Adding upstream version 1.64.0+dfsg1.upstream/1.64.0+dfsg1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

25 files changed, 12037 insertions, 0 deletions

diff --git a/library/std/src/io/buffered/bufreader.rs b/library/std/src/io/buffered/bufreader.rs
new file mode 100644
index 000000000..f7fbaa9c2
--- /dev/null
+++ b/library/std/src/io/buffered/bufreader.rs
@@ -0,0 +1,496 @@
+mod buffer;
+
+use crate::fmt;
+use crate::io::{
+    self, BufRead, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, SizeHint, DEFAULT_BUF_SIZE,
+};
+use buffer::Buffer;
+
+/// 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 <code>[Vec]\<u8></code>.
+///
+/// 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 {len} bytes long");
+///     Ok(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct BufReader<R> {
+    inner: R,
+    buf: Buffer,
+}
+
+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> {
+        BufReader { inner, buf: Buffer::with_capacity(capacity) }
+    }
+}
+
+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.buffer()
+    }
+
+    /// 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.capacity()
+    }
+
+    /// 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.buf.discard_buffer()
+    }
+}
+
+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.
+    #[stable(feature = "bufreader_seek_relative", since = "1.53.0")]
+    pub fn seek_relative(&mut self, offset: i64) -> io::Result<()> {
+        let pos = self.buf.pos() as u64;
+        if offset < 0 {
+            if let Some(_) = pos.checked_sub((-offset) as u64) {
+                self.buf.unconsume((-offset) as usize);
+                return Ok(());
+            }
+        } else if let Some(new_pos) = pos.checked_add(offset as u64) {
+            if new_pos <= self.buf.filled() as u64 {
+                self.buf.consume(offset 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.buf.pos() == self.buf.filled() && buf.len() >= self.capacity() {
+            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_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        // 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.buf.pos() == self.buf.filled() && buf.remaining() >= self.capacity() {
+            self.discard_buffer();
+            return self.inner.read_buf(buf);
+        }
+
+        let prev = buf.filled_len();
+
+        let mut rem = self.fill_buf()?;
+        rem.read_buf(buf)?;
+
+        self.consume(buf.filled_len() - prev); //slice impl of read_buf known to never unfill buf
+
+        Ok(())
+    }
+
+    // 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.buf.consume_with(buf.len(), |claimed| buf.copy_from_slice(claimed)) {
+            return Ok(());
+        }
+
+        crate::io::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.buf.pos() == self.buf.filled() && total_len >= self.capacity() {
+            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()
+    }
+
+    // The inner reader might have an optimized `read_to_end`. Drain our buffer and then
+    // delegate to the inner implementation.
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        let inner_buf = self.buffer();
+        buf.extend_from_slice(inner_buf);
+        let nread = inner_buf.len();
+        self.discard_buffer();
+        Ok(nread + self.inner.read_to_end(buf)?)
+    }
+
+    // The inner reader might have an optimized `read_to_end`. Drain our buffer and then
+    // delegate to the inner implementation.
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        // In the general `else` case below we must read bytes into a side buffer, check
+        // that they are valid UTF-8, and then append them to `buf`. This requires a
+        // potentially large memcpy.
+        //
+        // If `buf` is empty--the most common case--we can leverage `append_to_string`
+        // to read directly into `buf`'s internal byte buffer, saving an allocation and
+        // a memcpy.
+        if buf.is_empty() {
+            // `append_to_string`'s safety relies on the buffer only being appended to since
+            // it only checks the UTF-8 validity of new data. If there were existing content in
+            // `buf` then an untrustworthy reader (i.e. `self.inner`) could not only append
+            // bytes but also modify existing bytes and render them invalid. On the other hand,
+            // if `buf` is empty then by definition any writes must be appends and
+            // `append_to_string` will validate all of the new bytes.
+            unsafe { crate::io::append_to_string(buf, |b| self.read_to_end(b)) }
+        } else {
+            // We cannot append our byte buffer directly onto the `buf` String as there could
+            // be an incomplete UTF-8 sequence that has only been partially read. We must read
+            // everything into a side buffer first and then call `from_utf8` on the complete
+            // buffer.
+            let mut bytes = Vec::new();
+            self.read_to_end(&mut bytes)?;
+            let string = crate::str::from_utf8(&bytes).map_err(|_| {
+                io::const_io_error!(
+                    io::ErrorKind::InvalidData,
+                    "stream did not contain valid UTF-8",
+                )
+            })?;
+            *buf += string;
+            Ok(string.len())
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<R: Read> BufRead for BufReader<R> {
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        self.buf.fill_buf(&mut self.inner)
+    }
+
+    fn consume(&mut self, amt: usize) {
+        self.buf.consume(amt)
+    }
+}
+
+#[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.buf.filled() - self.buf.pos(), self.capacity()),
+            )
+            .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 <code>[SeekFrom::Current]\(_)</code> 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 <code>[SeekFrom::Current]\(n)</code>
+    /// 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 <code>[SeekFrom::Current]\(0)</code>.
+    ///
+    /// [`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.buf.filled() - self.buf.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.buf.filled() - self.buf.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))
+    }
+}
diff --git a/library/std/src/io/buffered/bufreader/buffer.rs b/library/std/src/io/buffered/bufreader/buffer.rs
new file mode 100644
index 000000000..8ae01f3b0
--- /dev/null
+++ b/library/std/src/io/buffered/bufreader/buffer.rs
@@ -0,0 +1,105 @@
+///! An encapsulation of `BufReader`'s buffer management logic.
+///
+/// This module factors out the basic functionality of `BufReader` in order to protect two core
+/// invariants:
+/// * `filled` bytes of `buf` are always initialized
+/// * `pos` is always <= `filled`
+/// Since this module encapsulates the buffer management logic, we can ensure that the range
+/// `pos..filled` is always a valid index into the initialized region of the buffer. This means
+/// that user code which wants to do reads from a `BufReader` via `buffer` + `consume` can do so
+/// without encountering any runtime bounds checks.
+use crate::cmp;
+use crate::io::{self, Read, ReadBuf};
+use crate::mem::MaybeUninit;
+
+pub struct Buffer {
+    // The buffer.
+    buf: Box<[MaybeUninit<u8>]>,
+    // The current seek offset into `buf`, must always be <= `filled`.
+    pos: usize,
+    // Each call to `fill_buf` sets `filled` to indicate how many bytes at the start of `buf` are
+    // initialized with bytes from a read.
+    filled: usize,
+}
+
+impl Buffer {
+    #[inline]
+    pub fn with_capacity(capacity: usize) -> Self {
+        let buf = Box::new_uninit_slice(capacity);
+        Self { buf, pos: 0, filled: 0 }
+    }
+
+    #[inline]
+    pub fn buffer(&self) -> &[u8] {
+        // SAFETY: self.pos and self.cap are valid, and self.cap => self.pos, and
+        // that region is initialized because those are all invariants of this type.
+        unsafe { MaybeUninit::slice_assume_init_ref(self.buf.get_unchecked(self.pos..self.filled)) }
+    }
+
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.buf.len()
+    }
+
+    #[inline]
+    pub fn filled(&self) -> usize {
+        self.filled
+    }
+
+    #[inline]
+    pub fn pos(&self) -> usize {
+        self.pos
+    }
+
+    #[inline]
+    pub fn discard_buffer(&mut self) {
+        self.pos = 0;
+        self.filled = 0;
+    }
+
+    #[inline]
+    pub fn consume(&mut self, amt: usize) {
+        self.pos = cmp::min(self.pos + amt, self.filled);
+    }
+
+    /// If there are `amt` bytes available in the buffer, pass a slice containing those bytes to
+    /// `visitor` and return true. If there are not enough bytes available, return false.
+    #[inline]
+    pub fn consume_with<V>(&mut self, amt: usize, mut visitor: V) -> bool
+    where
+        V: FnMut(&[u8]),
+    {
+        if let Some(claimed) = self.buffer().get(..amt) {
+            visitor(claimed);
+            // If the indexing into self.buffer() succeeds, amt must be a valid increment.
+            self.pos += amt;
+            true
+        } else {
+            false
+        }
+    }
+
+    #[inline]
+    pub fn unconsume(&mut self, amt: usize) {
+        self.pos = self.pos.saturating_sub(amt);
+    }
+
+    #[inline]
+    pub fn fill_buf(&mut self, mut reader: impl Read) -> io::Result<&[u8]> {
+        // If we've reached the end of our internal buffer then we need to fetch
+        // some more data from the reader.
+        // Branch using `>=` instead of the more correct `==`
+        // to tell the compiler that the pos..cap slice is always valid.
+        if self.pos >= self.filled {
+            debug_assert!(self.pos == self.filled);
+
+            let mut readbuf = ReadBuf::uninit(&mut self.buf);
+
+            reader.read_buf(&mut readbuf)?;
+
+            self.filled = readbuf.filled_len();
+            self.pos = 0;
+        }
+        Ok(self.buffer())
+    }
+}
diff --git a/library/std/src/io/buffered/bufwriter.rs b/library/std/src/io/buffered/bufwriter.rs
new file mode 100644
index 000000000..6acb937e7
--- /dev/null
+++ b/library/std/src/io/buffered/bufwriter.rs
@@ -0,0 +1,674 @@
+use crate::error;
+use crate::fmt;
+use crate::io::{
+    self, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
+};
+use crate::mem;
+use crate::ptr;
+
+/// 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 <code>[Vec]\<u8></code>.
+///
+/// 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
+#[stable(feature = "rust1", since = "1.0.0")]
+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());
+    /// ```
+    #[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 at least the specified buffer capacity.
+    ///
+    /// # Examples
+    ///
+    /// Creating a buffer with a buffer of at least 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, 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(in crate::io) 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(io::const_io_error!(
+                        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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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();
+    /// ```
+    #[stable(feature = "bufreader_buffer", since = "1.37.0")]
+    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(in crate::io) 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();
+    /// ```
+    #[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.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");
+    /// ```
+    #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+    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()
+    }
+}
+
+#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+/// 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.
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+    pub fn into_inner(self) -> Vec<u8> {
+        self.buf
+    }
+
+    const DESCRIPTION: &'static str =
+        "BufWriter inner writer panicked, what data remains unwritten is not known";
+}
+
+#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+impl error::Error for WriterPanicked {
+    #[allow(deprecated, deprecated_in_future)]
+    fn description(&self) -> &str {
+        Self::DESCRIPTION
+    }
+}
+
+#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+impl fmt::Display for WriterPanicked {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", Self::DESCRIPTION)
+    }
+}
+
+#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+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()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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())
+    }
+}
+
+#[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)
+            .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.panicked {
+            // dtors should not panic, so we ignore a failed flush
+            let _r = self.flush_buf();
+        }
+    }
+}
diff --git a/library/std/src/io/buffered/linewriter.rs b/library/std/src/io/buffered/linewriter.rs
new file mode 100644
index 000000000..a26a4ab33
--- /dev/null
+++ b/library/std/src/io/buffered/linewriter.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 at least the 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_non_exhaustive()
+    }
+}
diff --git a/library/std/src/io/buffered/linewritershim.rs b/library/std/src/io/buffered/linewritershim.rs
new file mode 100644
index 000000000..0175d2693
--- /dev/null
+++ b/library/std/src/io/buffered/linewritershim.rs
@@ -0,0 +1,276 @@
+use crate::io::{self, BufWriter, IoSlice, Write};
+use crate::sys_common::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)
+            }
+        }
+    }
+}
diff --git a/library/std/src/io/buffered/mod.rs b/library/std/src/io/buffered/mod.rs
new file mode 100644
index 000000000..100dab1e2
--- /dev/null
+++ b/library/std/src/io/buffered/mod.rs
@@ -0,0 +1,196 @@
+//! 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;
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::{bufreader::BufReader, bufwriter::BufWriter, linewriter::LineWriter};
+use linewritershim::LineWriterShim;
+
+#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+pub use bufwriter::WriterPanicked;
+
+/// 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
+    }
+
+    /// 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);
+    /// ```
+    #[stable(feature = "io_into_inner_error_parts", since = "1.55.0")]
+    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");
+    /// ```
+    #[stable(feature = "io_into_inner_error_parts", since = "1.55.0")]
+    pub fn into_parts(self) -> (Error, W) {
+        (self.1, 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)
+    }
+}
diff --git a/library/std/src/io/buffered/tests.rs b/library/std/src/io/buffered/tests.rs
new file mode 100644
index 000000000..fe45b1326
--- /dev/null
+++ b/library/std/src/io/buffered/tests.rs
@@ -0,0 +1,1039 @@
+use crate::io::prelude::*;
+use crate::io::{self, BufReader, BufWriter, ErrorKind, IoSlice, LineWriter, ReadBuf, SeekFrom};
+use crate::mem::MaybeUninit;
+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_read_buf() {
+    let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
+    let mut reader = BufReader::with_capacity(2, inner);
+
+    let mut buf = [MaybeUninit::uninit(); 3];
+    let mut buf = ReadBuf::uninit(&mut buf);
+
+    reader.read_buf(&mut buf).unwrap();
+
+    assert_eq!(buf.filled(), [5, 6, 7]);
+    assert_eq!(reader.buffer(), []);
+
+    let mut buf = [MaybeUninit::uninit(); 2];
+    let mut buf = ReadBuf::uninit(&mut buf);
+
+    reader.read_buf(&mut buf).unwrap();
+
+    assert_eq!(buf.filled(), [0, 1]);
+    assert_eq!(reader.buffer(), []);
+
+    let mut buf = [MaybeUninit::uninit(); 1];
+    let mut buf = ReadBuf::uninit(&mut buf);
+
+    reader.read_buf(&mut buf).unwrap();
+
+    assert_eq!(buf.filled(), [2]);
+    assert_eq!(reader.buffer(), [3]);
+
+    let mut buf = [MaybeUninit::uninit(); 3];
+    let mut buf = ReadBuf::uninit(&mut buf);
+
+    reader.read_buf(&mut buf).unwrap();
+
+    assert_eq!(buf.filled(), [3]);
+    assert_eq!(reader.buffer(), []);
+
+    reader.read_buf(&mut buf).unwrap();
+
+    assert_eq!(buf.filled(), [3, 4]);
+    assert_eq!(reader.buffer(), []);
+
+    buf.clear();
+
+    reader.read_buf(&mut buf).unwrap();
+
+    assert_eq!(buf.filled_len(), 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_reader_read_to_end_consumes_buffer() {
+    let data: &[u8] = &[0, 1, 2, 3, 4, 5, 6, 7];
+    let mut reader = BufReader::with_capacity(3, data);
+    let mut buf = Vec::new();
+    assert_eq!(reader.fill_buf().ok(), Some(&[0, 1, 2][..]));
+    assert_eq!(reader.read_to_end(&mut buf).ok(), Some(8));
+    assert_eq!(&buf, &[0, 1, 2, 3, 4, 5, 6, 7]);
+    assert!(reader.buffer().is_empty());
+}
+
+#[test]
+fn test_buffered_reader_read_to_string_consumes_buffer() {
+    let data: &[u8] = "deadbeef".as_bytes();
+    let mut reader = BufReader::with_capacity(3, data);
+    let mut buf = String::new();
+    assert_eq!(reader.fill_buf().ok(), Some("dea".as_bytes()));
+    assert_eq!(reader.read_to_string(&mut buf).ok(), Some(8));
+    assert_eq!(&buf, "deadbeef");
+    assert!(reader.buffer().is_empty());
+}
+
+#[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();
+            core::hint::black_box(&buf);
+        }
+    });
+}
+
+#[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>,
+
+    // 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 {
+            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())]);
+}
diff --git a/library/std/src/io/copy.rs b/library/std/src/io/copy.rs
new file mode 100644
index 000000000..1a10245e4
--- /dev/null
+++ b/library/std/src/io/copy.rs
@@ -0,0 +1,161 @@
+use super::{BufWriter, ErrorKind, Read, ReadBuf, Result, Write, DEFAULT_BUF_SIZE};
+use crate::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(())
+/// }
+/// ```
+///
+/// # Platform-specific behavior
+///
+/// On Linux (including Android), this function uses `copy_file_range(2)`,
+/// `sendfile(2)` or `splice(2)` syscalls to move data directly between file
+/// descriptors if possible.
+///
+/// Note that platform-specific behavior [may change in the future][changes].
+///
+/// [changes]: crate::io#platform-specific-behavior
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn copy<R: ?Sized, W: ?Sized>(reader: &mut R, writer: &mut W) -> Result<u64>
+where
+    R: Read,
+    W: Write,
+{
+    cfg_if::cfg_if! {
+        if #[cfg(any(target_os = "linux", target_os = "android"))] {
+            crate::sys::kernel_copy::copy_spec(reader, writer)
+        } else {
+            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)
+    }
+}
+
+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);
+        }
+
+        let mut len = 0;
+        let mut init = 0;
+
+        loop {
+            let buf = writer.buffer_mut();
+            let mut read_buf = ReadBuf::uninit(buf.spare_capacity_mut());
+
+            // SAFETY: init is either 0 or the initialized_len of the previous iteration
+            unsafe {
+                read_buf.assume_init(init);
+            }
+
+            if read_buf.capacity() >= DEFAULT_BUF_SIZE {
+                match reader.read_buf(&mut read_buf) {
+                    Ok(()) => {
+                        let bytes_read = read_buf.filled_len();
+
+                        if bytes_read == 0 {
+                            return Ok(len);
+                        }
+
+                        init = read_buf.initialized_len() - bytes_read;
+
+                        // SAFETY: ReadBuf guarantees all of its filled bytes are init
+                        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::uninit(); DEFAULT_BUF_SIZE];
+    let mut buf = ReadBuf::uninit(&mut buf);
+
+    let mut len = 0;
+
+    loop {
+        match reader.read_buf(&mut buf) {
+            Ok(()) => {}
+            Err(e) if e.kind() == ErrorKind::Interrupted => continue,
+            Err(e) => return Err(e),
+        };
+
+        if buf.filled().is_empty() {
+            break;
+        }
+
+        len += buf.filled().len() as u64;
+        writer.write_all(buf.filled())?;
+        buf.clear();
+    }
+
+    Ok(len)
+}
diff --git a/library/std/src/io/cursor.rs b/library/std/src/io/cursor.rs
new file mode 100644
index 000000000..f3fbfc447
--- /dev/null
+++ b/library/std/src/io/cursor.rs
@@ -0,0 +1,640 @@
+#[cfg(test)]
+mod tests;
+
+use crate::io::prelude::*;
+
+use crate::alloc::Allocator;
+use crate::cmp;
+use crate::io::{self, ErrorKind, IoSlice, IoSliceMut, ReadBuf, SeekFrom};
+
+/// A `Cursor` wraps an in-memory buffer and provides it with a
+/// [`Seek`] implementation.
+///
+/// `Cursor`s are used with in-memory buffers, anything implementing
+/// <code>[AsRef]<\[u8]></code>, 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 <code>Cursor<[Vec]\<u8>></code> and
+/// <code>Cursor<[&\[u8\]][bytes]></code>.
