read_line(&mut v).unwrap(), 1);
+    assert_eq!(v, "\n");
+    v.truncate(0);
+    assert_eq!(buf.read_line(&mut v).unwrap(), 0);
+    assert_eq!(v, "");
+}
+
+#[test]
+fn lines() {
+    let buf = Cursor::new(&b"12\r"[..]);
+    let mut s = buf.lines();
+    assert_eq!(s.next().unwrap().unwrap(), "12\r".to_string());
+    assert!(s.next().is_none());
+
+    let buf = Cursor::new(&b"12\r\n\n"[..]);
+    let mut s = buf.lines();
+    assert_eq!(s.next().unwrap().unwrap(), "12".to_string());
+    assert_eq!(s.next().unwrap().unwrap(), "".to_string());
+    assert!(s.next().is_none());
+}
+
+#[test]
+fn buf_read_has_data_left() {
+    let mut buf = Cursor::new(&b"abcd"[..]);
+    assert!(buf.has_data_left().unwrap());
+    buf.read_exact(&mut [0; 2]).unwrap();
+    assert!(buf.has_data_left().unwrap());
+    buf.read_exact(&mut [0; 2]).unwrap();
+    assert!(!buf.has_data_left().unwrap());
+}
+
+#[test]
+fn read_to_end() {
+    let mut c = Cursor::new(&b""[..]);
+    let mut v = Vec::new();
+    assert_eq!(c.read_to_end(&mut v).unwrap(), 0);
+    assert_eq!(v, []);
+
+    let mut c = Cursor::new(&b"1"[..]);
+    let mut v = Vec::new();
+    assert_eq!(c.read_to_end(&mut v).unwrap(), 1);
+    assert_eq!(v, b"1");
+
+    let cap = 1024 * 1024;
+    let data = (0..cap).map(|i| (i / 3) as u8).collect::<Vec<_>>();
+    let mut v = Vec::new();
+    let (a, b) = data.split_at(data.len() / 2);
+    assert_eq!(Cursor::new(a).read_to_end(&mut v).unwrap(), a.len());
+    assert_eq!(Cursor::new(b).read_to_end(&mut v).unwrap(), b.len());
+    assert_eq!(v, data);
+}
+
+#[test]
+fn read_to_string() {
+    let mut c = Cursor::new(&b""[..]);
+    let mut v = String::new();
+    assert_eq!(c.read_to_string(&mut v).unwrap(), 0);
+    assert_eq!(v, "");
+
+    let mut c = Cursor::new(&b"1"[..]);
+    let mut v = String::new();
+    assert_eq!(c.read_to_string(&mut v).unwrap(), 1);
+    assert_eq!(v, "1");
+
+    let mut c = Cursor::new(&b"\xff"[..]);
+    let mut v = String::new();
+    assert!(c.read_to_string(&mut v).is_err());
+}
+
+#[test]
+fn read_exact() {
+    let mut buf = [0; 4];
+
+    let mut c = Cursor::new(&b""[..]);
+    assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+
+    let mut c = Cursor::new(&b"123"[..]).chain(Cursor::new(&b"456789"[..]));
+    c.read_exact(&mut buf).unwrap();
+    assert_eq!(&buf, b"1234");
+    c.read_exact(&mut buf).unwrap();
+    assert_eq!(&buf, b"5678");
+    assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+}
+
+#[test]
+fn read_exact_slice() {
+    let mut buf = [0; 4];
+
+    let mut c = &b""[..];
+    assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+
+    let mut c = &b"123"[..];
+    assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+    // make sure the optimized (early returning) method is being used
+    assert_eq!(&buf, &[0; 4]);
+
+    let mut c = &b"1234"[..];
+    c.read_exact(&mut buf).unwrap();
+    assert_eq!(&buf, b"1234");
+
+    let mut c = &b"56789"[..];
+    c.read_exact(&mut buf).unwrap();
+    assert_eq!(&buf, b"5678");
+    assert_eq!(c, b"9");
+}
+
+#[test]
+fn take_eof() {
+    struct R;
+
+    impl Read for R {
+        fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
+            Err(io::Error::new_const(io::ErrorKind::Other, &""))
+        }
+    }
+    impl BufRead for R {
+        fn fill_buf(&mut self) -> io::Result<&[u8]> {
+            Err(io::Error::new_const(io::ErrorKind::Other, &""))
+        }
+        fn consume(&mut self, _amt: usize) {}
+    }
+
+    let mut buf = [0; 1];
+    assert_eq!(0, R.take(0).read(&mut buf).unwrap());
+    assert_eq!(b"", R.take(0).fill_buf().unwrap());
+}
+
+fn cmp_bufread<Br1: BufRead, Br2: BufRead>(mut br1: Br1, mut br2: Br2, exp: &[u8]) {
+    let mut cat = Vec::new();
+    loop {
+        let consume = {
+            let buf1 = br1.fill_buf().unwrap();
+            let buf2 = br2.fill_buf().unwrap();
+            let minlen = if buf1.len() < buf2.len() { buf1.len() } else { buf2.len() };
+            assert_eq!(buf1[..minlen], buf2[..minlen]);
+            cat.extend_from_slice(&buf1[..minlen]);
+            minlen
+        };
+        if consume == 0 {
+            break;
+        }
+        br1.consume(consume);
+        br2.consume(consume);
+    }
+    assert_eq!(br1.fill_buf().unwrap().len(), 0);
+    assert_eq!(br2.fill_buf().unwrap().len(), 0);
+    assert_eq!(&cat[..], &exp[..])
