Rollup merge of #76084 - Lucretiel:split-buffered, r=dtolnay

Refactor io/buffered.rs into submodules

This pull request splits `BufWriter`, `BufReader`, `LineWriter`, and `LineWriterShim` (along with their associated tests) into separate submodules. It contains no functional changes. This change is being made in anticipation of adding another type of buffered writer which can be switched between line- and block-buffering mode.

Part of a series of pull requests resolving #60673.

Author: Dylan-DPC

library/std/src/io/buffered.rs

@@ -0,0 +1,423 @@
+use crate::cmp;
+use crate::fmt;
+use crate::io::{self, BufRead, Initializer, IoSliceMut, Read, Seek, SeekFrom, DEFAULT_BUF_SIZE};
+
+/// 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.
+///
+/// [`TcpStream::read`]: Read::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(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R> {
+        unsafe {
+            let mut buffer = Vec::with_capacity(capacity);
+            buffer.set_len(capacity);
+            inner.initializer().initialize(&mut buffer);
+            BufReader { inner, buf: buffer.into_boxed_slice(), 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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "bufreader_buffer", since = "1.37.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "buffered_io_capacity", since = "1.46.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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.
+    #[unstable(feature = "bufreader_seek_relative", issue = "31100")]
+    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)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+    }
+
+    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()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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);
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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
+    /// #![feature(seek_convenience)]
+    /// 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",
+            )
+        })
+    }
+}

library/std/src/io/buffered/bufreader.rs

@@ -0,0 +1,387 @@
+use crate::fmt;
+use crate::io::{
+    self, Error, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
+};
+
+/// 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.
+///
+/// [`TcpStream::write`]: Write::write
+/// [`TcpStream`]: crate::net::TcpStream
+/// [`flush`]: Write::flush
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct BufWriter<W: Write> {
+    inner: Option<W>,
+    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());
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> {
+        BufWriter { inner: Some(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(super) 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);
+        let inner = self.inner.as_mut().unwrap();
+        while !guard.done() {
+            self.panicked = true;
+            let r = inner.write(guard.remaining());
+            self.panicked = false;
+
+            match r {
+                Ok(0) => {
+                    return Err(Error::new(
+                        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.buf.capacity() - self.buf.len();
+        let amt_to_buffer = available.min(buf.len());
+        self.buf.extend_from_slice(&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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn get_ref(&self) -> &W {
+        self.inner.as_ref().unwrap()
+    }
+
+    /// 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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn get_mut(&mut self) -> &mut W {
+        self.inner.as_mut().unwrap()
+    }
+
+    /// 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();
+    /// ```
+    #[stable(feature = "bufreader_buffer", since = "1.37.0")]
+    pub fn buffer(&self) -> &[u8] {
+        &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();
+    /// ```
+    #[stable(feature = "buffered_io_capacity", since = "1.46.0")]
+    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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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.inner.take().unwrap()),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<W: Write> Write for BufWriter<W> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        if self.buf.len() + buf.len() > self.buf.capacity() {
+            self.flush_buf()?;
+        }
+        // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
+        if buf.len() >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write(buf);
+            self.panicked = false;
+            r
+        } else {
+            self.buf.extend_from_slice(buf);
+            Ok(buf.len())
+        }
+    }
+
+    fn write_all(&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 self.buf.len() + buf.len() > self.buf.capacity() {
+            self.flush_buf()?;
+        }
+        // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
+        if buf.len() >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write_all(buf);
+            self.panicked = false;
+            r
+        } else {
+            self.buf.extend_from_slice(buf);
+            Ok(())
+        }
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let total_len = bufs.iter().map(|b| b.len()).sum::<usize>();
+        if self.buf.len() + total_len > self.buf.capacity() {
+            self.flush_buf()?;
+        }
+        // FIXME: Why no len > capacity? Why not buffer len == capacity? #72919
+        if total_len >= self.buf.capacity() {
+            self.panicked = true;
+            let r = self.get_mut().write_vectored(bufs);
+            self.panicked = false;
+            r
+        } else {
+            bufs.iter().for_each(|b| self.buf.extend_from_slice(b));
+            Ok(total_len)
+        }
+    }
+
+    fn is_write_vectored(&self) -> bool {
+        self.get_ref().is_write_vectored()
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        self.flush_buf().and_then(|()| self.get_mut().flush())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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.as_ref().unwrap())
+            .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
+            .finish()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<W: Write> Drop for BufWriter<W> {
+    fn drop(&mut self) {
+        if self.inner.is_some() && !self.panicked {
+            // dtors should not panic, so we ignore a failed flush
+            let _r = self.flush_buf();
+        }
+    }
+}