+///
+/// # 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 "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]);
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+#[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);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
+    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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
+    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();
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
+    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);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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(), &[]);
+    /// ```
+    #[unstable(feature = "cursor_remaining", issue = "86369")]
+    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());
+    /// ```
+    #[unstable(feature = "cursor_remaining", issue = "86369")]
+    pub fn is_empty(&self) -> bool {
+        self.pos >= self.inner.as_ref().len() as u64
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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;
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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),
+        };
+        match base_pos.checked_add_signed(offset) {
+            Some(n) => {
+                self.pos = n;
+                Ok(self.pos)
+            }
+            None => Err(io::const_io_error!(
+                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)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        let prev_filled = buf.filled_len();
+
+        Read::read_buf(&mut self.fill_buf()?, buf)?;
+
+        self.pos += (buf.filled_len() - prev_filled) as u64;
+
+        Ok(())
+    }
+
+    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(())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+}
+
+/// Reserves the required space, and pads the vec with 0s if necessary.
+fn reserve_and_pad<A: Allocator>(
+    pos_mut: &mut u64,
+    vec: &mut Vec<u8, A>,
+    buf_len: usize,
+) -> io::Result<usize> {
+    let pos: usize = (*pos_mut).try_into().map_err(|_| {
+        io::const_io_error!(
+            ErrorKind::InvalidInput,
+            "cursor position exceeds maximum possible vector length",
+        )
+    })?;
+
+    // For safety reasons, we don't want these numbers to overflow
+    // otherwise our allocation won't be enough
+    let desired_cap = pos.saturating_add(buf_len);
+    if desired_cap > vec.capacity() {
+        // We want our vec's total capacity
+        // to have room for (pos+buf_len) bytes. Reserve allocates
+        // based on additional elements from the length, so we need to
+        // reserve the difference
+        vec.reserve(desired_cap - vec.len());
+    }
+    // Pad if pos is above the current len.
+    if pos > vec.len() {
+        let diff = pos - vec.len();
+        // Unfortunately, `resize()` would suffice but the optimiser does not
+        // realise the `reserve` it does can be eliminated. So we do it manually
+        // to eliminate that extra branch
+        let spare = vec.spare_capacity_mut();
+        debug_assert!(spare.len() >= diff);
+        // Safety: we have allocated enough capacity for this.
+        // And we are only writing, not reading
+        unsafe {
+            spare.get_unchecked_mut(..diff).fill(core::mem::MaybeUninit::new(0));
+            vec.set_len(pos);
+        }
+    }
+
+    Ok(pos)
+}
+
+/// Writes the slice to the vec without allocating
+/// # Safety: vec must have buf.len() spare capacity
+unsafe fn vec_write_unchecked<A>(pos: usize, vec: &mut Vec<u8, A>, buf: &[u8]) -> usize
+where
+    A: Allocator,
+{
+    debug_assert!(vec.capacity() >= pos + buf.len());
+    vec.as_mut_ptr().add(pos).copy_from(buf.as_ptr(), buf.len());
+    pos + buf.len()
+}
+
+/// Resizing write implementation for [`Cursor`]
+///
+/// Cursor is allowed to have a pre-allocated and initialised
+/// vector body, but with a position of 0. This means the [`Write`]
+/// will overwrite the contents of the vec.
+///
+/// This also allows for the vec body to be empty, but with a position of N.
+/// This means that [`Write`] will pad the vec with 0 initially,
+/// before writing anything from that point
+fn vec_write<A>(pos_mut: &mut u64, vec: &mut Vec<u8, A>, buf: &[u8]) -> io::Result<usize>
+where
+    A: Allocator,
+{
+    let buf_len = buf.len();
+    let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
+
+    // Write the buf then progress the vec forward if necessary
+    // Safety: we have ensured that the capacity is available
+    // and that all bytes get written up to pos
+    unsafe {
+        pos = vec_write_unchecked(pos, vec, buf);
+        if pos > vec.len() {
+            vec.set_len(pos);
+        }
+    };
+
+    // Bump us forward
+    *pos_mut += buf_len as u64;
+    Ok(buf_len)
+}
+
+/// Resizing write_vectored implementation for [`Cursor`]
+///
+/// Cursor is allowed to have a pre-allocated and initialised
+/// vector body, but with a position of 0. This means the [`Write`]
+/// will overwrite the contents of the vec.
+///
+/// This also allows for the vec body to be empty, but with a position of N.
+/// This means that [`Write`] will pad the vec with 0 initially,
+/// before writing anything from that point
+fn vec_write_vectored<A>(
+    pos_mut: &mut u64,
+    vec: &mut Vec<u8, A>,
+    bufs: &[IoSlice<'_>],
+) -> io::Result<usize>
+where
+    A: Allocator,
+{
+    // For safety reasons, we don't want this sum to overflow ever.
+    // If this saturates, the reserve should panic to avoid any unsound writing.
+    let buf_len = bufs.iter().fold(0usize, |a, b| a.saturating_add(b.len()));
+    let mut pos = reserve_and_pad(pos_mut, vec, buf_len)?;
+
+    // Write the buf then progress the vec forward if necessary
+    // Safety: we have ensured that the capacity is available
+    // and that all bytes get written up to the last pos
+    unsafe {
+        for buf in bufs {
+            pos = vec_write_unchecked(pos, vec, buf);
+        }
+        if pos > vec.len() {
+            vec.set_len(pos);
+        }
+    }
+
+    // Bump us forward
+    *pos_mut += buf_len as u64;
+    Ok(buf_len)
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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(())
+    }
+}
+
+#[stable(feature = "cursor_mut_vec", since = "1.25.0")]
+impl<A> Write for Cursor<&mut Vec<u8, A>>
+where
+    A: Allocator,
+{
+    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(())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<A> Write for Cursor<Vec<u8, A>>
+where
+    A: Allocator,
+{
+    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(())
+    }
+}
+
+#[stable(feature = "cursor_box_slice", since = "1.5.0")]
+impl<A> Write for Cursor<Box<[u8], A>>
+where
+    A: Allocator,
+{
+    #[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(())
+    }
+}
+
+#[stable(feature = "cursor_array", since = "1.61.0")]
+impl<const N: usize> Write for Cursor<[u8; N]> {
+    #[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(())
+    }
+}
diff --git a/library/std/src/io/cursor/tests.rs b/library/std/src/io/cursor/tests.rs
new file mode 100644
index 000000000..d7c203c29
--- /dev/null
+++ b/library/std/src/io/cursor/tests.rs
@@ -0,0 +1,567 @@
+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());
+    writer.set_position(10);
+    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()[..10], &[0; 10]);
+    assert_eq!(&writer.get_ref()[10..], b);
+}
+
+#[test]
+fn test_mem_writer_preallocated() {
+    let mut writer = Cursor::new(vec![0, 0, 0, 0, 0, 0, 0, 0, 8, 9, 10]);
+    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);
+    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);
+}
+
+fn test_slice_writer<T>(writer: &mut Cursor<T>)
+where
+    T: AsRef<[u8]>,
+    Cursor<T>: Write,
+{
+    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().as_ref(), b);
+}
+
+fn test_slice_writer_vectored<T>(writer: &mut Cursor<T>)
+where
+    T: AsRef<[u8]>,
+    Cursor<T>: Write,
+{
+    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().as_ref(), b);
+}
+
+#[test]
+fn test_box_slice_writer() {
+    let mut writer = Cursor::new(vec![0u8; 9].into_boxed_slice());
+    test_slice_writer(&mut writer);
+}
+
+#[test]
+fn test_box_slice_writer_vectored() {
+    let mut writer = Cursor::new(vec![0u8; 9].into_boxed_slice());
+    test_slice_writer_vectored(&mut writer);
+}
+
+#[test]
+fn test_array_writer() {
+    let mut writer = Cursor::new([0u8; 9]);
+    test_slice_writer(&mut writer);
+}
+
+#[test]
+fn test_array_writer_vectored() {
+    let mut writer = Cursor::new([0u8; 9]);
+    test_slice_writer_vectored(&mut writer);
+}
+
+#[test]
+fn test_buf_writer() {
+    let mut buf = [0 as u8; 9];
+    let mut writer = Cursor::new(&mut buf[..]);
+    test_slice_writer(&mut writer);
+}
+
+#[test]
+fn test_buf_writer_vectored() {
+    let mut buf = [0 as u8; 9];
+    let mut writer = Cursor::new(&mut buf[..]);
+    test_slice_writer_vectored(&mut writer);
+}
+
+#[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();
+}
+
+#[bench]
+fn bench_write_vec(b: &mut test::Bencher) {
+    let slice = &[1; 128];
+
+    b.iter(|| {
+        let mut buf = b"some random data to overwrite".to_vec();
+        let mut cursor = Cursor::new(&mut buf);
+
+        let _ = cursor.write_all(slice);
+        test::black_box(&cursor);
+    })
+}
+
+#[bench]
+fn bench_write_vec_vectored(b: &mut test::Bencher) {
+    let slices = [
+        IoSlice::new(&[1; 128]),
+        IoSlice::new(&[2; 256]),
+        IoSlice::new(&[3; 512]),
+        IoSlice::new(&[4; 1024]),
+        IoSlice::new(&[5; 2048]),
+        IoSlice::new(&[6; 4096]),
+        IoSlice::new(&[7; 8192]),
+        IoSlice::new(&[8; 8192 * 2]),
+    ];
+
+    b.iter(|| {
+        let mut buf = b"some random data to overwrite".to_vec();
+        let mut cursor = Cursor::new(&mut buf);
+
+        let mut slices = slices;
+        let _ = cursor.write_all_vectored(&mut slices);
+        test::black_box(&cursor);
+    })
+}
diff --git a/library/std/src/io/error.rs b/library/std/src/io/error.rs
new file mode 100644
index 000000000..ff7fdcae1
--- /dev/null
+++ b/library/std/src/io/error.rs
@@ -0,0 +1,960 @@
+#[cfg(test)]
+mod tests;
+
+#[cfg(target_pointer_width = "64")]
+mod repr_bitpacked;
+#[cfg(target_pointer_width = "64")]
+use repr_bitpacked::Repr;
+
+#[cfg(not(target_pointer_width = "64"))]
+mod repr_unpacked;
+#[cfg(not(target_pointer_width = "64"))]
+use repr_unpacked::Repr;
+
+use crate::convert::From;
+use crate::error;
+use crate::fmt;
+use crate::result;
+use crate::sys;
+
+/// 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)
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+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
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Error {
+    repr: Repr,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(&self.repr, f)
+    }
+}
+
+// Only derive debug in tests, to make sure it
+// doesn't accidentally get printed.
+#[cfg_attr(test, derive(Debug))]
+enum ErrorData<C> {
+    Os(i32),
+    Simple(ErrorKind),
+    SimpleMessage(&'static SimpleMessage),
+    Custom(C),
+}
+
+// `#[repr(align(4))]` is probably redundant, it should have that value or
+// higher already. We include it just because repr_bitpacked.rs's encoding
+// requires an alignment >= 4 (note that `#[repr(align)]` will not reduce the
+// alignment required by the struct, only increase it).
+//
+// If we add more variants to ErrorData, this can be increased to 8, but it
+// should probably be behind `#[cfg_attr(target_pointer_width = "64", ...)]` or
+// whatever cfg we're using to enable the `repr_bitpacked` code, since only the
+// that version needs the alignment, and 8 is higher than the alignment we'll
+// have on 32 bit platforms.
+//
+// (For the sake of being explicit: the alignment requirement here only matters
+// if `error/repr_bitpacked.rs` is in use — for the unpacked repr it doesn't
+// matter at all)
+#[repr(align(4))]
+#[derive(Debug)]
+pub(crate) struct SimpleMessage {
+    kind: ErrorKind,
+    message: &'static str,
+}
+
+impl SimpleMessage {
+    pub(crate) const fn new(kind: ErrorKind, message: &'static str) -> Self {
+        Self { kind, message }
+    }
+}
+
+/// Create and return an `io::Error` for a given `ErrorKind` and constant
+/// message. This doesn't allocate.
+pub(crate) macro const_io_error($kind:expr, $message:expr $(,)?) {
+    $crate::io::error::Error::from_static_message({
+        const MESSAGE_DATA: $crate::io::error::SimpleMessage =
+            $crate::io::error::SimpleMessage::new($kind, $message);
+        &MESSAGE_DATA
+    })
+}
+
+// As with `SimpleMessage`: `#[repr(align(4))]` here is just because
+// repr_bitpacked's encoding requires it. In practice it almost certainly be
+// already be this high or higher.
+#[derive(Debug)]
+#[repr(align(4))]
+struct Custom {
+    kind: ErrorKind,
+    error: Box<dyn error::Error + Send + Sync>,
+}
+
+/// 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
+///
+/// # Handling errors and matching on `ErrorKind`
+///
+/// In application code, use `match` for the `ErrorKind` values you are
+/// expecting; use `_` to match "all other errors".
+///
+/// In comprehensive and thorough tests that want to verify that a test doesn't
+/// return any known incorrect error kind, you may want to cut-and-paste the
+/// current full list of errors from here into your test code, and then match
+/// `_` as the correct case. This seems counterintuitive, but it will make your
+/// tests more robust. In particular, if you want to verify that your code does
+/// produce an unrecognized error kind, the robust solution is to check for all
+/// the recognized error kinds and fail in those cases.
+#[derive(Clone, Copy, Debug, Eq, Hash, Ord, PartialEq, PartialOrd)]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[allow(deprecated)]
+#[non_exhaustive]
+pub enum ErrorKind {
+    /// An entity was not found, often a file.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    NotFound,
+    /// The operation lacked the necessary privileges to complete.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    PermissionDenied,
+    /// The connection was refused by the remote server.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    ConnectionRefused,
+    /// The connection was reset by the remote server.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    ConnectionReset,
+    /// The remote host is not reachable.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    HostUnreachable,
+    /// The network containing the remote host is not reachable.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    NetworkUnreachable,
+    /// The connection was aborted (terminated) by the remote server.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    ConnectionAborted,
+    /// The network operation failed because it was not connected yet.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    NotConnected,
+    /// A socket address could not be bound because the address is already in
+    /// use elsewhere.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    AddrInUse,
+    /// A nonexistent interface was requested or the requested address was not
+    /// local.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    AddrNotAvailable,
+    /// The system's networking is down.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    NetworkDown,
+    /// The operation failed because a pipe was closed.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    BrokenPipe,
+    /// An entity already exists, often a file.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    AlreadyExists,
+    /// The operation needs to block to complete, but the blocking operation was
+    /// requested to not occur.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    WouldBlock,
+    /// A filesystem object is, unexpectedly, not a directory.
+    ///
+    /// For example, a filesystem path was specified where one of the intermediate directory
+    /// components was, in fact, a plain file.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    NotADirectory,
+    /// The filesystem object is, unexpectedly, a directory.
+    ///
+    /// A directory was specified when a non-directory was expected.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    IsADirectory,
+    /// A non-empty directory was specified where an empty directory was expected.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    DirectoryNotEmpty,
+    /// The filesystem or storage medium is read-only, but a write operation was attempted.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    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.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    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.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    StaleNetworkFileHandle,
+    /// A parameter was incorrect.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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
+    #[stable(feature = "io_invalid_data", since = "1.2.0")]
+    InvalidData,
+    /// The I/O operation's timeout expired, causing it to be canceled.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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
+    #[stable(feature = "rust1", since = "1.0.0")]
+    WriteZero,
+    /// The underlying storage (typically, a filesystem) is full.
+    ///
+    /// This does not include out of quota errors.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    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`.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    NotSeekable,
+    /// Filesystem quota was exceeded.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    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.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    FileTooLarge,
+    /// Resource is busy.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    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.)
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    ExecutableFileBusy,
+    /// Deadlock (avoided).
+    ///
+    /// A file locking operation would result in deadlock.  This situation is typically detected, if
+    /// at all, on a best-effort basis.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    Deadlock,
+    /// Cross-device or cross-filesystem (hard) link or rename.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    CrossesDevices,
+    /// Too many (hard) links to the same filesystem object.
+    ///
+    /// The filesystem does not support making so many hardlinks to the same file.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    TooManyLinks,
+    /// A filename was invalid.
+    ///
+    /// This error can also cause if it exceeded the filename length limit.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    InvalidFilename,
+    /// 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.
+    #[unstable(feature = "io_error_more", issue = "86442")]
+    ArgumentListTooLong,
+    /// This operation was interrupted.
+    ///
+    /// Interrupted operations can typically be retried.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    Interrupted,
+
+    /// This operation is unsupported on this platform.
+    ///
+    /// This means that the operation can never succeed.
+    #[stable(feature = "unsupported_error", since = "1.53.0")]
+    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.
+    #[stable(feature = "read_exact", since = "1.6.0")]
+    UnexpectedEof,
+
+    /// An operation could not be completed, because it failed
+    /// to allocate enough memory.
+    #[stable(feature = "out_of_memory_error", since = "1.54.0")]
+    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.
+    ///
+    /// Errors from the standard library that do not fall under any of the I/O
+    /// error kinds cannot be `match`ed on, and will only match a wildcard (`_`) pattern.
+    /// New [`ErrorKind`]s might be added in the future for some of those.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    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.
+    #[unstable(feature = "io_error_uncategorized", issue = "none")]
+    #[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",
+            FilesystemLoop => "filesystem loop or indirection limit (e.g. symlink loop)",
+            FilesystemQuotaExceeded => "filesystem quota exceeded",
+            HostUnreachable => "host unreachable",
+            Interrupted => "operation interrupted",
+            InvalidData => "invalid data",
+            InvalidFilename => "invalid filename",
+            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",
+        }
+    }
+}
+
+#[stable(feature = "io_errorkind_display", since = "1.60.0")]
+impl fmt::Display for ErrorKind {
+    /// Shows a human-readable description of the `ErrorKind`.
+    ///
+    /// This is similar to `impl Display for Error`, but doesn't require first converting to Error.
+    ///
+    /// # Examples
+    /// ```
+    /// use std::io::ErrorKind;
+    /// assert_eq!("entity not found", ErrorKind::NotFound.to_string());
+    /// ```
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt.write_str(self.as_str())
+    }
+}
+
+/// Intended for use for errors not exposed to the user, where allocating onto
+/// the heap (for normal construction via Error::new) is too costly.
+#[stable(feature = "io_error_from_errorkind", since = "1.14.0")]
+impl From<ErrorKind> for Error {
+    /// Converts an [`ErrorKind`] into an [`Error`].
+    ///
+    /// This conversion creates 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::new_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`].
+    ///
+    /// If no extra payload is required, use the `From` conversion from
+    /// `ErrorKind`.
+    ///
+    /// # 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);
+    ///
+    /// // creating an error without payload
+    /// let eof_error = Error::from(ErrorKind::UnexpectedEof);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn new<E>(kind: ErrorKind, error: E) -> Error
+    where
+        E: Into<Box<dyn error::Error + Send + Sync>>,
+    {
+        Self::_new(kind, error.into())
+    }
+
+    /// Creates a new I/O error from an arbitrary error payload.
+    ///
+    /// This function is used to generically create I/O errors which do not
+    /// originate from the OS itself. It is a shortcut for [`Error::new`]
+    /// with [`ErrorKind::Other`].
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(io_error_other)]
+    ///
+    /// use std::io::Error;
+    ///
+    /// // errors can be created from strings
+    /// let custom_error = Error::other("oh no!");
+    ///
+    /// // errors can also be created from other errors
+    /// let custom_error2 = Error::other(custom_error);
+    /// ```
+    #[unstable(feature = "io_error_other", issue = "91946")]
+    pub fn other<E>(error: E) -> Error
+    where
+        E: Into<Box<dyn error::Error + Send + Sync>>,
+    {
+        Self::_new(ErrorKind::Other, error.into())
+    }
+
+    fn _new(kind: ErrorKind, error: Box<dyn error::Error + Send + Sync>) -> Error {
+        Error { repr: Repr::new_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.