+}
+
+#[test]
+fn chain_bufread() {
+    let testdata = b"ABCDEFGHIJKL";
+    let chain1 =
+        (&testdata[..3]).chain(&testdata[3..6]).chain(&testdata[6..9]).chain(&testdata[9..]);
+    let chain2 = (&testdata[..4]).chain(&testdata[4..8]).chain(&testdata[8..]);
+    cmp_bufread(chain1, chain2, &testdata[..]);
+}
+
+#[test]
+fn bufreader_size_hint() {
+    let testdata = b"ABCDEFGHIJKL";
+    let mut buf_reader = BufReader::new(&testdata[..]);
+    assert_eq!(buf_reader.buffer().len(), 0);
+
+    let buffer_length = testdata.len();
+    buf_reader.fill_buf().unwrap();
+
+    // Check that size hint matches buffer contents
+    let mut buffered_bytes = buf_reader.bytes();
+    let (lower_bound, _upper_bound) = buffered_bytes.size_hint();
+    assert_eq!(lower_bound, buffer_length);
+
+    // Check that size hint matches buffer contents after advancing
+    buffered_bytes.next().unwrap().unwrap();
+    let (lower_bound, _upper_bound) = buffered_bytes.size_hint();
+    assert_eq!(lower_bound, buffer_length - 1);
+}
+
+#[test]
+fn empty_size_hint() {
+    let size_hint = io::empty().bytes().size_hint();
+    assert_eq!(size_hint, (0, Some(0)));
+}
+
+#[test]
+fn slice_size_hint() {
+    let size_hint = (&[1, 2, 3]).bytes().size_hint();
+    assert_eq!(size_hint, (3, Some(3)));
+}
+
+#[test]
+fn take_size_hint() {
+    let size_hint = (&[1, 2, 3]).take(2).bytes().size_hint();
+    assert_eq!(size_hint, (2, Some(2)));
+
+    let size_hint = (&[1, 2, 3]).take(4).bytes().size_hint();
+    assert_eq!(size_hint, (3, Some(3)));
+
+    let size_hint = io::repeat(0).take(3).bytes().size_hint();
+    assert_eq!(size_hint, (3, Some(3)));
+}
+
+#[test]
+fn chain_empty_size_hint() {
+    let chain = io::empty().chain(io::empty());
+    let size_hint = chain.bytes().size_hint();
+    assert_eq!(size_hint, (0, Some(0)));
+}
+
+#[test]
+fn chain_size_hint() {
+    let testdata = b"ABCDEFGHIJKL";
+    let mut buf_reader_1 = BufReader::new(&testdata[..6]);
+    let mut buf_reader_2 = BufReader::new(&testdata[6..]);
+
+    buf_reader_1.fill_buf().unwrap();
+    buf_reader_2.fill_buf().unwrap();
+
+    let chain = buf_reader_1.chain(buf_reader_2);
+    let size_hint = chain.bytes().size_hint();
+    assert_eq!(size_hint, (testdata.len(), Some(testdata.len())));
+}
+
+#[test]
+fn chain_zero_length_read_is_not_eof() {
+    let a = b"A";
+    let b = b"B";
+    let mut s = String::new();
+    let mut chain = (&a[..]).chain(&b[..]);
+    chain.read(&mut []).unwrap();
+    chain.read_to_string(&mut s).unwrap();
+    assert_eq!("AB", s);
+}
+
+#[bench]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn bench_read_to_end(b: &mut test::Bencher) {
+    b.iter(|| {
+        let mut lr = repeat(1).take(10000000);
+        let mut vec = Vec::with_capacity(1024);
+        super::read_to_end(&mut lr, &mut vec)
+    });
+}
+
+#[test]
+fn seek_len() -> io::Result<()> {
+    let mut c = Cursor::new(vec![0; 15]);
+    assert_eq!(c.stream_len()?, 15);
+
+    c.seek(SeekFrom::End(0))?;
+    let old_pos = c.stream_position()?;
+    assert_eq!(c.stream_len()?, 15);