library/std/src/io/buffered/bufwriter.rs

@@ -0,0 +1,232 @@
+use crate::fmt;
+use crate::io::{self, 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(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn into_inner(self) -> Result<W, IntoInnerError<LineWriter<W>>> {
+        self.inner.into_inner().map_err(|err| err.new_wrapped(|inner| LineWriter { inner }))
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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()
+    }
+}

library/std/src/io/buffered/linewriter.rs

@@ -0,0 +1,270 @@
+use crate::io::{self, BufWriter, IoSlice, Write};
+use crate::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 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.buffer.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)
+            }
+        }
+    }
+}

library/std/src/io/buffered/linewritershim.rs

@@ -0,0 +1,151 @@
+//! Buffering wrappers for I/O traits
+
+mod bufreader;
+mod bufwriter;
+mod linewriter;
+mod linewritershim;
+
+#[cfg(test)]
+mod tests;
+
+use crate::error;
+use crate::fmt;
+use crate::io::Error;
+
+pub use bufreader::BufReader;
+pub use bufwriter::BufWriter;
+pub use linewriter::LineWriter;
+use 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)]
+#[stable(feature = "rust1", since = "1.0.0")]
+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.");
+    ///     }
+    /// };
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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()
+    ///     }
+    /// };
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn into_inner(self) -> W {
+        self.0
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<W> From<IntoInnerError<W>> for Error {
+    fn from(iie: IntoInnerError<W>) -> Error {
+        iie.1
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<W: Send + fmt::Debug> error::Error for IntoInnerError<W> {
+    #[allow(deprecated, deprecated_in_future)]
+    fn description(&self) -> &str {
+        error::Error::description(self.error())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<W> fmt::Display for IntoInnerError<W> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.error().fmt(f)
+    }
+}

library/std/src/io/buffered/mod.rs

@@ -13,7 +13,7 @@ error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satis
 LL |     let fp = BufWriter::new(fp);
    |              ^^^^^^^^^^^^^^ the trait `std::io::Write` is not implemented for `&dyn std::io::Write`
    | 
-  ::: $SRC_DIR/std/src/io/buffered.rs:LL:COL
+  ::: $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
    |
 LL | pub struct BufWriter<W: Write> {
    |                         ----- required by this bound in `BufWriter`
@@ -26,7 +26,7 @@ error[E0277]: the trait bound `&dyn std::io::Write: std::io::Write` is not satis
 LL |     let fp = BufWriter::new(fp);
    |              ^^^^^^^^^^^^^^^^^^ the trait `std::io::Write` is not implemented for `&dyn std::io::Write`
    | 
-  ::: $SRC_DIR/std/src/io/buffered.rs:LL:COL
+  ::: $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
    |
 LL | pub struct BufWriter<W: Write> {
    |                         ----- required by this bound in `BufWriter`
@@ -39,7 +39,7 @@ error[E0599]: no method named `write_fmt` found for struct `BufWriter<&dyn std::
 LL |     writeln!(fp, "hello world").unwrap();
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ method not found in `BufWriter<&dyn std::io::Write>`
    | 
-  ::: $SRC_DIR/std/src/io/buffered.rs:LL:COL
+  ::: $SRC_DIR/std/src/io/buffered/bufwriter.rs:LL:COL
    |
 LL | pub struct BufWriter<W: Write> {
    | ------------------------------ doesn't satisfy `BufWriter<&dyn std::io::Write>: std::io::Write`