+    ///
+    /// You should not use this directly, and instead use the `const_io_error!`
+    /// macro: `io::const_io_error!(ErrorKind::Something, "some_message")`.
+    ///
+    /// This function should maybe change to `from_static_message<const MSG: &'static
+    /// str>(kind: ErrorKind)` in the future, when const generics allow that.
+    #[inline]
+    pub(crate) const fn from_static_message(msg: &'static SimpleMessage) -> Error {
+        Self { repr: Repr::new_simple_message(msg) }
+    }
+
+    /// Returns an error representing the last OS error which occurred.
+    ///
+    /// This function reads the value of `errno` for the target platform (e.g.
+    /// `GetLastError` on Windows) and will return a corresponding instance of
+    /// [`Error`] for the error code.
+    ///
+    /// This should be called immediately after a call to a platform function,
+    /// otherwise the state of the error value is indeterminate. In particular,
+    /// other standard library functions may call platform functions that may
+    /// (or may not) reset the error value even if they succeed.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::Error;
+    ///
+    /// let os_error = Error::last_os_error();
+    /// println!("last OS error: {os_error:?}");
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    #[inline]
+    pub fn last_os_error() -> Error {
+        Error::from_raw_os_error(sys::os::errno() as i32)
+    }
+
+    /// 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);
+    /// # }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    #[inline]
+    pub fn from_raw_os_error(code: i32) -> Error {
+        Error { repr: Repr::new_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!"));
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    #[inline]
+    pub fn raw_os_error(&self) -> Option<i32> {
+        match self.repr.data() {
+            ErrorData::Os(i) => Some(i),
+            ErrorData::Custom(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::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!"));
+    /// }
+    /// ```
+    #[stable(feature = "io_error_inner", since = "1.3.0")]
+    #[must_use]
+    #[inline]
+    pub fn get_ref(&self) -> Option<&(dyn error::Error + Send + Sync + 'static)> {
+        match self.repr.data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(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())));
+    /// }
+    /// ```
+    #[stable(feature = "io_error_inner", since = "1.3.0")]
+    #[must_use]
+    #[inline]
+    pub fn get_mut(&mut self) -> Option<&mut (dyn error::Error + Send + Sync + 'static)> {
+        match self.repr.data_mut() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(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!"));
+    /// }
+    /// ```
+    #[stable(feature = "io_error_inner", since = "1.3.0")]
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[inline]
+    pub fn into_inner(self) -> Option<Box<dyn error::Error + Send + Sync>> {
+        match self.repr.into_data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(c) => Some(c.error),
+        }
+    }
+
+    /// Attempt to downgrade the inner error to `E` if any.
+    ///
+    /// If this [`Error`] was constructed via [`new`] then this function will
+    /// attempt to perform downgrade on it, otherwise it will return [`Err`].
+    ///
+    /// If downgrade succeeds, it will return [`Ok`], otherwise it will also
+    /// return [`Err`].
+    ///
+    /// [`new`]: Error::new
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(io_error_downcast)]
+    ///
+    /// use std::fmt;
+    /// use std::io;
+    /// use std::error::Error;
+    ///
+    /// #[derive(Debug)]
+    /// enum E {
+    ///     Io(io::Error),
+    ///     SomeOtherVariant,
+    /// }
+    ///
+    /// impl fmt::Display for E {
+    ///    // ...
+    /// #    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    /// #        todo!()
+    /// #    }
+    /// }
+    /// impl Error for E {}
+    ///
+    /// impl From<io::Error> for E {
+    ///     fn from(err: io::Error) -> E {
+    ///         err.downcast::<E>()
+    ///             .map(|b| *b)
+    ///             .unwrap_or_else(E::Io)
+    ///     }
+    /// }
+    /// ```
+    #[unstable(feature = "io_error_downcast", issue = "99262")]
+    pub fn downcast<E>(self) -> result::Result<Box<E>, Self>
+    where
+        E: error::Error + Send + Sync + 'static,
+    {
+        match self.repr.into_data() {
+            ErrorData::Custom(b) if b.error.is::<E>() => {
+                let res = (*b).error.downcast::<E>();
+
+                // downcast is a really trivial and is marked as inline, so
+                // it's likely be inlined here.
+                //
+                // And the compiler should be able to eliminate the branch
+                // that produces `Err` here since b.error.is::<E>()
+                // returns true.
+                Ok(res.unwrap())
+            }
+            repr_data => Err(Self { repr: Repr::new(repr_data) }),
+        }
+    }
+
+    /// 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!"));
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    #[must_use]
+    #[inline]
+    pub fn kind(&self) -> ErrorKind {
+        match self.repr.data() {
+            ErrorData::Os(code) => sys::decode_error_kind(code),
+            ErrorData::Custom(c) => c.kind,
+            ErrorData::Simple(kind) => kind,
+            ErrorData::SimpleMessage(m) => m.kind,
+        }
+    }
+}
+
+impl fmt::Debug for Repr {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.data() {
+            ErrorData::Os(code) => fmt
+                .debug_struct("Os")
+                .field("code", &code)
+                .field("kind", &sys::decode_error_kind(code))
+                .field("message", &sys::os::error_string(code))
+                .finish(),
+            ErrorData::Custom(c) => fmt::Debug::fmt(&c, fmt),
+            ErrorData::Simple(kind) => fmt.debug_tuple("Kind").field(&kind).finish(),
+            ErrorData::SimpleMessage(msg) => fmt
+                .debug_struct("Error")
+                .field("kind", &msg.kind)
+                .field("message", &msg.message)
+                .finish(),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl fmt::Display for Error {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self.repr.data() {
+            ErrorData::Os(code) => {
+                let detail = sys::os::error_string(code);
+                write!(fmt, "{detail} (os error {code})")
+            }
+            ErrorData::Custom(ref c) => c.error.fmt(fmt),
+            ErrorData::Simple(kind) => write!(fmt, "{}", kind.as_str()),
+            ErrorData::SimpleMessage(msg) => msg.message.fmt(fmt),
+        }
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl error::Error for Error {
+    #[allow(deprecated, deprecated_in_future)]
+    fn description(&self) -> &str {
+        match self.repr.data() {
+            ErrorData::Os(..) | ErrorData::Simple(..) => self.kind().as_str(),
+            ErrorData::SimpleMessage(msg) => msg.message,
+            ErrorData::Custom(c) => c.error.description(),
+        }
+    }
+
+    #[allow(deprecated)]
+    fn cause(&self) -> Option<&dyn error::Error> {
+        match self.repr.data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(c) => c.error.cause(),
+        }
+    }
+
+    fn source(&self) -> Option<&(dyn error::Error + 'static)> {
+        match self.repr.data() {
+            ErrorData::Os(..) => None,
+            ErrorData::Simple(..) => None,
+            ErrorData::SimpleMessage(..) => None,
+            ErrorData::Custom(c) => c.error.source(),
+        }
+    }
+}
+
+fn _assert_error_is_sync_send() {
+    fn _is_sync_send<T: Sync + Send>() {}
+    _is_sync_send::<Error>();
+}
diff --git a/library/std/src/io/error/repr_bitpacked.rs b/library/std/src/io/error/repr_bitpacked.rs
new file mode 100644
index 000000000..292bf4826
--- /dev/null
+++ b/library/std/src/io/error/repr_bitpacked.rs
@@ -0,0 +1,409 @@
+//! This is a densely packed error representation which is used on targets with
+//! 64-bit pointers.
+//!
+//! (Note that `bitpacked` vs `unpacked` here has no relationship to
+//! `#[repr(packed)]`, it just refers to attempting to use any available bits in
+//! a more clever manner than `rustc`'s default layout algorithm would).
+//!
+//! Conceptually, it stores the same data as the "unpacked" equivalent we use on
+//! other targets. Specifically, you can imagine it as an optimized version of
+//! the following enum (which is roughly equivalent to what's stored by
+//! `repr_unpacked::Repr`, e.g. `super::ErrorData<Box<Custom>>`):
+//!
+//! ```ignore (exposition-only)
+//! enum ErrorData {
+//!    Os(i32),
+//!    Simple(ErrorKind),
+//!    SimpleMessage(&'static SimpleMessage),
+//!    Custom(Box<Custom>),
+//! }
+//! ```
+//!
+//! However, it packs this data into a 64bit non-zero value.
+//!
+//! This optimization not only allows `io::Error` to occupy a single pointer,
+//! but improves `io::Result` as well, especially for situations like
+//! `io::Result<()>` (which is now 64 bits) or `io::Result<u64>` (which is now
+//! 128 bits), which are quite common.
+//!
+//! # Layout
+//! Tagged values are 64 bits, with the 2 least significant bits used for the
+//! tag. This means there are there are 4 "variants":
+//!
+//! - **Tag 0b00**: The first variant is equivalent to
+//!   `ErrorData::SimpleMessage`, and holds a `&'static SimpleMessage` directly.
+//!
+//!   `SimpleMessage` has an alignment >= 4 (which is requested with
+//!   `#[repr(align)]` and checked statically at the bottom of this file), which
+//!   means every `&'static SimpleMessage` should have the both tag bits as 0,
+//!   meaning its tagged and untagged representation are equivalent.
+//!
+//!   This means we can skip tagging it, which is necessary as this variant can
+//!   be constructed from a `const fn`, which probably cannot tag pointers (or
+//!   at least it would be difficult).
+//!
+//! - **Tag 0b01**: The other pointer variant holds the data for
+//!   `ErrorData::Custom` and the remaining 62 bits are used to store a
+//!   `Box<Custom>`. `Custom` also has alignment >= 4, so the bottom two bits
+//!   are free to use for the tag.
+//!
+//!   The only important thing to note is that `ptr::wrapping_add` and
+//!   `ptr::wrapping_sub` are used to tag the pointer, rather than bitwise
+//!   operations. This should preserve the pointer's provenance, which would
+//!   otherwise be lost.
+//!
+//! - **Tag 0b10**: Holds the data for `ErrorData::Os(i32)`. We store the `i32`
+//!   in the pointer's most significant 32 bits, and don't use the bits `2..32`
+//!   for anything. Using the top 32 bits is just to let us easily recover the
+//!   `i32` code with the correct sign.
+//!
+//! - **Tag 0b11**: Holds the data for `ErrorData::Simple(ErrorKind)`. This
+//!   stores the `ErrorKind` in the top 32 bits as well, although it doesn't
+//!   occupy nearly that many. Most of the bits are unused here, but it's not
+//!   like we need them for anything else yet.
+//!
+//! # Use of `NonNull<()>`
+//!
+//! Everything is stored in a `NonNull<()>`, which is odd, but actually serves a
+//! purpose.
+//!
+//! Conceptually you might think of this more like:
+//!
+//! ```ignore (exposition-only)
+//! union Repr {
+//!     // holds integer (Simple/Os) variants, and
+//!     // provides access to the tag bits.
+//!     bits: NonZeroU64,
+//!     // Tag is 0, so this is stored untagged.
+//!     msg: &'static SimpleMessage,
+//!     // Tagged (offset) `Box<Custom>` pointer.
+//!     tagged_custom: NonNull<()>,
+//! }
+//! ```
+//!
+//! But there are a few problems with this:
+//!
+//! 1. Union access is equivalent to a transmute, so this representation would
+//!    require we transmute between integers and pointers in at least one
+//!    direction, which may be UB (and even if not, it is likely harder for a
+//!    compiler to reason about than explicit ptr->int operations).
+//!
+//! 2. Even if all fields of a union have a niche, the union itself doesn't,
+//!    although this may change in the future. This would make things like
+//!    `io::Result<()>` and `io::Result<usize>` larger, which defeats part of
+//!    the motivation of this bitpacking.
+//!
+//! Storing everything in a `NonZeroUsize` (or some other integer) would be a
+//! bit more traditional for pointer tagging, but it would lose provenance
+//! information, couldn't be constructed from a `const fn`, and would probably
+//! run into other issues as well.
+//!
+//! The `NonNull<()>` seems like the only alternative, even if it's fairly odd
+//! to use a pointer type to store something that may hold an integer, some of
+//! the time.
+
+use super::{Custom, ErrorData, ErrorKind, SimpleMessage};
+use alloc::boxed::Box;
+use core::marker::PhantomData;
+use core::mem::{align_of, size_of};
+use core::ptr::{self, NonNull};
+
+// The 2 least-significant bits are used as tag.
+const TAG_MASK: usize = 0b11;
+const TAG_SIMPLE_MESSAGE: usize = 0b00;
+const TAG_CUSTOM: usize = 0b01;
+const TAG_OS: usize = 0b10;
+const TAG_SIMPLE: usize = 0b11;
+
+/// The internal representation.
+///
+/// See the module docs for more, this is just a way to hack in a check that we
+/// indeed are not unwind-safe.
+///
+/// ```compile_fail,E0277
+/// fn is_unwind_safe<T: core::panic::UnwindSafe>() {}
+/// is_unwind_safe::<std::io::Error>();
+/// ```
+#[repr(transparent)]
+pub(super) struct Repr(NonNull<()>, PhantomData<ErrorData<Box<Custom>>>);
+
+// All the types `Repr` stores internally are Send + Sync, and so is it.
+unsafe impl Send for Repr {}
+unsafe impl Sync for Repr {}
+
+impl Repr {
+    pub(super) fn new(dat: ErrorData<Box<Custom>>) -> Self {
+        match dat {
+            ErrorData::Os(code) => Self::new_os(code),
+            ErrorData::Simple(kind) => Self::new_simple(kind),
+            ErrorData::SimpleMessage(simple_message) => Self::new_simple_message(simple_message),
+            ErrorData::Custom(b) => Self::new_custom(b),
+        }
+    }
+
+    pub(super) fn new_custom(b: Box<Custom>) -> Self {
+        let p = Box::into_raw(b).cast::<u8>();
+        // Should only be possible if an allocator handed out a pointer with
+        // wrong alignment.
+        debug_assert_eq!(p.addr() & TAG_MASK, 0);
+        // Note: We know `TAG_CUSTOM <= size_of::<Custom>()` (static_assert at
+        // end of file), and both the start and end of the expression must be
+        // valid without address space wraparound due to `Box`'s semantics.
+        //
+        // This means it would be correct to implement this using `ptr::add`
+        // (rather than `ptr::wrapping_add`), but it's unclear this would give
+        // any benefit, so we just use `wrapping_add` instead.
+        let tagged = p.wrapping_add(TAG_CUSTOM).cast::<()>();
+        // Safety: `TAG_CUSTOM + p` is the same as `TAG_CUSTOM | p`,
+        // because `p`'s alignment means it isn't allowed to have any of the
+        // `TAG_BITS` set (you can verify that addition and bitwise-or are the
+        // same when the operands have no bits in common using a truth table).
+        //
+        // Then, `TAG_CUSTOM | p` is not zero, as that would require
+        // `TAG_CUSTOM` and `p` both be zero, and neither is (as `p` came from a
+        // box, and `TAG_CUSTOM` just... isn't zero -- it's `0b01`). Therefore,
+        // `TAG_CUSTOM + p` isn't zero and so `tagged` can't be, and the
+        // `new_unchecked` is safe.
+        let res = Self(unsafe { NonNull::new_unchecked(tagged) }, PhantomData);
+        // quickly smoke-check we encoded the right thing (This generally will
+        // only run in libstd's tests, unless the user uses -Zbuild-std)
+        debug_assert!(matches!(res.data(), ErrorData::Custom(_)), "repr(custom) encoding failed");
+        res
+    }
+
+    #[inline]
+    pub(super) fn new_os(code: i32) -> Self {
+        let utagged = ((code as usize) << 32) | TAG_OS;
+        // Safety: `TAG_OS` is not zero, so the result of the `|` is not 0.
+        let res = Self(unsafe { NonNull::new_unchecked(ptr::invalid_mut(utagged)) }, PhantomData);
+        // quickly smoke-check we encoded the right thing (This generally will
+        // only run in libstd's tests, unless the user uses -Zbuild-std)
+        debug_assert!(
+            matches!(res.data(), ErrorData::Os(c) if c == code),
+            "repr(os) encoding failed for {code}"
+        );
+        res
+    }
+
+    #[inline]
+    pub(super) fn new_simple(kind: ErrorKind) -> Self {
+        let utagged = ((kind as usize) << 32) | TAG_SIMPLE;
+        // Safety: `TAG_SIMPLE` is not zero, so the result of the `|` is not 0.
+        let res = Self(unsafe { NonNull::new_unchecked(ptr::invalid_mut(utagged)) }, PhantomData);
+        // quickly smoke-check we encoded the right thing (This generally will
+        // only run in libstd's tests, unless the user uses -Zbuild-std)
+        debug_assert!(
+            matches!(res.data(), ErrorData::Simple(k) if k == kind),
+            "repr(simple) encoding failed {:?}",
+            kind,
+        );
+        res
+    }
+
+    #[inline]
+    pub(super) const fn new_simple_message(m: &'static SimpleMessage) -> Self {
+        // Safety: References are never null.
+        Self(unsafe { NonNull::new_unchecked(m as *const _ as *mut ()) }, PhantomData)
+    }
+
+    #[inline]
+    pub(super) fn data(&self) -> ErrorData<&Custom> {
+        // Safety: We're a Repr, decode_repr is fine.
+        unsafe { decode_repr(self.0, |c| &*c) }
+    }
+
+    #[inline]
+    pub(super) fn data_mut(&mut self) -> ErrorData<&mut Custom> {
+        // Safety: We're a Repr, decode_repr is fine.
+        unsafe { decode_repr(self.0, |c| &mut *c) }
+    }
+
+    #[inline]
+    pub(super) fn into_data(self) -> ErrorData<Box<Custom>> {
+        let this = core::mem::ManuallyDrop::new(self);
+        // Safety: We're a Repr, decode_repr is fine. The `Box::from_raw` is
+        // safe because we prevent double-drop using `ManuallyDrop`.
+        unsafe { decode_repr(this.0, |p| Box::from_raw(p)) }
+    }
+}
+
+impl Drop for Repr {
+    #[inline]
+    fn drop(&mut self) {
+        // Safety: We're a Repr, decode_repr is fine. The `Box::from_raw` is
+        // safe because we're being dropped.
+        unsafe {
+            let _ = decode_repr(self.0, |p| Box::<Custom>::from_raw(p));
+        }
+    }
+}
+
+// Shared helper to decode a `Repr`'s internal pointer into an ErrorData.
+//
+// Safety: `ptr`'s bits should be encoded as described in the document at the
+// top (it should `some_repr.0`)
+#[inline]
+unsafe fn decode_repr<C, F>(ptr: NonNull<()>, make_custom: F) -> ErrorData<C>
+where
+    F: FnOnce(*mut Custom) -> C,
+{
+    let bits = ptr.as_ptr().addr();
+    match bits & TAG_MASK {
+        TAG_OS => {
+            let code = ((bits as i64) >> 32) as i32;
+            ErrorData::Os(code)
+        }
+        TAG_SIMPLE => {
+            let kind_bits = (bits >> 32) as u32;
+            let kind = kind_from_prim(kind_bits).unwrap_or_else(|| {
+                debug_assert!(false, "Invalid io::error::Repr bits: `Repr({:#018x})`", bits);
+                // This means the `ptr` passed in was not valid, which violates
+                // the unsafe contract of `decode_repr`.