+    assert_eq!(c.stream_position()?, old_pos);
+
+    c.seek(SeekFrom::Start(7))?;
+    c.seek(SeekFrom::Current(2))?;
+    let old_pos = c.stream_position()?;
+    assert_eq!(c.stream_len()?, 15);
+    assert_eq!(c.stream_position()?, old_pos);
+
+    Ok(())
+}
+
+#[test]
+fn seek_position() -> io::Result<()> {
+    // All `asserts` are duplicated here to make sure the method does not
+    // change anything about the seek state.
+    let mut c = Cursor::new(vec![0; 15]);
+    assert_eq!(c.stream_position()?, 0);
+    assert_eq!(c.stream_position()?, 0);
+
+    c.seek(SeekFrom::End(0))?;
+    assert_eq!(c.stream_position()?, 15);
+    assert_eq!(c.stream_position()?, 15);
+
+    c.seek(SeekFrom::Start(7))?;
+    c.seek(SeekFrom::Current(2))?;
+    assert_eq!(c.stream_position()?, 9);
+    assert_eq!(c.stream_position()?, 9);
+
+    c.seek(SeekFrom::End(-3))?;
+    c.seek(SeekFrom::Current(1))?;
+    c.seek(SeekFrom::Current(-5))?;
+    assert_eq!(c.stream_position()?, 8);
+    assert_eq!(c.stream_position()?, 8);
+
+    c.rewind()?;
+    assert_eq!(c.stream_position()?, 0);
+    assert_eq!(c.stream_position()?, 0);
+
+    Ok(())
+}
+
+// A simple example reader which uses the default implementation of
+// read_to_end.
+struct ExampleSliceReader<'a> {
+    slice: &'a [u8],
+}
+
+impl<'a> Read for ExampleSliceReader<'a> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let len = cmp::min(self.slice.len(), buf.len());
+        buf[..len].copy_from_slice(&self.slice[..len]);
+        self.slice = &self.slice[len..];
+        Ok(len)
+    }
+}
+
+#[test]
+fn test_read_to_end_capacity() -> io::Result<()> {
+    let input = &b"foo"[..];
+
+    // read_to_end() generally needs to over-allocate, both for efficiency
+    // and so that it can distinguish EOF. Assert that this is the case
+    // with this simple ExampleSliceReader struct, which uses the default
+    // implementation of read_to_end. Even though vec1 is allocated with
+    // exactly enough capacity for the read, read_to_end will allocate more
+    // space here.
+    let mut vec1 = Vec::with_capacity(input.len());
+    ExampleSliceReader { slice: input }.read_to_end(&mut vec1)?;
+    assert_eq!(vec1.len(), input.len());
+    assert!(vec1.capacity() > input.len(), "allocated more");
+
+    // However, std::io::Take includes an implementation of read_to_end
+    // that will not allocate when the limit has already been reached. In
+    // this case, vec2 never grows.
+    let mut vec2 = Vec::with_capacity(input.len());
+    ExampleSliceReader { slice: input }.take(input.len() as u64).read_to_end(&mut vec2)?;
+    assert_eq!(vec2.len(), input.len());
+    assert_eq!(vec2.capacity(), input.len(), "did not allocate more");
+
+    Ok(())
+}
+
+#[test]
+fn io_slice_mut_advance_slices() {
+    let mut buf1 = [1; 8];
+    let mut buf2 = [2; 16];
+    let mut buf3 = [3; 8];
+    let mut bufs = &mut [
+        IoSliceMut::new(&mut buf1),
+        IoSliceMut::new(&mut buf2),
+        IoSliceMut::new(&mut buf3),
+    ][..];
+
+    // Only in a single buffer..