+                //
+                // Using this rather than unwrap meaningfully improves the code
+                // for callers which only care about one variant (usually
+                // `Custom`)
+                core::hint::unreachable_unchecked();
+            });
+            ErrorData::Simple(kind)
+        }
+        TAG_SIMPLE_MESSAGE => ErrorData::SimpleMessage(&*ptr.cast::<SimpleMessage>().as_ptr()),
+        TAG_CUSTOM => {
+            // It would be correct for us to use `ptr::sub` here (see the
+            // comment above the `wrapping_add` call in `new_custom` for why),
+            // but it isn't clear that it makes a difference, so we don't.
+            let custom = ptr.as_ptr().cast::<u8>().wrapping_sub(TAG_CUSTOM).cast::<Custom>();
+            ErrorData::Custom(make_custom(custom))
+        }
+        _ => {
+            // Can't happen, and compiler can tell
+            unreachable!();
+        }
+    }
+}
+
+// This compiles to the same code as the check+transmute, but doesn't require
+// unsafe, or to hard-code max ErrorKind or its size in a way the compiler
+// couldn't verify.
+#[inline]
+fn kind_from_prim(ek: u32) -> Option<ErrorKind> {
+    macro_rules! from_prim {
+        ($prim:expr => $Enum:ident { $($Variant:ident),* $(,)? }) => {{
+            // Force a compile error if the list gets out of date.
+            const _: fn(e: $Enum) = |e: $Enum| match e {
+                $($Enum::$Variant => ()),*
+            };
+            match $prim {
+                $(v if v == ($Enum::$Variant as _) => Some($Enum::$Variant),)*
+                _ => None,
+            }
+        }}
+    }
+    from_prim!(ek => ErrorKind {
+        NotFound,
+        PermissionDenied,
+        ConnectionRefused,
+        ConnectionReset,
+        HostUnreachable,
+        NetworkUnreachable,
+        ConnectionAborted,
+        NotConnected,
+        AddrInUse,
+        AddrNotAvailable,
+        NetworkDown,
+        BrokenPipe,
+        AlreadyExists,
+        WouldBlock,
+        NotADirectory,
+        IsADirectory,
+        DirectoryNotEmpty,
+        ReadOnlyFilesystem,
+        FilesystemLoop,
+        StaleNetworkFileHandle,
+        InvalidInput,
+        InvalidData,
+        TimedOut,
+        WriteZero,
+        StorageFull,
+        NotSeekable,
+        FilesystemQuotaExceeded,
+        FileTooLarge,
+        ResourceBusy,
+        ExecutableFileBusy,
+        Deadlock,
+        CrossesDevices,
+        TooManyLinks,
+        InvalidFilename,
+        ArgumentListTooLong,
+        Interrupted,
+        Other,
+        UnexpectedEof,
+        Unsupported,
+        OutOfMemory,
+        Uncategorized,
+    })
+}
+
+// Some static checking to alert us if a change breaks any of the assumptions
+// that our encoding relies on for correctness and soundness. (Some of these are
+// a bit overly thorough/cautious, admittedly)
+//
+// If any of these are hit on a platform that libstd supports, we should likely
+// just use `repr_unpacked.rs` there instead (unless the fix is easy).
+macro_rules! static_assert {
+    ($condition:expr) => {
+        const _: () = assert!($condition);
+    };
+    (@usize_eq: $lhs:expr, $rhs:expr) => {
+        const _: [(); $lhs] = [(); $rhs];
+    };
+}
+
+// The bitpacking we use requires pointers be exactly 64 bits.
+static_assert!(@usize_eq: size_of::<NonNull<()>>(), 8);
+
+// We also require pointers and usize be the same size.
+static_assert!(@usize_eq: size_of::<NonNull<()>>(), size_of::<usize>());
+
+// `Custom` and `SimpleMessage` need to be thin pointers.
+static_assert!(@usize_eq: size_of::<&'static SimpleMessage>(), 8);
+static_assert!(@usize_eq: size_of::<Box<Custom>>(), 8);
+
+static_assert!((TAG_MASK + 1).is_power_of_two());
+// And they must have sufficient alignment.
+static_assert!(align_of::<SimpleMessage>() >= TAG_MASK + 1);
+static_assert!(align_of::<Custom>() >= TAG_MASK + 1);
+
+static_assert!(@usize_eq: (TAG_MASK & TAG_SIMPLE_MESSAGE), TAG_SIMPLE_MESSAGE);
+static_assert!(@usize_eq: (TAG_MASK & TAG_CUSTOM), TAG_CUSTOM);
+static_assert!(@usize_eq: (TAG_MASK & TAG_OS), TAG_OS);
+static_assert!(@usize_eq: (TAG_MASK & TAG_SIMPLE), TAG_SIMPLE);
+
+// This is obviously true (`TAG_CUSTOM` is `0b01`), but in `Repr::new_custom` we
+// offset a pointer by this value, and expect it to both be within the same
+// object, and to not wrap around the address space. See the comment in that
+// function for further details.
+//
+// Actually, at the moment we use `ptr::wrapping_add`, not `ptr::add`, so this
+// check isn't needed for that one, although the assertion that we don't
+// actually wrap around in that wrapping_add does simplify the safety reasoning
+// elsewhere considerably.
+static_assert!(size_of::<Custom>() >= TAG_CUSTOM);
+
+// These two store a payload which is allowed to be zero, so they must be
+// non-zero to preserve the `NonNull`'s range invariant.
+static_assert!(TAG_OS != 0);
+static_assert!(TAG_SIMPLE != 0);
+// We can't tag `SimpleMessage`s, the tag must be 0.
+static_assert!(@usize_eq: TAG_SIMPLE_MESSAGE, 0);
+
+// Check that the point of all of this still holds.
+//
+// We'd check against `io::Error`, but *technically* it's allowed to vary,
+// as it's not `#[repr(transparent)]`/`#[repr(C)]`. We could add that, but
+// the `#[repr()]` would show up in rustdoc, which might be seen as a stable
+// commitment.
+static_assert!(@usize_eq: size_of::<Repr>(), 8);
+static_assert!(@usize_eq: size_of::<Option<Repr>>(), 8);
+static_assert!(@usize_eq: size_of::<Result<(), Repr>>(), 8);
+static_assert!(@usize_eq: size_of::<Result<usize, Repr>>(), 16);
diff --git a/library/std/src/io/error/repr_unpacked.rs b/library/std/src/io/error/repr_unpacked.rs
new file mode 100644
index 000000000..d6ad55b99
--- /dev/null
+++ b/library/std/src/io/error/repr_unpacked.rs
@@ -0,0 +1,54 @@
+//! This is a fairly simple unpacked error representation that's used on
+//! non-64bit targets, where the packed 64 bit representation wouldn't work, and
+//! would have no benefit.
+
+use super::{Custom, ErrorData, ErrorKind, SimpleMessage};
+use alloc::boxed::Box;
+
+type Inner = ErrorData<Box<Custom>>;
+
+pub(super) struct Repr(Inner);
+
+impl Repr {
+    #[inline]
+    pub(super) fn new(dat: ErrorData<Box<Custom>>) -> Self {
+        Self(dat)
+    }
+    pub(super) fn new_custom(b: Box<Custom>) -> Self {
+        Self(Inner::Custom(b))
+    }
+    #[inline]
+    pub(super) fn new_os(code: i32) -> Self {
+        Self(Inner::Os(code))
+    }
+    #[inline]
+    pub(super) fn new_simple(kind: ErrorKind) -> Self {
+        Self(Inner::Simple(kind))
+    }
+    #[inline]
+    pub(super) const fn new_simple_message(m: &'static SimpleMessage) -> Self {
+        Self(Inner::SimpleMessage(m))
+    }
+    #[inline]
+    pub(super) fn into_data(self) -> ErrorData<Box<Custom>> {
+        self.0
+    }
+    #[inline]
+    pub(super) fn data(&self) -> ErrorData<&Custom> {
+        match &self.0 {
+            Inner::Os(c) => ErrorData::Os(*c),
+            Inner::Simple(k) => ErrorData::Simple(*k),
+            Inner::SimpleMessage(m) => ErrorData::SimpleMessage(*m),
+            Inner::Custom(m) => ErrorData::Custom(&*m),
+        }
+    }
+    #[inline]
+    pub(super) fn data_mut(&mut self) -> ErrorData<&mut Custom> {
+        match &mut self.0 {
+            Inner::Os(c) => ErrorData::Os(*c),
+            Inner::Simple(k) => ErrorData::Simple(*k),
+            Inner::SimpleMessage(m) => ErrorData::SimpleMessage(*m),
+            Inner::Custom(m) => ErrorData::Custom(&mut *m),
+        }
+    }
+}
diff --git a/library/std/src/io/error/tests.rs b/library/std/src/io/error/tests.rs
new file mode 100644
index 000000000..c897a5e87
--- /dev/null
+++ b/library/std/src/io/error/tests.rs
@@ -0,0 +1,194 @@
+use super::{const_io_error, Custom, Error, ErrorData, ErrorKind, Repr, SimpleMessage};
+use crate::assert_matches::assert_matches;
+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::new_custom(Box::new(Custom {
+            kind: ErrorKind::InvalidInput,
+            error: Box::new(Error { repr: super::Repr::new_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 = const_io_error!(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"));
+}
+
+#[test]
+fn test_os_packing() {
+    for code in -20i32..20i32 {
+        let e = Error::from_raw_os_error(code);
+        assert_eq!(e.raw_os_error(), Some(code));
+        assert_matches!(
+            e.repr.data(),
+            ErrorData::Os(c) if c == code,
+        );
+    }
+}
+
+#[test]
+fn test_errorkind_packing() {
+    assert_eq!(Error::from(ErrorKind::NotFound).kind(), ErrorKind::NotFound);
+    assert_eq!(Error::from(ErrorKind::PermissionDenied).kind(), ErrorKind::PermissionDenied);
+    assert_eq!(Error::from(ErrorKind::Uncategorized).kind(), ErrorKind::Uncategorized);
+    // Check that the innards look like like what we want.
+    assert_matches!(
+        Error::from(ErrorKind::OutOfMemory).repr.data(),
+        ErrorData::Simple(ErrorKind::OutOfMemory),
+    );
+}
+
+#[test]
+fn test_simple_message_packing() {
+    use super::{ErrorKind::*, SimpleMessage};
+    macro_rules! check_simple_msg {
+        ($err:expr, $kind:ident, $msg:literal) => {{
+            let e = &$err;
+            // Check that the public api is right.
+            assert_eq!(e.kind(), $kind);
+            assert!(format!("{e:?}").contains($msg));
+            // and we got what we expected
+            assert_matches!(
+                e.repr.data(),
+                ErrorData::SimpleMessage(SimpleMessage { kind: $kind, message: $msg })
+            );
+        }};
+    }
+
+    let not_static = const_io_error!(Uncategorized, "not a constant!");
+    check_simple_msg!(not_static, Uncategorized, "not a constant!");
+
+    const CONST: Error = const_io_error!(NotFound, "definitely a constant!");
+    check_simple_msg!(CONST, NotFound, "definitely a constant!");
+
+    static STATIC: Error = const_io_error!(BrokenPipe, "a constant, sort of!");
+    check_simple_msg!(STATIC, BrokenPipe, "a constant, sort of!");
+}
+
+#[derive(Debug, PartialEq)]
+struct Bojji(bool);
+impl error::Error for Bojji {}
+impl fmt::Display for Bojji {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "ah! {:?}", self)
+    }
+}
+
+#[test]
+fn test_custom_error_packing() {
+    use super::Custom;
+    let test = Error::new(ErrorKind::Uncategorized, Bojji(true));
+    assert_matches!(
+        test.repr.data(),
+        ErrorData::Custom(Custom {
+            kind: ErrorKind::Uncategorized,
+            error,
+        }) if error.downcast_ref::<Bojji>().as_deref() == Some(&Bojji(true)),
+    );
+}
+
+#[derive(Debug)]
+struct E;
+
+impl fmt::Display for E {
+    fn fmt(&self, _f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        Ok(())
+    }
+}
+
+impl error::Error for E {}
+
+#[test]
+fn test_std_io_error_downcast() {
+    // Case 1: custom error, downcast succeeds
+    let io_error = Error::new(ErrorKind::Other, Bojji(true));
+    let e: Box<Bojji> = io_error.downcast().unwrap();
+    assert!(e.0);
+
+    // Case 2: custom error, downcast fails
+    let io_error = Error::new(ErrorKind::Other, Bojji(true));
+    let io_error = io_error.downcast::<E>().unwrap_err();
+
+    //   ensures that the custom error is intact
+    assert_eq!(ErrorKind::Other, io_error.kind());
+    let e: Box<Bojji> = io_error.downcast().unwrap();
+    assert!(e.0);
+
+    // Case 3: os error
+    let errno = 20;
+    let io_error = Error::from_raw_os_error(errno);
+    let io_error = io_error.downcast::<E>().unwrap_err();
+
+    assert_eq!(errno, io_error.raw_os_error().unwrap());
+
+    // Case 4: simple
+    let kind = ErrorKind::OutOfMemory;
+    let io_error: Error = kind.into();
+    let io_error = io_error.downcast::<E>().unwrap_err();
+
+    assert_eq!(kind, io_error.kind());
+
+    // Case 5: simple message
+    const SIMPLE_MESSAGE: SimpleMessage =
+        SimpleMessage { kind: ErrorKind::Other, message: "simple message error test" };
+    let io_error = Error::from_static_message(&SIMPLE_MESSAGE);
+    let io_error = io_error.downcast::<E>().unwrap_err();
+
+    assert_eq!(SIMPLE_MESSAGE.kind, io_error.kind());
+    assert_eq!(SIMPLE_MESSAGE.message, &*format!("{io_error}"));
+}
diff --git a/library/std/src/io/impls.rs b/library/std/src/io/impls.rs
new file mode 100644
index 000000000..950725473
--- /dev/null
+++ b/library/std/src/io/impls.rs
@@ -0,0 +1,458 @@
+#[cfg(test)]
+mod tests;
+
+use crate::alloc::Allocator;
+use crate::cmp;
+use crate::collections::VecDeque;
+use crate::fmt;
+use crate::io::{
+    self, BufRead, ErrorKind, IoSlice, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, Write,
+};
+use crate::mem;
+
+// =============================================================================
+// Forwarding implementations
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        (**self).read_buf(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]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (**self).read_to_end(buf)
+    }
+
+    #[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)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+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()
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        (**self).read_buf(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]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        (**self).read_to_end(buf)
+    }
+
+    #[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)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+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)
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+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()
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+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.
+#[stable(feature = "rust1", since = "1.0.0")]
+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_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        let amt = cmp::min(buf.remaining(), self.len());
+        let (a, b) = self.split_at(amt);
+
+        buf.append(a);
+
+        *self = b;
+        Ok(())
+    }
+
+    #[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]
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        if buf.len() > self.len() {
+            return Err(io::const_io_error!(
+                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(())
+    }
+
+    #[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)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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`.
+#[stable(feature = "rust1", since = "1.0.0")]
+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(io::const_io_error!(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.
+#[stable(feature = "rust1", since = "1.0.0")]
+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(())
+    }
+}
+
+/// Read is implemented for `VecDeque<u8>` by consuming bytes from the front of the `VecDeque`.
+#[stable(feature = "vecdeque_read_write", since = "1.63.0")]
+impl<A: Allocator> Read for VecDeque<u8, A> {
+    /// Fill `buf` with the contents of the "front" slice as returned by
+    /// [`as_slices`][`VecDeque::as_slices`]. If the contained byte slices of the `VecDeque` are
+    /// discontiguous, multiple calls to `read` will be needed to read the entire content.
+    #[inline]
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        let (ref mut front, _) = self.as_slices();
+        let n = Read::read(front, buf)?;
+        self.drain(..n);
+        Ok(n)
+    }
+
+    #[inline]
+    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        let (ref mut front, _) = self.as_slices();
+        let n = cmp::min(buf.remaining(), front.len());
+        Read::read_buf(front, buf)?;
+        self.drain(..n);
+        Ok(())
+    }
+}
+
+/// Write is implemented for `VecDeque<u8>` by appending to the `VecDeque`, growing it as needed.
+#[stable(feature = "vecdeque_read_write", since = "1.63.0")]
+impl<A: Allocator> Write for VecDeque<u8, A> {
+    #[inline]
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.extend(buf);
+        Ok(buf.len())
+    }
+
+    #[inline]
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.extend(buf);
+        Ok(())
+    }
+
+    #[inline]
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
diff --git a/library/std/src/io/impls/tests.rs b/library/std/src/io/impls/tests.rs
new file mode 100644
index 000000000..d1cd84a67
--- /dev/null
+++ b/library/std/src/io/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);
+        }
+    })
+}
diff --git a/library/std/src/io/mod.rs b/library/std/src/io/mod.rs
new file mode 100644
index 000000000..96addbd1a
--- /dev/null
+++ b/library/std/src/io/mod.rs
@@ -0,0 +1,2827 @@
+//! Traits, helpers, and type definitions for core I/O functionality.
+//!
+//! The `std::io` module contains a number of common things you'll need
+//! when doing input and output. The most core part of this module is
+//! the [`Read`] and [`Write`] traits, which provide the
+//! most general interface for reading and writing input and output.
+//!
+//! # Read and Write
+//!
+//! Because they are traits, [`Read`] and [`Write`] are implemented by a number
+//! of other types, and you can implement them for your types too. As such,
+//! you'll see a few different types of I/O throughout the documentation in
+//! this module: [`File`]s, [`TcpStream`]s, and sometimes even [`Vec<T>`]s. For
+//! example, [`Read`] adds a [`read`][`Read::read`] method, which we can use on
+//! [`File`]s:
+//!
+//! ```no_run
+//! use std::io;
+//! use std::io::prelude::*;
+//! use std::fs::File;
+//!
+//! fn main() -> io::Result<()> {
+//!     let mut f = File::open("foo.txt")?;
+//!     let mut buffer = [0; 10];
+//!
+//!     // read up to 10 bytes
+//!     let n = f.read(&mut buffer)?;
+//!
+//!     println!("The bytes: {:?}", &buffer[..n]);
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [`Read`] and [`Write`] are so important, implementors of the two traits have a
+//! nickname: readers and writers. So you'll sometimes see 'a reader' instead
+//! of 'a type that implements the [`Read`] trait'. Much easier!
+//!
+//! ## Seek and BufRead
+//!
+//! Beyond that, there are two important traits that are provided: [`Seek`]
+//! and [`BufRead`]. Both of these build on top of a reader to control
+//! how the reading happens. [`Seek`] lets you control where the next byte is
+//! coming from:
+//!
+//! ```no_run
+//! use std::io;
+//! use std::io::prelude::*;
+//! use std::io::SeekFrom;
+//! use std::fs::File;
+//!
+//! fn main() -> io::Result<()> {
+//!     let mut f = File::open("foo.txt")?;
+//!     let mut buffer = [0; 10];
+//!
+//!     // skip to the last 10 bytes of the file
+//!     f.seek(SeekFrom::End(-10))?;
+//!
+//!     // read up to 10 bytes
+//!     let n = f.read(&mut buffer)?;
+//!
+//!     println!("The bytes: {:?}", &buffer[..n]);
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [`BufRead`] uses an internal buffer to provide a number of other ways to read, but
+//! to show it off, we'll need to talk about buffers in general. Keep reading!
+//!
+//! ## BufReader and BufWriter
+//!
+//! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
+//! making near-constant calls to the operating system. To help with this,
+//! `std::io` comes with two structs, [`BufReader`] and [`BufWriter`], which wrap
+//! readers and writers. The wrapper uses a buffer, reducing the number of
+//! calls and providing nicer methods for accessing exactly what you want.
+//!
+//! For example, [`BufReader`] works with the [`BufRead`] trait to add extra
+//! methods to any reader:
+//!
+//! ```no_run
+//! use std::io;
+//! use std::io::prelude::*;
+//! use std::io::BufReader;
+//! use std::fs::File;
+//!
+//! fn main() -> io::Result<()> {
+//!     let f = File::open("foo.txt")?;
+//!     let mut reader = BufReader::new(f);
+//!     let mut buffer = String::new();
+//!
+//!     // read a line into buffer
+//!     reader.read_line(&mut buffer)?;
+//!
+//!     println!("{buffer}");
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
+//! to [`write`][`Write::write`]:
+//!
+//! ```no_run
+//! use std::io;
+//! use std::io::prelude::*;
+//! use std::io::BufWriter;
+//! use std::fs::File;
+//!
+//! fn main() -> io::Result<()> {
+//!     let f = File::create("foo.txt")?;
+//!     {
+//!         let mut writer = BufWriter::new(f);
+//!
+//!         // write a byte to the buffer
+//!         writer.write(&[42])?;
+//!
+//!     } // the buffer is flushed once writer goes out of scope
+//!
+//!     Ok(())
+//! }
+//! ```
+//!
+//! ## Standard input and output
+//!
+//! A very common source of input is standard input:
+//!
+//! ```no_run
+//! use std::io;
+//!
+//! fn main() -> io::Result<()> {
+//!     let mut input = String::new();
+//!
+//!     io::stdin().read_line(&mut input)?;
+//!
+//!     println!("You typed: {}", input.trim());
+//!     Ok(())
+//! }
+//! ```
+//!
+//! Note that you cannot use the [`?` operator] in functions that do not return
+//! a [`Result<T, E>`][`Result`]. Instead, you can call [`.unwrap()`]
+//! or `match` on the return value to catch any possible errors:
+//!
+//! ```no_run
+//! use std::io;
+//!
+//! let mut input = String::new();
+//!
+//! io::stdin().read_line(&mut input).unwrap();
+//! ```
+//!
+//! And a very common source of output is standard output:
+//!
+//! ```no_run
+//! use std::io;
+//! use std::io::prelude::*;
+//!
+//! fn main() -> io::Result<()> {
+//!     io::stdout().write(&[42])?;
+//!     Ok(())
+//! }
+//! ```
+//!
+//! Of course, using [`io::stdout`] directly is less common than something like
+//! [`println!`].
+//!
+//! ## Iterator types
+//!
+//! A large number of the structures provided by `std::io` are for various
+//! ways of iterating over I/O. For example, [`Lines`] is used to split over
+//! lines:
+//!
+//! ```no_run
+//! use std::io;
+//! use std::io::prelude::*;
+//! use std::io::BufReader;
+//! use std::fs::File;
+//!
+//! fn main() -> io::Result<()> {
+//!     let f = File::open("foo.txt")?;
+//!     let reader = BufReader::new(f);
+//!
+//!     for line in reader.lines() {
+//!         println!("{}", line?);
+//!     }
+//!     Ok(())
+//! }
+//! ```
+//!
+//! ## Functions
+//!
+//! There are a number of [functions][functions-list] that offer access to various
+//! features. For example, we can use three of these functions to copy everything
+//! from standard input to standard output:
+//!
+//! ```no_run
+//! use std::io;
+//!
+//! fn main() -> io::Result<()> {
+//!     io::copy(&mut io::stdin(), &mut io::stdout())?;
+//!     Ok(())
+//! }
+//! ```
+//!
+//! [functions-list]: #functions-1
+//!
+//! ## io::Result
+//!
+//! Last, but certainly not least, is [`io::Result`]. This type is used
+//! as the return type of many `std::io` functions that can cause an error, and
+//! can be returned from your own functions as well. Many of the examples in this
+//! module use the [`?` operator]:
+//!
+//! ```
+//! use std::io;
+//!
+//! fn read_input() -> io::Result<()> {
+//!     let mut input = String::new();
+//!
+//!     io::stdin().read_line(&mut input)?;
+//!
+//!     println!("You typed: {}", input.trim());
+//!
+//!     Ok(())
+//! }
+//! ```
+//!
+//! The return type of `read_input()`, [`io::Result<()>`][`io::Result`], is a very
+//! common type for functions which don't have a 'real' return value, but do want to
+//! return errors if they happen. In this case, the only purpose of this function is
+//! to read the line and print it, so we use `()`.
+//!
+//! ## Platform-specific behavior
+//!
+//! Many I/O functions throughout the standard library are documented to indicate
+//! what various library or syscalls they are delegated to. This is done to help
+//! applications both understand what's happening under the hood as well as investigate
+//! any possibly unclear semantics. Note, however, that this is informative, not a binding
+//! contract. The implementation of many of these functions are subject to change over
+//! time and may call fewer or more syscalls/library functions.
+//!
+//! [`File`]: crate::fs::File
+//! [`TcpStream`]: crate::net::TcpStream
+//! [`io::stdout`]: stdout
+//! [`io::Result`]: self::Result
+//! [`?` operator]: ../../book/appendix-02-operators.html
+//! [`Result`]: crate::result::Result
+//! [`.unwrap()`]: crate::result::Result::unwrap
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+#[cfg(test)]
+mod tests;
+
+use crate::cmp;
+use crate::fmt;
+use crate::mem::replace;
+use crate::ops::{Deref, DerefMut};
+use crate::slice;
+use crate::str;
+use crate::sys;
+use crate::sys_common::memchr;
+
+#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
+pub use self::buffered::WriterPanicked;
+#[unstable(feature = "internal_output_capture", issue = "none")]
+#[doc(no_inline, hidden)]
+pub use self::stdio::set_output_capture;
+#[unstable(feature = "print_internals", issue = "none")]
+pub use self::stdio::{_eprint, _print};
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use self::{
+    buffered::{BufReader, BufWriter, IntoInnerError, LineWriter},
+    copy::copy,
+    cursor::Cursor,
+    error::{Error, ErrorKind, Result},
+    stdio::{stderr, stdin, stdout, Stderr, StderrLock, Stdin, StdinLock, Stdout, StdoutLock},
+    util::{empty, repeat, sink, Empty, Repeat, Sink},
+};
+
+#[unstable(feature = "read_buf", issue = "78485")]
+pub use self::readbuf::ReadBuf;
+pub(crate) use error::const_io_error;
+
+mod buffered;
+pub(crate) mod copy;
+mod cursor;
+mod error;
+mod impls;
+pub mod prelude;
+mod readbuf;
+mod stdio;
+mod util;
+
+const DEFAULT_BUF_SIZE: usize = crate::sys_common::io::DEFAULT_BUF_SIZE;
+
+pub(crate) use stdio::cleanup;
+
+struct Guard<'a> {
+    buf: &'a mut Vec<u8>,
+    len: usize,
+}
+
+impl Drop for Guard<'_> {
+    fn drop(&mut self) {
+        unsafe {
+            self.buf.set_len(self.len);
+        }
+    }
+}
+
+// Several `read_to_string` and `read_line` methods in the standard library will
+// append data into a `String` buffer, but we need to be pretty careful when
+// doing this. The implementation will just call `.as_mut_vec()` and then
+// delegate to a byte-oriented reading method, but we must ensure that when
+// returning we never leave `buf` in a state such that it contains invalid UTF-8
+// in its bounds.
+//
+// To this end, we use an RAII guard (to protect against panics) which updates
+// the length of the string when it is dropped. This guard initially truncates
+// the string to the prior length and only after we've validated that the
+// new contents are valid UTF-8 do we allow it to set a longer length.
+//
+// The unsafety in this function is twofold:
+//
+// 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
+//    checks.
+// 2. We're passing a raw buffer to the function `f`, and it is expected that
+//    the function only *appends* bytes to the buffer. We'll get undefined
+//    behavior if existing bytes are overwritten to have non-UTF-8 data.
+pub(crate) unsafe fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
+where
+    F: FnOnce(&mut Vec<u8>) -> Result<usize>,
+{
+    let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
+    let ret = f(g.buf);
+    if str::from_utf8(&g.buf[g.len..]).is_err() {
+        ret.and_then(|_| {
+            Err(error::const_io_error!(
+                ErrorKind::InvalidData,
+                "stream did not contain valid UTF-8"
+            ))
+        })
+    } else {
+        g.len = g.buf.len();
+        ret
+    }
+}
+
+// This uses an adaptive system to extend the vector when it fills. We want to
+// avoid paying to allocate and zero a huge chunk of memory if the reader only
+// has 4 bytes while still making large reads if the reader does have a ton
+// of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
+// time is 4,500 times (!) slower than a default reservation size of 32 if the
+// reader has a very small amount of data to return.
+pub(crate) fn default_read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
+    let start_len = buf.len();
+    let start_cap = buf.capacity();
+
+    let mut initialized = 0; // Extra initialized bytes from previous loop iteration
+    loop {
+        if buf.len() == buf.capacity() {
+            buf.reserve(32); // buf is full, need more space
+        }
+
+        let mut read_buf = ReadBuf::uninit(buf.spare_capacity_mut());
+
+        // SAFETY: These bytes were initialized but not filled in the previous loop
+        unsafe {
+            read_buf.assume_init(initialized);
+        }
+
+        match r.read_buf(&mut read_buf) {
+            Ok(()) => {}
+            Err(e) if e.kind() == ErrorKind::Interrupted => continue,
+            Err(e) => return Err(e),
+        }
+
+        if read_buf.filled_len() == 0 {
+            return Ok(buf.len() - start_len);
+        }
+
+        // store how much was initialized but not filled
+        initialized = read_buf.initialized_len() - read_buf.filled_len();
+        let new_len = read_buf.filled_len() + buf.len();
+
+        // SAFETY: ReadBuf's invariants mean this much memory is init
+        unsafe {
+            buf.set_len(new_len);
+        }
+
+        if buf.len() == buf.capacity() && buf.capacity() == start_cap {
+            // The buffer might be an exact fit. Let's read into a probe buffer
+            // and see if it returns `Ok(0)`. If so, we've avoided an
+            // unnecessary doubling of the capacity. But if not, append the
+            // probe buffer to the primary buffer and let its capacity grow.
+            let mut probe = [0u8; 32];
+
+            loop {
+                match r.read(&mut probe) {
+                    Ok(0) => return Ok(buf.len() - start_len),
+                    Ok(n) => {
+                        buf.extend_from_slice(&probe[..n]);
+                        break;
+                    }
+                    Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+                    Err(e) => return Err(e),
+                }
+            }
+        }
+    }
+}
+
+pub(crate) fn default_read_to_string<R: Read + ?Sized>(
+    r: &mut R,
+    buf: &mut String,
+) -> Result<usize> {
+    // Note that we do *not* call `r.read_to_end()` here. We are passing
+    // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
+    // method to fill it up. An arbitrary implementation could overwrite the
+    // entire contents of the vector, not just append to it (which is what
+    // we are expecting).
+    //
+    // To prevent extraneously checking the UTF-8-ness of the entire buffer
+    // we pass it to our hardcoded `default_read_to_end` implementation which
+    // we know is guaranteed to only read data into the end of the buffer.
+    unsafe { append_to_string(buf, |b| default_read_to_end(r, b)) }
+}
+
+pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
+where
+    F: FnOnce(&mut [u8]) -> Result<usize>,
+{
+    let buf = bufs.iter_mut().find(|b| !b.is_empty()).map_or(&mut [][..], |b| &mut **b);
+    read(buf)
+}
+
+pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
+where
+    F: FnOnce(&[u8]) -> Result<usize>,
+{
+    let buf = bufs.iter().find(|b| !b.is_empty()).map_or(&[][..], |b| &**b);
+    write(buf)
+}
+
+pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {
+    while !buf.is_empty() {
+        match this.read(buf) {
+            Ok(0) => break,
+            Ok(n) => {
+                let tmp = buf;
+                buf = &mut tmp[n..];
+            }
+            Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+            Err(e) => return Err(e),
+        }
+    }
+    if !buf.is_empty() {
+        Err(error::const_io_error!(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
+    } else {
+        Ok(())
+    }
+}
+
+pub(crate) fn default_read_buf<F>(read: F, buf: &mut ReadBuf<'_>) -> Result<()>
+where
+    F: FnOnce(&mut [u8]) -> Result<usize>,
+{
+    let n = read(buf.initialize_unfilled())?;
+    buf.add_filled(n);
+    Ok(())
+}
+
+/// The `Read` trait allows for reading bytes from a source.
+///
+/// Implementors of the `Read` trait are called 'readers'.
+///
+/// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
+/// will attempt to pull bytes from this source into a provided buffer. A
+/// number of other methods are implemented in terms of [`read()`], giving
+/// implementors a number of ways to read bytes while only needing to implement
+/// a single method.
+///
+/// Readers are intended to be composable with one another. Many implementors
+/// throughout [`std::io`] take and provide types which implement the `Read`
+/// trait.
+///
+/// Please note that each call to [`read()`] may involve a system call, and
+/// therefore, using something that implements [`BufRead`], such as
+/// [`BufReader`], will be more efficient.
+///
+/// # Examples
+///
+/// [`File`]s implement `Read`:
+///
+/// ```no_run
+/// use std::io;
+/// use std::io::prelude::*;
+/// use std::fs::File;
+///
+/// fn main() -> io::Result<()> {
+///     let mut f = File::open("foo.txt")?;
+///     let mut buffer = [0; 10];
+///
+///     // read up to 10 bytes
+///     f.read(&mut buffer)?;
+///
+///     let mut buffer = Vec::new();
+///     // read the whole file
+///     f.read_to_end(&mut buffer)?;
+///
+///     // read into a String, so that you don't need to do the conversion.
+///     let mut buffer = String::new();
+///     f.read_to_string(&mut buffer)?;
+///
+///     // and more! See the other methods for more details.
+///     Ok(())
+/// }
+/// ```
+///
+/// Read from [`&str`] because [`&[u8]`][prim@slice] implements `Read`:
+///
+/// ```no_run
+/// # use std::io;
+/// use std::io::prelude::*;
+///
+/// fn main() -> io::Result<()> {
+///     let mut b = "This string will be read".as_bytes();
+///     let mut buffer = [0; 10];
+///
+///     // read up to 10 bytes
+///     b.read(&mut buffer)?;
+///
+///     // etc... it works exactly as a File does!
+///     Ok(())
+/// }
+/// ```
+///
+/// [`read()`]: Read::read
+/// [`&str`]: prim@str
+/// [`std::io`]: self
+/// [`File`]: crate::fs::File
+#[stable(feature = "rust1", since = "1.0.0")]
+#[doc(notable_trait)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "IoRead")]
+pub trait Read {
+    /// Pull some bytes from this source into the specified buffer, returning
+    /// how many bytes were read.
+    ///
+    /// This function does not provide any guarantees about whether it blocks
+    /// waiting for data, but if an object needs to block for a read and cannot,
+    /// it will typically signal this via an [`Err`] return value.
+    ///
+    /// If the return value of this method is [`Ok(n)`], then implementations must
+    /// guarantee that `0 <= n <= buf.len()`. A nonzero `n` value indicates
+    /// that the buffer `buf` has been filled in with `n` bytes of data from this
+    /// source. If `n` is `0`, then it can indicate one of two scenarios:
+    ///
+    /// 1. This reader has reached its "end of file" and will likely no longer
+    ///    be able to produce bytes. Note that this does not mean that the
+    ///    reader will *always* no longer be able to produce bytes. As an example,
+    ///    on Linux, this method will call the `recv` syscall for a [`TcpStream`],
+    ///    where returning zero indicates the connection was shut down correctly. While
+    ///    for [`File`], it is possible to reach the end of file and get zero as result,
+    ///    but if more data is appended to the file, future calls to `read` will return
+    ///    more data.
+    /// 2. The buffer specified was 0 bytes in length.
+    ///
+    /// It is not an error if the returned value `n` is smaller than the buffer size,
+    /// even when the reader is not at the end of the stream yet.
+    /// This may happen for example because fewer bytes are actually available right now
+    /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
+    ///
+    /// As this trait is safe to implement, callers cannot rely on `n <= buf.len()` for safety.
+    /// Extra care needs to be taken when `unsafe` functions are used to access the read bytes.
+    /// Callers have to ensure that no unchecked out-of-bounds accesses are possible even if
+    /// `n > buf.len()`.
+    ///
+    /// No guarantees are provided about the contents of `buf` when this
+    /// function is called, implementations cannot rely on any property of the
+    /// contents of `buf` being true. It is recommended that *implementations*
+    /// only write data to `buf` instead of reading its contents.
+    ///
+    /// Correspondingly, however, *callers* of this method must not assume any guarantees
+    /// about how the implementation uses `buf`. The trait is safe to implement,
+    /// so it is possible that the code that's supposed to write to the buffer might also read
+    /// from it. It is your responsibility to make sure that `buf` is initialized
+    /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
+    /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
+    ///
+    /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
+    ///
+    /// # Errors
+    ///
+    /// If this function encounters any form of I/O or other error, an error
+    /// variant will be returned. If an error is returned then it must be
+    /// guaranteed that no bytes were read.
+    ///
+    /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
+    /// operation should be retried if there is nothing else to do.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`Ok(n)`]: Ok
+    /// [`File`]: crate::fs::File
+    /// [`TcpStream`]: crate::net::TcpStream
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt")?;
+    ///     let mut buffer = [0; 10];
+    ///
+    ///     // read up to 10 bytes
+    ///     let n = f.read(&mut buffer[..])?;
+    ///
+    ///     println!("The bytes: {:?}", &buffer[..n]);
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
+
+    /// Like `read`, except that it reads into a slice of buffers.
+    ///
+    /// Data is copied to fill each buffer in order, with the final buffer
+    /// written to possibly being only partially filled. This method must
+    /// behave equivalently to a single call to `read` with concatenated
+    /// buffers.
+    ///
+    /// The default implementation calls `read` with either the first nonempty
+    /// buffer provided, or an empty one if none exists.
+    #[stable(feature = "iovec", since = "1.36.0")]
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
+        default_read_vectored(|b| self.read(b), bufs)
+    }
+
+    /// Determines if this `Read`er has an efficient `read_vectored`
+    /// implementation.
+    ///
+    /// If a `Read`er does not override the default `read_vectored`
+    /// implementation, code using it may want to avoid the method all together
+    /// and coalesce writes into a single buffer for higher performance.
+    ///
+    /// The default implementation returns `false`.
+    #[unstable(feature = "can_vector", issue = "69941")]
+    fn is_read_vectored(&self) -> bool {
+        false
+    }
+
+    /// Read all bytes until EOF in this source, placing them into `buf`.
+    ///
+    /// All bytes read from this source will be appended to the specified buffer
+    /// `buf`. This function will continuously call [`read()`] to append more data to
+    /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
+    /// non-[`ErrorKind::Interrupted`] kind.
+    ///
+    /// If successful, this function will return the total number of bytes read.
+    ///
+    /// # Errors
+    ///
+    /// If this function encounters an error of the kind
+    /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
+    /// will continue.