+    IoSliceMut::advance_slices(&mut bufs, 1);
+    assert_eq!(bufs[0].deref(), [1; 7].as_ref());
+    assert_eq!(bufs[1].deref(), [2; 16].as_ref());
+    assert_eq!(bufs[2].deref(), [3; 8].as_ref());
+
+    // Removing a buffer, leaving others as is.
+    IoSliceMut::advance_slices(&mut bufs, 7);
+    assert_eq!(bufs[0].deref(), [2; 16].as_ref());
+    assert_eq!(bufs[1].deref(), [3; 8].as_ref());
+
+    // Removing a buffer and removing from the next buffer.
+    IoSliceMut::advance_slices(&mut bufs, 18);
+    assert_eq!(bufs[0].deref(), [3; 6].as_ref());
+}
+
+#[test]
+fn io_slice_mut_advance_slices_empty_slice() {
+    let mut empty_bufs = &mut [][..];
+    // Shouldn't panic.
+    IoSliceMut::advance_slices(&mut empty_bufs, 1);
+}
+
+#[test]
+fn io_slice_mut_advance_slices_beyond_total_length() {
+    let mut buf1 = [1; 8];
+    let mut bufs = &mut [IoSliceMut::new(&mut buf1)][..];
+
+    // Going beyond the total length should be ok.
+    IoSliceMut::advance_slices(&mut bufs, 9);
+    assert!(bufs.is_empty());
+}
+
+#[test]
+fn io_slice_advance_slices() {
+    let buf1 = [1; 8];
+    let buf2 = [2; 16];
+    let buf3 = [3; 8];
+    let mut bufs = &mut [IoSlice::new(&buf1), IoSlice::new(&buf2), IoSlice::new(&buf3)][..];
+
+    // Only in a single buffer..
+    IoSlice::advance_slices(&mut bufs, 1);
+    assert_eq!(bufs[0].deref(), [1; 7].as_ref());
+    assert_eq!(bufs[1].deref(), [2; 16].as_ref());
+    assert_eq!(bufs[2].deref(), [3; 8].as_ref());
+
+    // Removing a buffer, leaving others as is.
+    IoSlice::advance_slices(&mut bufs, 7);
+    assert_eq!(bufs[0].deref(), [2; 16].as_ref());
+    assert_eq!(bufs[1].deref(), [3; 8].as_ref());
+
+    // Removing a buffer and removing from the next buffer.
+    IoSlice::advance_slices(&mut bufs, 18);
+    assert_eq!(bufs[0].deref(), [3; 6].as_ref());
+}
+
+#[test]
+fn io_slice_advance_slices_empty_slice() {
+    let mut empty_bufs = &mut [][..];
+    // Shouldn't panic.
+    IoSlice::advance_slices(&mut empty_bufs, 1);
+}
+
+#[test]
+fn io_slice_advance_slices_beyond_total_length() {
+    let buf1 = [1; 8];
+    let mut bufs = &mut [IoSlice::new(&buf1)][..];
+
+    // Going beyond the total length should be ok.
+    IoSlice::advance_slices(&mut bufs, 9);
+    assert!(bufs.is_empty());
+}
+
+/// Create a new writer that reads from at most `n_bufs` and reads
+/// `per_call` bytes (in total) per call to write.
+fn test_writer(n_bufs: usize, per_call: usize) -> TestWriter {
+    TestWriter { n_bufs, per_call, written: Vec::new() }
+}
+
+struct TestWriter {
+    n_bufs: usize,
+    per_call: usize,
+    written: Vec<u8>,
+}
+
+impl Write for TestWriter {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.write_vectored(&[IoSlice::new(buf)])
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let mut left = self.per_call;
+        let mut written = 0;
+        for buf in bufs.iter().take(self.n_bufs) {
+            let n = min(left, buf.len());
+            self.written.extend_from_slice(&buf[0..n]);
+            left -= n;
+            written += n;
+        }
+        Ok(written)
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+#[test]
+fn test_writer_read_from_one_buf() {
+    let mut writer = test_writer(1, 2);
+
+    assert_eq!(writer.write(&[]).unwrap(), 0);
+    assert_eq!(writer.write_vectored(&[]).unwrap(), 0);
+
+    // Read at most 2 bytes.