+    ///
+    /// If any other read error is encountered then this function immediately
+    /// returns. Any bytes which have already been read will be appended to
+    /// `buf`.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`read()`]: Read::read
+    /// [`Ok(0)`]: Ok
+    /// [`File`]: crate::fs::File
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt")?;
+    ///     let mut buffer = Vec::new();
+    ///
+    ///     // read the whole file
+    ///     f.read_to_end(&mut buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// (See also the [`std::fs::read`] convenience function for reading from a
+    /// file.)
+    ///
+    /// [`std::fs::read`]: crate::fs::read
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
+        default_read_to_end(self, buf)
+    }
+
+    /// Read all bytes until EOF in this source, appending them to `buf`.
+    ///
+    /// If successful, this function returns the number of bytes which were read
+    /// and appended to `buf`.
+    ///
+    /// # Errors
+    ///
+    /// If the data in this stream is *not* valid UTF-8 then an error is
+    /// returned and `buf` is unchanged.
+    ///
+    /// See [`read_to_end`] for other error semantics.
+    ///
+    /// [`read_to_end`]: Read::read_to_end
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`File`]: crate::fs::File
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt")?;
+    ///     let mut buffer = String::new();
+    ///
+    ///     f.read_to_string(&mut buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// (See also the [`std::fs::read_to_string`] convenience function for
+    /// reading from a file.)
+    ///
+    /// [`std::fs::read_to_string`]: crate::fs::read_to_string
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
+        default_read_to_string(self, buf)
+    }
+
+    /// Read the exact number of bytes required to fill `buf`.
+    ///
+    /// This function reads as many bytes as necessary to completely fill the
+    /// specified buffer `buf`.
+    ///
+    /// No guarantees are provided about the contents of `buf` when this
+    /// function is called, implementations cannot rely on any property of the
+    /// contents of `buf` being true. It is recommended that implementations
+    /// only write data to `buf` instead of reading its contents. The
+    /// documentation on [`read`] has a more detailed explanation on this
+    /// subject.
+    ///
+    /// # Errors
+    ///
+    /// If this function encounters an error of the kind
+    /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
+    /// will continue.
+    ///
+    /// If this function encounters an "end of file" before completely filling
+    /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
+    /// The contents of `buf` are unspecified in this case.
+    ///
+    /// If any other read error is encountered then this function immediately
+    /// returns. The contents of `buf` are unspecified in this case.
+    ///
+    /// If this function returns an error, it is unspecified how many bytes it
+    /// has read, but it will never read more than would be necessary to
+    /// completely fill the buffer.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`read`]: Read::read
+    /// [`File`]: crate::fs::File
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt")?;
+    ///     let mut buffer = [0; 10];
+    ///
+    ///     // read exactly 10 bytes
+    ///     f.read_exact(&mut buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "read_exact", since = "1.6.0")]
+    fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
+        default_read_exact(self, buf)
+    }
+
+    /// Pull some bytes from this source into the specified buffer.
+    ///
+    /// This is equivalent to the [`read`](Read::read) method, except that it is passed a [`ReadBuf`] rather than `[u8]` to allow use
+    /// with uninitialized buffers. The new data will be appended to any existing contents of `buf`.
+    ///
+    /// The default implementation delegates to `read`.
+    #[unstable(feature = "read_buf", issue = "78485")]
+    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
+        default_read_buf(|b| self.read(b), buf)
+    }
+
+    /// Read the exact number of bytes required to fill `buf`.
+    ///
+    /// This is equivalent to the [`read_exact`](Read::read_exact) method, except that it is passed a [`ReadBuf`] rather than `[u8]` to
+    /// allow use with uninitialized buffers.
+    #[unstable(feature = "read_buf", issue = "78485")]
+    fn read_buf_exact(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
+        while buf.remaining() > 0 {
+            let prev_filled = buf.filled().len();
+            match self.read_buf(buf) {
+                Ok(()) => {}
+                Err(e) if e.kind() == ErrorKind::Interrupted => continue,
+                Err(e) => return Err(e),
+            }
+
+            if buf.filled().len() == prev_filled {
+                return Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill buffer"));
+            }
+        }
+
+        Ok(())
+    }
+
+    /// Creates a "by reference" adaptor for this instance of `Read`.
+    ///
+    /// The returned adapter also implements `Read` and will simply borrow this
+    /// current reader.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`File`]: crate::fs::File
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::Read;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt")?;
+    ///     let mut buffer = Vec::new();
+    ///     let mut other_buffer = Vec::new();
+    ///
+    ///     {
+    ///         let reference = f.by_ref();
+    ///
+    ///         // read at most 5 bytes
+    ///         reference.take(5).read_to_end(&mut buffer)?;
+    ///
+    ///     } // drop our &mut reference so we can use f again
+    ///
+    ///     // original file still usable, read the rest
+    ///     f.read_to_end(&mut other_buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn by_ref(&mut self) -> &mut Self
+    where
+        Self: Sized,
+    {
+        self
+    }
+
+    /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
+    ///
+    /// The returned type implements [`Iterator`] where the [`Item`] is
+    /// <code>[Result]<[u8], [io::Error]></code>.
+    /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
+    /// otherwise. EOF is mapped to returning [`None`] from this iterator.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`Item`]: Iterator::Item
+    /// [`File`]: crate::fs::File "fs::File"
+    /// [Result]: crate::result::Result "Result"
+    /// [io::Error]: self::Error "io::Error"
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let f = File::open("foo.txt")?;
+    ///
+    ///     for byte in f.bytes() {
+    ///         println!("{}", byte.unwrap());
+    ///     }
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn bytes(self) -> Bytes<Self>
+    where
+        Self: Sized,
+    {
+        Bytes { inner: self }
+    }
+
+    /// Creates an adapter which will chain this stream with another.
+    ///
+    /// The returned `Read` instance will first read all bytes from this object
+    /// until EOF is encountered. Afterwards the output is equivalent to the
+    /// output of `next`.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`File`]: crate::fs::File
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let f1 = File::open("foo.txt")?;
+    ///     let f2 = File::open("bar.txt")?;
+    ///
+    ///     let mut handle = f1.chain(f2);
+    ///     let mut buffer = String::new();
+    ///
+    ///     // read the value into a String. We could use any Read method here,
+    ///     // this is just one example.
+    ///     handle.read_to_string(&mut buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn chain<R: Read>(self, next: R) -> Chain<Self, R>
+    where
+        Self: Sized,
+    {
+        Chain { first: self, second: next, done_first: false }
+    }
+
+    /// Creates an adapter which will read at most `limit` bytes from it.
+    ///
+    /// This function returns a new instance of `Read` which will read at most
+    /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
+    /// read errors will not count towards the number of bytes read and future
+    /// calls to [`read()`] may succeed.
+    ///
+    /// # Examples
+    ///
+    /// [`File`]s implement `Read`:
+    ///
+    /// [`File`]: crate::fs::File
+    /// [`Ok(0)`]: Ok
+    /// [`read()`]: Read::read
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let f = File::open("foo.txt")?;
+    ///     let mut buffer = [0; 5];
+    ///
+    ///     // read at most five bytes
+    ///     let mut handle = f.take(5);
+    ///
+    ///     handle.read(&mut buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn take(self, limit: u64) -> Take<Self>
+    where
+        Self: Sized,
+    {
+        Take { inner: self, limit }
+    }
+}
+
+/// Read all bytes from a [reader][Read] into a new [`String`].
+///
+/// This is a convenience function for [`Read::read_to_string`]. Using this
+/// function avoids having to create a variable first and provides more type
+/// safety since you can only get the buffer out if there were no errors. (If you
+/// use [`Read::read_to_string`] you have to remember to check whether the read
+/// succeeded because otherwise your buffer will be empty or only partially full.)
+///
+/// # Performance
+///
+/// The downside of this function's increased ease of use and type safety is
+/// that it gives you less control over performance. For example, you can't
+/// pre-allocate memory like you can using [`String::with_capacity`] and
+/// [`Read::read_to_string`]. Also, you can't re-use the buffer if an error
+/// occurs while reading.
+///
+/// In many cases, this function's performance will be adequate and the ease of use
+/// and type safety tradeoffs will be worth it. However, there are cases where you
+/// need more control over performance, and in those cases you should definitely use
+/// [`Read::read_to_string`] directly.
+///
+/// Note that in some special cases, such as when reading files, this function will
+/// pre-allocate memory based on the size of the input it is reading. In those
+/// cases, the performance should be as good as if you had used
+/// [`Read::read_to_string`] with a manually pre-allocated buffer.
+///
+/// # Errors
+///
+/// This function forces you to handle errors because the output (the `String`)
+/// is wrapped in a [`Result`]. See [`Read::read_to_string`] for the errors
+/// that can occur. If any error occurs, you will get an [`Err`], so you
+/// don't have to worry about your buffer being empty or partially full.
+///
+/// # Examples
+///
+/// ```no_run
+/// #![feature(io_read_to_string)]
+///
+/// # use std::io;
+/// fn main() -> io::Result<()> {
+///     let stdin = io::read_to_string(io::stdin())?;
+///     println!("Stdin was:");
+///     println!("{stdin}");
+///     Ok(())
+/// }
+/// ```
+#[unstable(feature = "io_read_to_string", issue = "80218")]
+pub fn read_to_string<R: Read>(mut reader: R) -> Result<String> {
+    let mut buf = String::new();
+    reader.read_to_string(&mut buf)?;
+    Ok(buf)
+}
+
+/// A buffer type used with `Read::read_vectored`.
+///
+/// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
+/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
+/// Windows.
+#[stable(feature = "iovec", since = "1.36.0")]
+#[repr(transparent)]
+pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);
+
+#[stable(feature = "iovec-send-sync", since = "1.44.0")]
+unsafe impl<'a> Send for IoSliceMut<'a> {}
+
+#[stable(feature = "iovec-send-sync", since = "1.44.0")]
+unsafe impl<'a> Sync for IoSliceMut<'a> {}
+
+#[stable(feature = "iovec", since = "1.36.0")]
+impl<'a> fmt::Debug for IoSliceMut<'a> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(self.0.as_slice(), fmt)
+    }
+}
+
+impl<'a> IoSliceMut<'a> {
+    /// Creates a new `IoSliceMut` wrapping a byte slice.
+    ///
+    /// # Panics
+    ///
+    /// Panics on Windows if the slice is larger than 4GB.
+    #[stable(feature = "iovec", since = "1.36.0")]
+    #[inline]
+    pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
+        IoSliceMut(sys::io::IoSliceMut::new(buf))
+    }
+
+    /// Advance the internal cursor of the slice.
+    ///
+    /// Also see [`IoSliceMut::advance_slices`] to advance the cursors of
+    /// multiple buffers.
+    ///
+    /// # Panics
+    ///
+    /// Panics when trying to advance beyond the end of the slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(io_slice_advance)]
+    ///
+    /// use std::io::IoSliceMut;
+    /// use std::ops::Deref;
+    ///
+    /// let mut data = [1; 8];
+    /// let mut buf = IoSliceMut::new(&mut data);
+    ///
+    /// // Mark 3 bytes as read.
+    /// buf.advance(3);
+    /// assert_eq!(buf.deref(), [1; 5].as_ref());
+    /// ```
+    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[inline]
+    pub fn advance(&mut self, n: usize) {
+        self.0.advance(n)
+    }
+
+    /// Advance a slice of slices.
+    ///
+    /// Shrinks the slice to remove any `IoSliceMut`s that are fully advanced over.
+    /// If the cursor ends up in the middle of an `IoSliceMut`, it is modified
+    /// to start at that cursor.
+    ///
+    /// For example, if we have a slice of two 8-byte `IoSliceMut`s, and we advance by 10 bytes,
+    /// the result will only include the second `IoSliceMut`, advanced by 2 bytes.
+    ///
+    /// # Panics
+    ///
+    /// Panics when trying to advance beyond the end of the slices.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(io_slice_advance)]
+    ///
+    /// use std::io::IoSliceMut;
+    /// use std::ops::Deref;
+    ///
+    /// 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),
+    /// ][..];
+    ///
+    /// // Mark 10 bytes as read.
+    /// IoSliceMut::advance_slices(&mut bufs, 10);
+    /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
+    /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
+    /// ```
+    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[inline]
+    pub fn advance_slices(bufs: &mut &mut [IoSliceMut<'a>], n: usize) {
+        // Number of buffers to remove.
+        let mut remove = 0;
+        // Total length of all the to be removed buffers.
+        let mut accumulated_len = 0;
+        for buf in bufs.iter() {
+            if accumulated_len + buf.len() > n {
+                break;
+            } else {
+                accumulated_len += buf.len();
+                remove += 1;
+            }
+        }
+
+        *bufs = &mut replace(bufs, &mut [])[remove..];
+        if bufs.is_empty() {
+            assert!(n == accumulated_len, "advancing io slices beyond their length");
+        } else {
+            bufs[0].advance(n - accumulated_len)
+        }
+    }
+}
+
+#[stable(feature = "iovec", since = "1.36.0")]
+impl<'a> Deref for IoSliceMut<'a> {
+    type Target = [u8];
+
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        self.0.as_slice()
+    }
+}
+
+#[stable(feature = "iovec", since = "1.36.0")]
+impl<'a> DerefMut for IoSliceMut<'a> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut [u8] {
+        self.0.as_mut_slice()
+    }
+}
+
+/// A buffer type used with `Write::write_vectored`.
+///
+/// It is semantically a wrapper around a `&[u8]`, but is guaranteed to be
+/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
+/// Windows.
+#[stable(feature = "iovec", since = "1.36.0")]
+#[derive(Copy, Clone)]
+#[repr(transparent)]
+pub struct IoSlice<'a>(sys::io::IoSlice<'a>);
+
+#[stable(feature = "iovec-send-sync", since = "1.44.0")]
+unsafe impl<'a> Send for IoSlice<'a> {}
+
+#[stable(feature = "iovec-send-sync", since = "1.44.0")]
+unsafe impl<'a> Sync for IoSlice<'a> {}
+
+#[stable(feature = "iovec", since = "1.36.0")]
+impl<'a> fmt::Debug for IoSlice<'a> {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        fmt::Debug::fmt(self.0.as_slice(), fmt)
+    }
+}
+
+impl<'a> IoSlice<'a> {
+    /// Creates a new `IoSlice` wrapping a byte slice.
+    ///
+    /// # Panics
+    ///
+    /// Panics on Windows if the slice is larger than 4GB.
+    #[stable(feature = "iovec", since = "1.36.0")]
+    #[must_use]
+    #[inline]
+    pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
+        IoSlice(sys::io::IoSlice::new(buf))
+    }
+
+    /// Advance the internal cursor of the slice.
+    ///
+    /// Also see [`IoSlice::advance_slices`] to advance the cursors of multiple
+    /// buffers.
+    ///
+    /// # Panics
+    ///
+    /// Panics when trying to advance beyond the end of the slice.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(io_slice_advance)]
+    ///
+    /// use std::io::IoSlice;
+    /// use std::ops::Deref;
+    ///
+    /// let data = [1; 8];
+    /// let mut buf = IoSlice::new(&data);
+    ///
+    /// // Mark 3 bytes as read.
+    /// buf.advance(3);
+    /// assert_eq!(buf.deref(), [1; 5].as_ref());
+    /// ```
+    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[inline]
+    pub fn advance(&mut self, n: usize) {
+        self.0.advance(n)
+    }
+
+    /// Advance a slice of slices.
+    ///
+    /// Shrinks the slice to remove any `IoSlice`s that are fully advanced over.
+    /// If the cursor ends up in the middle of an `IoSlice`, it is modified
+    /// to start at that cursor.
+    ///
+    /// For example, if we have a slice of two 8-byte `IoSlice`s, and we advance by 10 bytes,
+    /// the result will only include the second `IoSlice`, advanced by 2 bytes.
+    ///
+    /// # Panics
+    ///
+    /// Panics when trying to advance beyond the end of the slices.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(io_slice_advance)]
+    ///
+    /// use std::io::IoSlice;
+    /// use std::ops::Deref;
+    ///
+    /// 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),
+    /// ][..];
+    ///
+    /// // Mark 10 bytes as written.
+    /// IoSlice::advance_slices(&mut bufs, 10);
+    /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
+    /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
+    #[unstable(feature = "io_slice_advance", issue = "62726")]
+    #[inline]
+    pub fn advance_slices(bufs: &mut &mut [IoSlice<'a>], n: usize) {
+        // Number of buffers to remove.
+        let mut remove = 0;
+        // Total length of all the to be removed buffers.
+        let mut accumulated_len = 0;
+        for buf in bufs.iter() {
+            if accumulated_len + buf.len() > n {
+                break;
+            } else {
+                accumulated_len += buf.len();
+                remove += 1;
+            }
+        }
+
+        *bufs = &mut replace(bufs, &mut [])[remove..];
+        if bufs.is_empty() {
+            assert!(n == accumulated_len, "advancing io slices beyond their length");
+        } else {
+            bufs[0].advance(n - accumulated_len)
+        }
+    }
+}
+
+#[stable(feature = "iovec", since = "1.36.0")]
+impl<'a> Deref for IoSlice<'a> {
+    type Target = [u8];
+
+    #[inline]
+    fn deref(&self) -> &[u8] {
+        self.0.as_slice()
+    }
+}
+
+/// A trait for objects which are byte-oriented sinks.
+///
+/// Implementors of the `Write` trait are sometimes called 'writers'.
+///
+/// Writers are defined by two required methods, [`write`] and [`flush`]:
+///
+/// * The [`write`] method will attempt to write some data into the object,
+///   returning how many bytes were successfully written.
+///
+/// * The [`flush`] method is useful for adapters and explicit buffers
+///   themselves for ensuring that all buffered data has been pushed out to the
+///   'true sink'.
+///
+/// Writers are intended to be composable with one another. Many implementors
+/// throughout [`std::io`] take and provide types which implement the `Write`
+/// trait.
+///
+/// [`write`]: Write::write
+/// [`flush`]: Write::flush
+/// [`std::io`]: self
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::io::prelude::*;
+/// use std::fs::File;
+///
+/// fn main() -> std::io::Result<()> {
+///     let data = b"some bytes";
+///
+///     let mut pos = 0;
+///     let mut buffer = File::create("foo.txt")?;
+///
+///     while pos < data.len() {
+///         let bytes_written = buffer.write(&data[pos..])?;
+///         pos += bytes_written;
+///     }
+///     Ok(())
+/// }
+/// ```
+///
+/// The trait also provides convenience methods like [`write_all`], which calls
+/// `write` in a loop until its entire input has been written.
+///
+/// [`write_all`]: Write::write_all
+#[stable(feature = "rust1", since = "1.0.0")]
+#[doc(notable_trait)]
+#[cfg_attr(not(test), rustc_diagnostic_item = "IoWrite")]
+pub trait Write {
+    /// Write a buffer into this writer, returning how many bytes were written.
+    ///
+    /// This function will attempt to write the entire contents of `buf`, but
+    /// the entire write might not succeed, or the write may also generate an
+    /// error. A call to `write` represents *at most one* attempt to write to
+    /// any wrapped object.
+    ///
+    /// Calls to `write` are not guaranteed to block waiting for data to be
+    /// written, and a write which would otherwise block can be indicated through
+    /// an [`Err`] variant.
+    ///
+    /// If the return value is [`Ok(n)`] then it must be guaranteed that
+    /// `n <= buf.len()`. A return value of `0` typically means that the
+    /// underlying object is no longer able to accept bytes and will likely not
+    /// be able to in the future as well, or that the buffer provided is empty.
+    ///
+    /// # Errors
+    ///
+    /// Each call to `write` may generate an I/O error indicating that the
+    /// operation could not be completed. If an error is returned then no bytes
+    /// in the buffer were written to this writer.
+    ///
+    /// It is **not** considered an error if the entire buffer could not be
+    /// written to this writer.
+    ///
+    /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
+    /// write operation should be retried if there is nothing else to do.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut buffer = File::create("foo.txt")?;
+    ///
+    ///     // Writes some prefix of the byte string, not necessarily all of it.
+    ///     buffer.write(b"some bytes")?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// [`Ok(n)`]: Ok
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn write(&mut self, buf: &[u8]) -> Result<usize>;
+
+    /// Like [`write`], except that it writes from a slice of buffers.
+    ///
+    /// Data is copied from each buffer in order, with the final buffer
+    /// read from possibly being only partially consumed. This method must
+    /// behave as a call to [`write`] with the buffers concatenated would.
+    ///
+    /// The default implementation calls [`write`] with either the first nonempty
+    /// buffer provided, or an empty one if none exists.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::IoSlice;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let data1 = [1; 8];
+    ///     let data2 = [15; 8];
+    ///     let io_slice1 = IoSlice::new(&data1);
+    ///     let io_slice2 = IoSlice::new(&data2);
+    ///
+    ///     let mut buffer = File::create("foo.txt")?;
+    ///
+    ///     // Writes some prefix of the byte string, not necessarily all of it.
+    ///     buffer.write_vectored(&[io_slice1, io_slice2])?;
+    ///     Ok(())
+    /// }
+    /// ```
+    ///
+    /// [`write`]: Write::write
+    #[stable(feature = "iovec", since = "1.36.0")]
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
+        default_write_vectored(|b| self.write(b), bufs)
+    }
+
+    /// Determines if this `Write`r has an efficient [`write_vectored`]
+    /// implementation.
+    ///
+    /// If a `Write`r does not override the default [`write_vectored`]
+    /// implementation, code using it may want to avoid the method all together
+    /// and coalesce writes into a single buffer for higher performance.
+    ///
+    /// The default implementation returns `false`.
+    ///
+    /// [`write_vectored`]: Write::write_vectored
+    #[unstable(feature = "can_vector", issue = "69941")]
+    fn is_write_vectored(&self) -> bool {
+        false
+    }
+
+    /// Flush this output stream, ensuring that all intermediately buffered
+    /// contents reach their destination.
+    ///
+    /// # Errors
+    ///
+    /// It is considered an error if not all bytes could be written due to
+    /// I/O errors or EOF being reached.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::prelude::*;
+    /// use std::io::BufWriter;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut buffer = BufWriter::new(File::create("foo.txt")?);
+    ///
+    ///     buffer.write_all(b"some bytes")?;
+    ///     buffer.flush()?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn flush(&mut self) -> Result<()>;
+
+    /// Attempts to write an entire buffer into this writer.
+    ///
+    /// This method will continuously call [`write`] until there is no more data
+    /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
+    /// returned. This method will not return until the entire buffer has been
+    /// successfully written or such an error occurs. The first error that is
+    /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
+    /// returned.
+    ///
+    /// If the buffer contains no data, this will never call [`write`].
+    ///
+    /// # Errors
+    ///
+    /// This function will return the first error of
+    /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
+    ///
+    /// [`write`]: Write::write
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut buffer = File::create("foo.txt")?;
+    ///
+    ///     buffer.write_all(b"some bytes")?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
+        while !buf.is_empty() {
+            match self.write(buf) {
+                Ok(0) => {
+                    return Err(error::const_io_error!(
+                        ErrorKind::WriteZero,
+                        "failed to write whole buffer",
+                    ));
+                }
+                Ok(n) => buf = &buf[n..],
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(())
+    }
+
+    /// Attempts to write multiple buffers into this writer.
+    ///
+    /// This method will continuously call [`write_vectored`] until there is no
+    /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
+    /// kind is returned. This method will not return until all buffers have
+    /// been successfully written or such an error occurs. The first error that
+    /// is not of [`ErrorKind::Interrupted`] kind generated from this method
+    /// will be returned.
+    ///
+    /// If the buffer contains no data, this will never call [`write_vectored`].
+    ///
+    /// # Notes
+    ///
+    /// Unlike [`write_vectored`], this takes a *mutable* reference to
+    /// a slice of [`IoSlice`]s, not an immutable one. That's because we need to
+    /// modify the slice to keep track of the bytes already written.
+    ///
+    /// Once this function returns, the contents of `bufs` are unspecified, as
+    /// this depends on how many calls to [`write_vectored`] were necessary. It is
+    /// best to understand this function as taking ownership of `bufs` and to
+    /// not use `bufs` afterwards. The underlying buffers, to which the
+    /// [`IoSlice`]s point (but not the [`IoSlice`]s themselves), are unchanged and
+    /// can be reused.
+    ///
+    /// [`write_vectored`]: Write::write_vectored
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(write_all_vectored)]
+    /// # fn main() -> std::io::Result<()> {
+    ///
+    /// use std::io::{Write, IoSlice};
+    ///
+    /// let mut writer = Vec::new();
+    /// let bufs = &mut [
+    ///     IoSlice::new(&[1]),
+    ///     IoSlice::new(&[2, 3]),
+    ///     IoSlice::new(&[4, 5, 6]),
+    /// ];
+    ///
+    /// writer.write_all_vectored(bufs)?;
+    /// // Note: the contents of `bufs` is now undefined, see the Notes section.
+    ///
+    /// assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
+    /// # Ok(()) }
+    /// ```
+    #[unstable(feature = "write_all_vectored", issue = "70436")]
+    fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
+        // Guarantee that bufs is empty if it contains no data,
+        // to avoid calling write_vectored if there is no data to be written.
+        IoSlice::advance_slices(&mut bufs, 0);
+        while !bufs.is_empty() {
+            match self.write_vectored(bufs) {
+                Ok(0) => {
+                    return Err(error::const_io_error!(
+                        ErrorKind::WriteZero,
+                        "failed to write whole buffer",
+                    ));
+                }
+                Ok(n) => IoSlice::advance_slices(&mut bufs, n),
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(())
+    }
+
+    /// Writes a formatted string into this writer, returning any error
+    /// encountered.
+    ///
+    /// This method is primarily used to interface with the
+    /// [`format_args!()`] macro, and it is rare that this should
+    /// explicitly be called. The [`write!()`] macro should be favored to
+    /// invoke this method instead.
+    ///
+    /// This function internally uses the [`write_all`] method on
+    /// this trait and hence will continuously write data so long as no errors
+    /// are received. This also means that partial writes are not indicated in
+    /// this signature.
+    ///
+    /// [`write_all`]: Write::write_all
+    ///
+    /// # Errors
+    ///
+    /// This function will return any I/O error reported while formatting.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut buffer = File::create("foo.txt")?;
+    ///
+    ///     // this call
+    ///     write!(buffer, "{:.*}", 2, 1.234567)?;
+    ///     // turns into this:
+    ///     buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
+        // Create a shim which translates a Write to a fmt::Write and saves
+        // off I/O errors. instead of discarding them
+        struct Adapter<'a, T: ?Sized + 'a> {
+            inner: &'a mut T,
+            error: Result<()>,
+        }
+
+        impl<T: Write + ?Sized> fmt::Write for Adapter<'_, T> {
+            fn write_str(&mut self, s: &str) -> fmt::Result {
+                match self.inner.write_all(s.as_bytes()) {
+                    Ok(()) => Ok(()),
+                    Err(e) => {
+                        self.error = Err(e);
+                        Err(fmt::Error)
+                    }
+                }
+            }
+        }
+
+        let mut output = Adapter { inner: self, error: Ok(()) };
+        match fmt::write(&mut output, fmt) {
+            Ok(()) => Ok(()),
+            Err(..) => {
+                // check if the error came from the underlying `Write` or not
+                if output.error.is_err() {
+                    output.error
+                } else {
+                    Err(error::const_io_error!(ErrorKind::Uncategorized, "formatter error"))
+                }
+            }
+        }
+    }
+
+    /// Creates a "by reference" adapter for this instance of `Write`.
+    ///
+    /// The returned adapter also implements `Write` and will simply borrow this
+    /// current writer.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::Write;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> std::io::Result<()> {
+    ///     let mut buffer = File::create("foo.txt")?;
+    ///
+    ///     let reference = buffer.by_ref();
+    ///
+    ///     // we can use reference just like our original buffer
+    ///     reference.write_all(b"some bytes")?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn by_ref(&mut self) -> &mut Self
+    where
+        Self: Sized,
+    {
+        self
+    }
+}
+
+/// The `Seek` trait provides a cursor which can be moved within a stream of
+/// bytes.
+///
+/// The stream typically has a fixed size, allowing seeking relative to either
+/// end or the current offset.
+///
+/// # Examples
+///
+/// [`File`]s implement `Seek`:
+///
+/// [`File`]: crate::fs::File
+///
+/// ```no_run
+/// use std::io;
+/// use std::io::prelude::*;
+/// use std::fs::File;
+/// use std::io::SeekFrom;
+///
+/// fn main() -> io::Result<()> {
+///     let mut f = File::open("foo.txt")?;
+///
+///     // move the cursor 42 bytes from the start of the file
+///     f.seek(SeekFrom::Start(42))?;
+///     Ok(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait Seek {
+    /// Seek to an offset, in bytes, in a stream.
+    ///
+    /// A seek beyond the end of a stream is allowed, but behavior is defined
+    /// by the implementation.
+    ///
+    /// If the seek operation completed successfully,
+    /// this method returns the new position from the start of the stream.
+    /// That position can be used later with [`SeekFrom::Start`].
+    ///
+    /// # Errors
+    ///
+    /// Seeking can fail, for example because it might involve flushing a buffer.
+    ///
+    /// Seeking to a negative offset is considered an error.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
+
+    /// Rewind to the beginning of a stream.
+    ///
+    /// This is a convenience method, equivalent to `seek(SeekFrom::Start(0))`.
+    ///
+    /// # Errors
+    ///
+    /// Rewinding can fail, for example because it might involve flushing a buffer.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// use std::io::{Read, Seek, Write};
+    /// use std::fs::OpenOptions;
+    ///
+    /// let mut f = OpenOptions::new()
+    ///     .write(true)
+    ///     .read(true)
+    ///     .create(true)
+    ///     .open("foo.txt").unwrap();
+    ///
+    /// let hello = "Hello!\n";
+    /// write!(f, "{hello}").unwrap();
+    /// f.rewind().unwrap();
+    ///
+    /// let mut buf = String::new();
+    /// f.read_to_string(&mut buf).unwrap();
+    /// assert_eq!(&buf, hello);
+    /// ```
+    #[stable(feature = "seek_rewind", since = "1.55.0")]
+    fn rewind(&mut self) -> Result<()> {
+        self.seek(SeekFrom::Start(0))?;
+        Ok(())
+    }
+
+    /// Returns the length of this stream (in bytes).
+    ///
+    /// This method is implemented using up to three seek operations. If this
+    /// method returns successfully, the seek position is unchanged (i.e. the
+    /// position before calling this method is the same as afterwards).
+    /// However, if this method returns an error, the seek position is
+    /// unspecified.
+    ///
+    /// If you need to obtain the length of *many* streams and you don't care
+    /// about the seek position afterwards, you can reduce the number of seek
+    /// operations by simply calling `seek(SeekFrom::End(0))` and using its
+    /// return value (it is also the stream length).
+    ///
+    /// Note that length of a stream can change over time (for example, when
+    /// data is appended to a file). So calling this method multiple times does
+    /// not necessarily return the same length each time.
+    ///
+    /// # Example
+    ///
+    /// ```no_run
+    /// #![feature(seek_stream_len)]
+    /// use std::{
+    ///     io::{self, Seek},
+    ///     fs::File,
+    /// };
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut f = File::open("foo.txt")?;
+    ///
+    ///     let len = f.stream_len()?;
+    ///     println!("The file is currently {len} bytes long");
+    ///     Ok(())
+    /// }
+    /// ```
+    #[unstable(feature = "seek_stream_len", issue = "59359")]
+    fn stream_len(&mut self) -> Result<u64> {
+        let old_pos = self.stream_position()?;
+        let len = self.seek(SeekFrom::End(0))?;
+
+        // Avoid seeking a third time when we were already at the end of the
+        // stream. The branch is usually way cheaper than a seek operation.
+        if old_pos != len {
+            self.seek(SeekFrom::Start(old_pos))?;
+        }
+
+        Ok(len)
+    }
+
+    /// Returns the current seek position from the start of the stream.
+    ///
+    /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
+    ///
+    /// # 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(())
+    /// }
+    /// ```
+    #[stable(feature = "seek_convenience", since = "1.51.0")]
+    fn stream_position(&mut self) -> Result<u64> {
+        self.seek(SeekFrom::Current(0))
+    }
+}
+
+/// Enumeration of possible methods to seek within an I/O object.
+///
+/// It is used by the [`Seek`] trait.
+#[derive(Copy, PartialEq, Eq, Clone, Debug)]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub enum SeekFrom {
+    /// Sets the offset to the provided number of bytes.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    Start(#[stable(feature = "rust1", since = "1.0.0")] u64),
+
+    /// Sets the offset to the size of this object plus the specified number of
+    /// bytes.
+    ///
+    /// It is possible to seek beyond the end of an object, but it's an error to
+    /// seek before byte 0.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    End(#[stable(feature = "rust1", since = "1.0.0")] i64),
+
+    /// Sets the offset to the current position plus the specified number of
+    /// bytes.
+    ///
+    /// It is possible to seek beyond the end of an object, but it's an error to
+    /// seek before byte 0.
+    #[stable(feature = "rust1", since = "1.0.0")]
+    Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
+}
+
+fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
+    let mut read = 0;
+    loop {
+        let (done, used) = {
+            let available = match r.fill_buf() {
+                Ok(n) => n,
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+                Err(e) => return Err(e),
+            };
+            match memchr::memchr(delim, available) {
+                Some(i) => {
+                    buf.extend_from_slice(&available[..=i]);
+                    (true, i + 1)
+                }
+                None => {
+                    buf.extend_from_slice(available);
+                    (false, available.len())
+                }
+            }
+        };
+        r.consume(used);
+        read += used;
+        if done || used == 0 {
+            return Ok(read);
+        }
+    }
+}
+
+/// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
+/// to perform extra ways of reading.
+///
+/// For example, reading line-by-line is inefficient without using a buffer, so
+/// if you want to read by line, you'll need `BufRead`, which includes a
+/// [`read_line`] method as well as a [`lines`] iterator.
+///
+/// # Examples
+///
+/// A locked standard input implements `BufRead`:
+///
+/// ```no_run
+/// use std::io;
+/// use std::io::prelude::*;
+///
+/// let stdin = io::stdin();
+/// for line in stdin.lock().lines() {
+///     println!("{}", line.unwrap());
+/// }
+/// ```
+///
+/// If you have something that implements [`Read`], you can use the [`BufReader`
+/// type][`BufReader`] to turn it into a `BufRead`.
+///
+/// For example, [`File`] implements [`Read`], but not `BufRead`.
+/// [`BufReader`] to the rescue!
+///
+/// [`File`]: crate::fs::File
+/// [`read_line`]: BufRead::read_line
+/// [`lines`]: BufRead::lines
+///
+/// ```no_run
+/// use std::io::{self, BufReader};
+/// use std::io::prelude::*;
+/// use std::fs::File;
+///
+/// fn main() -> io::Result<()> {
+///     let f = File::open("foo.txt")?;
+///     let f = BufReader::new(f);
+///
+///     for line in f.lines() {
+///         println!("{}", line.unwrap());
+///     }
+///
+///     Ok(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub trait BufRead: Read {
+    /// Returns the contents of the internal buffer, filling it with more data
+    /// from the inner reader if it is empty.
+    ///
+    /// This function is a lower-level call. It needs to be paired with the
+    /// [`consume`] method to function properly. When calling this
+    /// method, none of the contents will be "read" in the sense that later
+    /// calling `read` may return the same contents. As such, [`consume`] must
+    /// be called with the number of bytes that are consumed from this buffer to
+    /// ensure that the bytes are never returned twice.
+    ///
+    /// [`consume`]: BufRead::consume
+    ///
+    /// An empty buffer returned indicates that the stream has reached EOF.
+    ///
+    /// # Errors
+    ///
+    /// This function will return an I/O error if the underlying reader was
+    /// read, but returned an error.
+    ///
+    /// # Examples
+    ///
+    /// A locked standard input implements `BufRead`:
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    ///
+    /// let stdin = io::stdin();
+    /// let mut stdin = stdin.lock();
+    ///
+    /// let buffer = stdin.fill_buf().unwrap();
+    ///
+    /// // work with buffer
+    /// println!("{buffer:?}");
+    ///
+    /// // ensure the bytes we worked with aren't returned again later
+    /// let length = buffer.len();
+    /// stdin.consume(length);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn fill_buf(&mut self) -> Result<&[u8]>;
+
+    /// Tells this buffer that `amt` bytes have been consumed from the buffer,
+    /// so they should no longer be returned in calls to `read`.
+    ///
+    /// This function is a lower-level call. It needs to be paired with the
+    /// [`fill_buf`] method to function properly. This function does
+    /// not perform any I/O, it simply informs this object that some amount of
+    /// its buffer, returned from [`fill_buf`], has been consumed and should
+    /// no longer be returned. As such, this function may do odd things if
+    /// [`fill_buf`] isn't called before calling it.
+    ///
+    /// The `amt` must be `<=` the number of bytes in the buffer returned by
+    /// [`fill_buf`].
+    ///
+    /// # Examples
+    ///
+    /// Since `consume()` is meant to be used with [`fill_buf`],
+    /// that method's example includes an example of `consume()`.
+    ///
+    /// [`fill_buf`]: BufRead::fill_buf
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn consume(&mut self, amt: usize);
+
+    /// Check if the underlying `Read` has any data left to be read.
+    ///
+    /// This function may fill the buffer to check for data,
+    /// so this functions returns `Result<bool>`, not `bool`.
+    ///
+    /// Default implementation calls `fill_buf` and checks that
+    /// returned slice is empty (which means that there is no data left,
+    /// since EOF is reached).
+    ///
+    /// Examples
+    ///
+    /// ```
+    /// #![feature(buf_read_has_data_left)]
+    /// use std::io;
+    /// use std::io::prelude::*;
+    ///
+    /// let stdin = io::stdin();
+    /// let mut stdin = stdin.lock();
+    ///
+    /// while stdin.has_data_left().unwrap() {
+    ///     let mut line = String::new();
+    ///     stdin.read_line(&mut line).unwrap();
+    ///     // work with line
+    ///     println!("{line:?}");
+    /// }
+    /// ```
+    #[unstable(feature = "buf_read_has_data_left", reason = "recently added", issue = "86423")]
+    fn has_data_left(&mut self) -> Result<bool> {
+        self.fill_buf().map(|b| !b.is_empty())
+    }
+
+    /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
+    ///
+    /// This function will read bytes from the underlying stream until the
+    /// delimiter or EOF is found. Once found, all bytes up to, and including,
+    /// the delimiter (if found) will be appended to `buf`.
+    ///
+    /// If successful, this function will return the total number of bytes read.
+    ///
+    /// This function is blocking and should be used carefully: it is possible for
+    /// an attacker to continuously send bytes without ever sending the delimiter
+    /// or EOF.