+    assert_eq!(writer.write(&[1, 1, 1]).unwrap(), 2);
+    let bufs = &[IoSlice::new(&[2, 2, 2])];
+    assert_eq!(writer.write_vectored(bufs).unwrap(), 2);
+
+    // Only read from first buf.
+    let bufs = &[IoSlice::new(&[3]), IoSlice::new(&[4, 4])];
+    assert_eq!(writer.write_vectored(bufs).unwrap(), 1);
+
+    assert_eq!(writer.written, &[1, 1, 2, 2, 3]);
+}
+
+#[test]
+fn test_writer_read_from_multiple_bufs() {
+    let mut writer = test_writer(3, 3);
+
+    // Read at most 3 bytes from two buffers.
+    let bufs = &[IoSlice::new(&[1]), IoSlice::new(&[2, 2, 2])];
+    assert_eq!(writer.write_vectored(bufs).unwrap(), 3);
+
+    // Read at most 3 bytes from three buffers.
+    let bufs = &[IoSlice::new(&[3]), IoSlice::new(&[4]), IoSlice::new(&[5, 5])];
+    assert_eq!(writer.write_vectored(bufs).unwrap(), 3);
+
+    assert_eq!(writer.written, &[1, 2, 2, 3, 4, 5]);
+}
+
+#[test]
+fn test_write_all_vectored() {
+    #[rustfmt::skip] // Becomes unreadable otherwise.
+    let tests: Vec<(_, &'static [u8])> = vec![
+        (vec![], &[]),
+        (vec![IoSlice::new(&[]), IoSlice::new(&[])], &[]),
+        (vec![IoSlice::new(&[1])], &[1]),
+        (vec![IoSlice::new(&[1, 2])], &[1, 2]),
+        (vec![IoSlice::new(&[1, 2, 3])], &[1, 2, 3]),
+        (vec![IoSlice::new(&[1, 2, 3, 4])], &[1, 2, 3, 4]),
+        (vec![IoSlice::new(&[1, 2, 3, 4, 5])], &[1, 2, 3, 4, 5]),
+        (vec![IoSlice::new(&[1]), IoSlice::new(&[2])], &[1, 2]),
+        (vec![IoSlice::new(&[1]), IoSlice::new(&[2, 2])], &[1, 2, 2]),
+        (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2])], &[1, 1, 2, 2]),
+        (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2, 2])], &[1, 1, 2, 2, 2]),
+        (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2, 2])], &[1, 1, 2, 2, 2]),
+        (vec![IoSlice::new(&[1, 1, 1]), IoSlice::new(&[2, 2, 2])], &[1, 1, 1, 2, 2, 2]),
+        (vec![IoSlice::new(&[1, 1, 1]), IoSlice::new(&[2, 2, 2, 2])], &[1, 1, 1, 2, 2, 2, 2]),
+        (vec![IoSlice::new(&[1, 1, 1, 1]), IoSlice::new(&[2, 2, 2, 2])], &[1, 1, 1, 1, 2, 2, 2, 2]),
+        (vec![IoSlice::new(&[1]), IoSlice::new(&[2]), IoSlice::new(&[3])], &[1, 2, 3]),
+        (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2]), IoSlice::new(&[3, 3])], &[1, 1, 2, 2, 3, 3]),
+        (vec![IoSlice::new(&[1]), IoSlice::new(&[2, 2]), IoSlice::new(&[3, 3, 3])], &[1, 2, 2, 3, 3, 3]),
+        (vec![IoSlice::new(&[1, 1, 1]), IoSlice::new(&[2, 2, 2]), IoSlice::new(&[3, 3, 3])], &[1, 1, 1, 2, 2, 2, 3, 3, 3]),
+    ];
+
+    let writer_configs = &[(1, 1), (1, 2), (1, 3), (2, 2), (2, 3), (3, 3)];
+
+    for (n_bufs, per_call) in writer_configs.iter().copied() {
+        for (mut input, wanted) in tests.clone().into_iter() {
+            let mut writer = test_writer(n_bufs, per_call);
+            assert!(writer.write_all_vectored(&mut *input).is_ok());
+            assert_eq!(&*writer.written, &*wanted);
+        }
+    }
+}

src/util.rs

@@ -0,0 +1,239 @@
+#![allow(missing_copy_implementations)]
+
+#[cfg(test)]
+mod tests;
+
+use core::fmt;
+use crate::{
+    self as io, Initializer, IoSlice, IoSliceMut, Read, Seek, SeekFrom, SizeHint, Write,
+};
+#[cfg(feature="collections")] use crate::BufRead;
+
+/// A reader which is always at EOF.
+///
+/// This struct is generally created by calling [`empty()`]. Please see
+/// the documentation of [`empty()`] for more details.
+#[non_exhaustive]
+#[derive(Copy, Clone, Default)]
+pub struct Empty;
+
+/// Constructs a new handle to an empty reader.
+///
+/// All reads from the returned reader will return [`Ok`]`(0)`.
+///
+/// # Examples
+///
+/// A slightly sad example of not reading anything into a buffer:
+///
+/// ```
+/// use std::io::{self, Read};
+///
+/// let mut buffer = String::new();
+/// io::empty().read_to_string(&mut buffer).unwrap();
+/// assert!(buffer.is_empty());
+/// ```
+pub const fn empty() -> Empty {
+    Empty
+}
+
+impl Read for Empty {
+    #[inline]
+    fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
+        Ok(0)
+    }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
+    }
+}
+
+#[cfg(feature="collections")]
+impl BufRead for Empty {
+    #[inline]
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        Ok(&[])
+    }
+    #[inline]
+    fn consume(&mut self, _n: usize) {}
+}
+
+impl Seek for Empty {
+    fn seek(&mut self, _pos: SeekFrom) -> io::Result<u64> {
+        Ok(0)
+    }
+
+    fn stream_len(&mut self) -> io::Result<u64> {
+        Ok(0)
+    }
+
+    fn stream_position(&mut self) -> io::Result<u64> {
+        Ok(0)
+    }
+}
+
+impl fmt::Debug for Empty {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Empty").finish_non_exhaustive()
+    }
+}
+
+impl SizeHint for Empty {
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        Some(0)
+    }
+}
+
+/// A reader which yields one byte over and over and over and over and over and...
+///
+/// This struct is generally created by calling [`repeat()`]. Please
+/// see the documentation of [`repeat()`] for more details.
+pub struct Repeat {
+    byte: u8,
+}
+
+/// Creates an instance of a reader that infinitely repeats one byte.
+///
+/// All reads from this reader will succeed by filling the specified buffer with
+/// the given byte.
+///
+/// # Examples
+///
+/// ```
+/// use std::io::{self, Read};
+///
+/// let mut buffer = [0; 3];
+/// io::repeat(0b101).read_exact(&mut buffer).unwrap();
+/// assert_eq!(buffer, [0b101, 0b101, 0b101]);
+/// ```
+pub const fn repeat(byte: u8) -> Repeat {
+    Repeat { byte }
+}
+
+impl Read for Repeat {
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        for slot in &mut *buf {
+            *slot = self.byte;
+        }
+        Ok(buf.len())
+    }
+
+    #[inline]
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        let mut nwritten = 0;
+        for buf in bufs {
+            nwritten += self.read(buf)?;
+        }
+        Ok(nwritten)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
+    }
+}
+
+impl SizeHint for Repeat {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        usize::MAX
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        None
+    }
+}
+
+impl fmt::Debug for Repeat {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Repeat").finish_non_exhaustive()
+    }
+}
+
+/// A writer which will move data into the void.
+///
+/// This struct is generally created by calling [`sink`]. Please
+/// see the documentation of [`sink()`] for more details.
+#[non_exhaustive]
+#[derive(Copy, Clone, Default)]
+pub struct Sink;
+
+/// Creates an instance of a writer which will successfully consume all data.
+///
+/// All calls to [`write`] on the returned instance will return `Ok(buf.len())`
+/// and the contents of the buffer will not be inspected.
+///
+/// [`write`]: Write::write
+///
+/// # Examples
+///
+/// ```rust
+/// use std::io::{self, Write};
+///
+/// let buffer = vec![1, 2, 3, 5, 8];
+/// let num_bytes = io::sink().write(&buffer).unwrap();
+/// assert_eq!(num_bytes, 5);
+/// ```
+pub const fn sink() -> Sink {
+    Sink
+}
+
+impl Write for Sink {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        Ok(buf.len())
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let total_len = bufs.iter().map(|b| b.len()).sum();
+        Ok(total_len)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+impl Write for &Sink {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        Ok(buf.len())
+    }
+
+    #[inline]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let total_len = bufs.iter().map(|b| b.len()).sum();
+        Ok(total_len)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        true
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+impl fmt::Debug for Sink {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Sink").finish_non_exhaustive()
+    }
+}

src/util/tests.rs

@@ -0,0 +1,120 @@
+use crate::cmp::{max, min};
+use crate::io::prelude::*;
+use crate::io::{
+    copy, empty, repeat, sink, BufWriter, Empty, Repeat, Result, SeekFrom, Sink, DEFAULT_BUF_SIZE,
+};
+
+#[test]
+fn copy_copies() {
+    let mut r = repeat(0).take(4);
+    let mut w = sink();
+    assert_eq!(copy(&mut r, &mut w).unwrap(), 4);
+
+    let mut r = repeat(0).take(1 << 17);
+    assert_eq!(copy(&mut r as &mut dyn Read, &mut w as &mut dyn Write).unwrap(), 1 << 17);
+}
+
+struct ShortReader {
+    cap: usize,
+    read_size: usize,
+    observed_buffer: usize,
+}
+
+impl Read for ShortReader {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        let bytes = min(self.cap, self.read_size);
+        self.cap -= bytes;
+        self.observed_buffer = max(self.observed_buffer, buf.len());
+        Ok(bytes)
+    }
+}
+
+struct WriteObserver {
+    observed_buffer: usize,
+}
+
+impl Write for WriteObserver {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        self.observed_buffer = max(self.observed_buffer, buf.len());
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> Result<()> {
+        Ok(())
+    }
+}
+
+#[test]
+fn copy_specializes_bufwriter() {
+    let cap = 117 * 1024;
+    let buf_sz = 16 * 1024;
+    let mut r = ShortReader { cap, observed_buffer: 0, read_size: 1337 };
+    let mut w = BufWriter::with_capacity(buf_sz, WriteObserver { observed_buffer: 0 });
+    assert_eq!(
+        copy(&mut r, &mut w).unwrap(),
+        cap as u64,
+        "expected the whole capacity to be copied"
+    );
+    assert_eq!(r.observed_buffer, buf_sz, "expected a large buffer to be provided to the reader");
+    assert!(w.get_mut().observed_buffer > DEFAULT_BUF_SIZE, "expected coalesced writes");
+}
+
+#[test]
+fn sink_sinks() {
+    let mut s = sink();
+    assert_eq!(s.write(&[]).unwrap(), 0);
+    assert_eq!(s.write(&[0]).unwrap(), 1);
+    assert_eq!(s.write(&[0; 1024]).unwrap(), 1024);
+    assert_eq!(s.by_ref().write(&[0; 1024]).unwrap(), 1024);
+}
+
+#[test]
+fn empty_reads() {
+    let mut e = empty();
+    assert_eq!(e.read(&mut []).unwrap(), 0);
+    assert_eq!(e.read(&mut [0]).unwrap(), 0);
+    assert_eq!(e.read(&mut [0; 1024]).unwrap(), 0);
+    assert_eq!(e.by_ref().read(&mut [0; 1024]).unwrap(), 0);
+}
+
+#[test]
+fn empty_seeks() {
+    let mut e = empty();
+    assert!(matches!(e.seek(SeekFrom::Start(0)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Start(1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Start(u64::MAX)), Ok(0)));
+
+    assert!(matches!(e.seek(SeekFrom::End(i64::MIN)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(-1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(0)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::End(i64::MAX)), Ok(0)));
+
+    assert!(matches!(e.seek(SeekFrom::Current(i64::MIN)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(-1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(0)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(1)), Ok(0)));
+    assert!(matches!(e.seek(SeekFrom::Current(i64::MAX)), Ok(0)));
+}
+
+#[test]
+fn repeat_repeats() {
+    let mut r = repeat(4);
+    let mut b = [0; 1024];
+    assert_eq!(r.read(&mut b).unwrap(), 1024);
+    assert!(b.iter().all(|b| *b == 4));
+}
+
+#[test]
+fn take_some_bytes() {
+    assert_eq!(repeat(4).take(100).bytes().count(), 100);
+    assert_eq!(repeat(4).take(100).bytes().next().unwrap().unwrap(), 4);
+    assert_eq!(repeat(1).take(10).chain(repeat(2).take(10)).bytes().count(), 20);
+}
+
+#[allow(dead_code)]
+fn const_utils() {
+    const _: Empty = empty();
+    const _: Repeat = repeat(b'c');
+    const _: Sink = sink();
+}