+    ///
+    /// # Errors
+    ///
+    /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
+    /// will otherwise return any errors returned by [`fill_buf`].
+    ///
+    /// If an I/O error is encountered then all bytes read so far will be
+    /// present in `buf` and its length will have been adjusted appropriately.
+    ///
+    /// [`fill_buf`]: BufRead::fill_buf
+    ///
+    /// # Examples
+    ///
+    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
+    /// this example, we use [`Cursor`] to read all the bytes in a byte slice
+    /// in hyphen delimited segments:
+    ///
+    /// ```
+    /// use std::io::{self, BufRead};
+    ///
+    /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
+    /// let mut buf = vec![];
+    ///
+    /// // cursor is at 'l'
+    /// let num_bytes = cursor.read_until(b'-', &mut buf)
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 6);
+    /// assert_eq!(buf, b"lorem-");
+    /// buf.clear();
+    ///
+    /// // cursor is at 'i'
+    /// let num_bytes = cursor.read_until(b'-', &mut buf)
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 5);
+    /// assert_eq!(buf, b"ipsum");
+    /// buf.clear();
+    ///
+    /// // cursor is at EOF
+    /// let num_bytes = cursor.read_until(b'-', &mut buf)
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 0);
+    /// assert_eq!(buf, b"");
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
+        read_until(self, byte, buf)
+    }
+
+    /// Read all bytes until a newline (the `0xA` byte) is reached, and append
+    /// them to the provided buffer. You do not need to clear the buffer before
+    /// appending.
+    ///
+    /// This function will read bytes from the underlying stream until the
+    /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
+    /// up to, and including, the delimiter (if found) will be appended to
+    /// `buf`.
+    ///
+    /// If successful, this function will return the total number of bytes read.
+    ///
+    /// If this function returns [`Ok(0)`], the stream has reached EOF.
+    ///
+    /// This function is blocking and should be used carefully: it is possible for
+    /// an attacker to continuously send bytes without ever sending a newline
+    /// or EOF.
+    ///
+    /// [`Ok(0)`]: Ok
+    ///
+    /// # Errors
+    ///
+    /// This function has the same error semantics as [`read_until`] and will
+    /// also return an error if the read bytes are not valid UTF-8. If an I/O
+    /// error is encountered then `buf` may contain some bytes already read in
+    /// the event that all data read so far was valid UTF-8.
+    ///
+    /// [`read_until`]: BufRead::read_until
+    ///
+    /// # Examples
+    ///
+    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
+    /// this example, we use [`Cursor`] to read all the lines in a byte slice:
+    ///
+    /// ```
+    /// use std::io::{self, BufRead};
+    ///
+    /// let mut cursor = io::Cursor::new(b"foo\nbar");
+    /// let mut buf = String::new();
+    ///
+    /// // cursor is at 'f'
+    /// let num_bytes = cursor.read_line(&mut buf)
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 4);
+    /// assert_eq!(buf, "foo\n");
+    /// buf.clear();
+    ///
+    /// // cursor is at 'b'
+    /// let num_bytes = cursor.read_line(&mut buf)
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 3);
+    /// assert_eq!(buf, "bar");
+    /// buf.clear();
+    ///
+    /// // cursor is at EOF
+    /// let num_bytes = cursor.read_line(&mut buf)
+    ///     .expect("reading from cursor won't fail");
+    /// assert_eq!(num_bytes, 0);
+    /// assert_eq!(buf, "");
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn read_line(&mut self, buf: &mut String) -> Result<usize> {
+        // Note that we are not calling the `.read_until` method here, but
+        // rather our hardcoded implementation. For more details as to why, see
+        // the comments in `read_to_end`.
+        unsafe { append_to_string(buf, |b| read_until(self, b'\n', b)) }
+    }
+
+    /// Returns an iterator over the contents of this reader split on the byte
+    /// `byte`.
+    ///
+    /// The iterator returned from this function will return instances of
+    /// <code>[io::Result]<[Vec]\<u8>></code>. Each vector returned will *not* have
+    /// the delimiter byte at the end.
+    ///
+    /// This function will yield errors whenever [`read_until`] would have
+    /// also yielded an error.
+    ///
+    /// [io::Result]: self::Result "io::Result"
+    /// [`read_until`]: BufRead::read_until
+    ///
+    /// # Examples
+    ///
+    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
+    /// this example, we use [`Cursor`] to iterate over all hyphen delimited
+    /// segments in a byte slice
+    ///
+    /// ```
+    /// use std::io::{self, BufRead};
+    ///
+    /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
+    ///
+    /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
+    /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
+    /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
+    /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
+    /// assert_eq!(split_iter.next(), None);
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn split(self, byte: u8) -> Split<Self>
+    where
+        Self: Sized,
+    {
+        Split { buf: self, delim: byte }
+    }
+
+    /// Returns an iterator over the lines of this reader.
+    ///
+    /// The iterator returned from this function will yield instances of
+    /// <code>[io::Result]<[String]></code>. Each string returned will *not* have a newline
+    /// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
+    ///
+    /// [io::Result]: self::Result "io::Result"
+    ///
+    /// # Examples
+    ///
+    /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
+    /// this example, we use [`Cursor`] to iterate over all the lines in a byte
+    /// slice.
+    ///
+    /// ```
+    /// use std::io::{self, BufRead};
+    ///
+    /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
+    ///
+    /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
+    /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
+    /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
+    /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
+    /// assert_eq!(lines_iter.next(), None);
+    /// ```
+    ///
+    /// # Errors
+    ///
+    /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
+    #[stable(feature = "rust1", since = "1.0.0")]
+    fn lines(self) -> Lines<Self>
+    where
+        Self: Sized,
+    {
+        Lines { buf: self }
+    }
+}
+
+/// Adapter to chain together two readers.
+///
+/// This struct is generally created by calling [`chain`] on a reader.
+/// Please see the documentation of [`chain`] for more details.
+///
+/// [`chain`]: Read::chain
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
+pub struct Chain<T, U> {
+    first: T,
+    second: U,
+    done_first: bool,
+}
+
+impl<T, U> Chain<T, U> {
+    /// Consumes the `Chain`, returning the wrapped readers.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut foo_file = File::open("foo.txt")?;
+    ///     let mut bar_file = File::open("bar.txt")?;
+    ///
+    ///     let chain = foo_file.chain(bar_file);
+    ///     let (foo_file, bar_file) = chain.into_inner();
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
+    pub fn into_inner(self) -> (T, U) {
+        (self.first, self.second)
+    }
+
+    /// Gets references to the underlying readers in this `Chain`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut foo_file = File::open("foo.txt")?;
+    ///     let mut bar_file = File::open("bar.txt")?;
+    ///
+    ///     let chain = foo_file.chain(bar_file);
+    ///     let (foo_file, bar_file) = chain.get_ref();
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
+    pub fn get_ref(&self) -> (&T, &U) {
+        (&self.first, &self.second)
+    }
+
+    /// Gets mutable references to the underlying readers in this `Chain`.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying readers as doing so may corrupt the internal state of this
+    /// `Chain`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut foo_file = File::open("foo.txt")?;
+    ///     let mut bar_file = File::open("bar.txt")?;
+    ///
+    ///     let mut chain = foo_file.chain(bar_file);
+    ///     let (foo_file, bar_file) = chain.get_mut();
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
+    pub fn get_mut(&mut self) -> (&mut T, &mut U) {
+        (&mut self.first, &mut self.second)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Read, U: Read> Read for Chain<T, U> {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        if !self.done_first {
+            match self.first.read(buf)? {
+                0 if !buf.is_empty() => self.done_first = true,
+                n => return Ok(n),
+            }
+        }
+        self.second.read(buf)
+    }
+
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
+        if !self.done_first {
+            match self.first.read_vectored(bufs)? {
+                0 if bufs.iter().any(|b| !b.is_empty()) => self.done_first = true,
+                n => return Ok(n),
+            }
+        }
+        self.second.read_vectored(bufs)
+    }
+}
+
+#[stable(feature = "chain_bufread", since = "1.9.0")]
+impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
+    fn fill_buf(&mut self) -> Result<&[u8]> {
+        if !self.done_first {
+            match self.first.fill_buf()? {
+                buf if buf.is_empty() => {
+                    self.done_first = true;
+                }
+                buf => return Ok(buf),
+            }
+        }
+        self.second.fill_buf()
+    }
+
+    fn consume(&mut self, amt: usize) {
+        if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
+    }
+}
+
+impl<T, U> SizeHint for Chain<T, U> {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        SizeHint::lower_bound(&self.first) + SizeHint::lower_bound(&self.second)
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        match (SizeHint::upper_bound(&self.first), SizeHint::upper_bound(&self.second)) {
+            (Some(first), Some(second)) => first.checked_add(second),
+            _ => None,
+        }
+    }
+}
+
+/// Reader adapter which limits the bytes read from an underlying reader.
+///
+/// This struct is generally created by calling [`take`] on a reader.
+/// Please see the documentation of [`take`] for more details.
+///
+/// [`take`]: Read::take
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
+pub struct Take<T> {
+    inner: T,
+    limit: u64,
+}
+
+impl<T> Take<T> {
+    /// Returns the number of bytes that can be read before this instance will
+    /// return EOF.
+    ///
+    /// # Note
+    ///
+    /// This instance may reach `EOF` after reading fewer bytes than indicated by
+    /// this method if the underlying [`Read`] instance reaches EOF.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let f = File::open("foo.txt")?;
+    ///
+    ///     // read at most five bytes
+    ///     let handle = f.take(5);
+    ///
+    ///     println!("limit: {}", handle.limit());
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn limit(&self) -> u64 {
+        self.limit
+    }
+
+    /// Sets the number of bytes that can be read before this instance will
+    /// return EOF. This is the same as constructing a new `Take` instance, so
+    /// the amount of bytes read and the previous limit value don't matter when
+    /// calling this method.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let f = File::open("foo.txt")?;
+    ///
+    ///     // read at most five bytes
+    ///     let mut handle = f.take(5);
+    ///     handle.set_limit(10);
+    ///
+    ///     assert_eq!(handle.limit(), 10);
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "take_set_limit", since = "1.27.0")]
+    pub fn set_limit(&mut self, limit: u64) {
+        self.limit = limit;
+    }
+
+    /// Consumes the `Take`, returning the wrapped reader.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut file = File::open("foo.txt")?;
+    ///
+    ///     let mut buffer = [0; 5];
+    ///     let mut handle = file.take(5);
+    ///     handle.read(&mut buffer)?;
+    ///
+    ///     let file = handle.into_inner();
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "io_take_into_inner", since = "1.15.0")]
+    pub fn into_inner(self) -> T {
+        self.inner
+    }
+
+    /// Gets a reference to the underlying reader.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut file = File::open("foo.txt")?;
+    ///
+    ///     let mut buffer = [0; 5];
+    ///     let mut handle = file.take(5);
+    ///     handle.read(&mut buffer)?;
+    ///
+    ///     let file = handle.get_ref();
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
+    pub fn get_ref(&self) -> &T {
+        &self.inner
+    }
+
+    /// Gets a mutable reference to the underlying reader.
+    ///
+    /// Care should be taken to avoid modifying the internal I/O state of the
+    /// underlying reader as doing so may corrupt the internal limit of this
+    /// `Take`.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    /// use std::io::prelude::*;
+    /// use std::fs::File;
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut file = File::open("foo.txt")?;
+    ///
+    ///     let mut buffer = [0; 5];
+    ///     let mut handle = file.take(5);
+    ///     handle.read(&mut buffer)?;
+    ///
+    ///     let file = handle.get_mut();
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
+    pub fn get_mut(&mut self) -> &mut T {
+        &mut self.inner
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: Read> Read for Take<T> {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
+        // Don't call into inner reader at all at EOF because it may still block
+        if self.limit == 0 {
+            return Ok(0);
+        }
+
+        let max = cmp::min(buf.len() as u64, self.limit) as usize;
+        let n = self.inner.read(&mut buf[..max])?;
+        assert!(n as u64 <= self.limit, "number of read bytes exceeds limit");
+        self.limit -= n as u64;
+        Ok(n)
+    }
+
+    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> Result<()> {
+        // Don't call into inner reader at all at EOF because it may still block
+        if self.limit == 0 {
+            return Ok(());
+        }
+
+        let prev_filled = buf.filled_len();
+
+        if self.limit <= buf.remaining() as u64 {
+            // if we just use an as cast to convert, limit may wrap around on a 32 bit target
+            let limit = cmp::min(self.limit, usize::MAX as u64) as usize;
+
+            let extra_init = cmp::min(limit as usize, buf.initialized_len() - buf.filled_len());
+
+            // SAFETY: no uninit data is written to ibuf
+            let ibuf = unsafe { &mut buf.unfilled_mut()[..limit] };
+
+            let mut sliced_buf = ReadBuf::uninit(ibuf);
+
+            // SAFETY: extra_init bytes of ibuf are known to be initialized
+            unsafe {
+                sliced_buf.assume_init(extra_init);
+            }
+
+            self.inner.read_buf(&mut sliced_buf)?;
+
+            let new_init = sliced_buf.initialized_len();
+            let filled = sliced_buf.filled_len();
+
+            // sliced_buf / ibuf must drop here
+
+            // SAFETY: new_init bytes of buf's unfilled buffer have been initialized
+            unsafe {
+                buf.assume_init(new_init);
+            }
+
+            buf.add_filled(filled);
+
+            self.limit -= filled as u64;
+        } else {
+            self.inner.read_buf(buf)?;
+
+            //inner may unfill
+            self.limit -= buf.filled_len().saturating_sub(prev_filled) as u64;
+        }
+
+        Ok(())
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<T: BufRead> BufRead for Take<T> {
+    fn fill_buf(&mut self) -> Result<&[u8]> {
+        // Don't call into inner reader at all at EOF because it may still block
+        if self.limit == 0 {
+            return Ok(&[]);
+        }
+
+        let buf = self.inner.fill_buf()?;
+        let cap = cmp::min(buf.len() as u64, self.limit) as usize;
+        Ok(&buf[..cap])
+    }
+
+    fn consume(&mut self, amt: usize) {
+        // Don't let callers reset the limit by passing an overlarge value
+        let amt = cmp::min(amt as u64, self.limit) as usize;
+        self.limit -= amt as u64;
+        self.inner.consume(amt);
+    }
+}
+
+impl<T> SizeHint for Take<T> {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        cmp::min(SizeHint::lower_bound(&self.inner) as u64, self.limit) as usize
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        match SizeHint::upper_bound(&self.inner) {
+            Some(upper_bound) => Some(cmp::min(upper_bound as u64, self.limit) as usize),
+            None => self.limit.try_into().ok(),
+        }
+    }
+}
+
+/// An iterator over `u8` values of a reader.
+///
+/// This struct is generally created by calling [`bytes`] on a reader.
+/// Please see the documentation of [`bytes`] for more details.
+///
+/// [`bytes`]: Read::bytes
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
+pub struct Bytes<R> {
+    inner: R,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<R: Read> Iterator for Bytes<R> {
+    type Item = Result<u8>;
+
+    fn next(&mut self) -> Option<Result<u8>> {
+        let mut byte = 0;
+        loop {
+            return match self.inner.read(slice::from_mut(&mut byte)) {
+                Ok(0) => None,
+                Ok(..) => Some(Ok(byte)),
+                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
+                Err(e) => Some(Err(e)),
+            };
+        }
+    }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        SizeHint::size_hint(&self.inner)
+    }
+}
+
+trait SizeHint {
+    fn lower_bound(&self) -> usize;
+
+    fn upper_bound(&self) -> Option<usize>;
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        (self.lower_bound(), self.upper_bound())
+    }
+}
+
+impl<T> SizeHint for T {
+    #[inline]
+    default fn lower_bound(&self) -> usize {
+        0
+    }
+
+    #[inline]
+    default fn upper_bound(&self) -> Option<usize> {
+        None
+    }
+}
+
+impl<T> SizeHint for &mut T {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        SizeHint::lower_bound(*self)
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        SizeHint::upper_bound(*self)
+    }
+}
+
+impl<T> SizeHint for Box<T> {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        SizeHint::lower_bound(&**self)
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        SizeHint::upper_bound(&**self)
+    }
+}
+
+impl SizeHint for &[u8] {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        self.len()
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        Some(self.len())
+    }
+}
+
+/// An iterator over the contents of an instance of `BufRead` split on a
+/// particular byte.
+///
+/// This struct is generally created by calling [`split`] on a `BufRead`.
+/// Please see the documentation of [`split`] for more details.
+///
+/// [`split`]: BufRead::split
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
+pub struct Split<B> {
+    buf: B,
+    delim: u8,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B: BufRead> Iterator for Split<B> {
+    type Item = Result<Vec<u8>>;
+
+    fn next(&mut self) -> Option<Result<Vec<u8>>> {
+        let mut buf = Vec::new();
+        match self.buf.read_until(self.delim, &mut buf) {
+            Ok(0) => None,
+            Ok(_n) => {
+                if buf[buf.len() - 1] == self.delim {
+                    buf.pop();
+                }
+                Some(Ok(buf))
+            }
+            Err(e) => Some(Err(e)),
+        }
+    }
+}
+
+/// An iterator over the lines of an instance of `BufRead`.
+///
+/// This struct is generally created by calling [`lines`] on a `BufRead`.
+/// Please see the documentation of [`lines`] for more details.
+///
+/// [`lines`]: BufRead::lines
+#[stable(feature = "rust1", since = "1.0.0")]
+#[derive(Debug)]
+pub struct Lines<B> {
+    buf: B,
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl<B: BufRead> Iterator for Lines<B> {
+    type Item = Result<String>;
+
+    fn next(&mut self) -> Option<Result<String>> {
+        let mut buf = String::new();
+        match self.buf.read_line(&mut buf) {
+            Ok(0) => None,
+            Ok(_n) => {
+                if buf.ends_with('\n') {
+                    buf.pop();
+                    if buf.ends_with('\r') {
+                        buf.pop();
+                    }
+                }
+                Some(Ok(buf))
+            }
+            Err(e) => Some(Err(e)),
+        }
+    }
+}
diff --git a/library/std/src/io/prelude.rs b/library/std/src/io/prelude.rs
new file mode 100644
index 000000000..d80643101
--- /dev/null
+++ b/library/std/src/io/prelude.rs
@@ -0,0 +1,14 @@
+//! 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::*;
+//! ```
+
+#![stable(feature = "rust1", since = "1.0.0")]
+
+#[stable(feature = "rust1", since = "1.0.0")]
+pub use super::{BufRead, Read, Seek, Write};
diff --git a/library/std/src/io/readbuf.rs b/library/std/src/io/readbuf.rs
new file mode 100644
index 000000000..78d1113f8
--- /dev/null
+++ b/library/std/src/io/readbuf.rs
@@ -0,0 +1,249 @@
+#![unstable(feature = "read_buf", issue = "78485")]
+
+#[cfg(test)]
+mod tests;
+
+use crate::cmp;
+use crate::fmt::{self, Debug, Formatter};
+use crate::mem::MaybeUninit;
+
+/// A wrapper around a byte buffer that is incrementally filled and initialized.
+///
+/// This type is a sort of "double cursor". It tracks three regions in the buffer: a region at the beginning of the
+/// buffer that has been logically filled with data, a region that has been initialized at some point but not yet
+/// logically filled, and a region at the end that is fully uninitialized. The filled region is guaranteed to be a
+/// subset of the initialized region.
+///
+/// In summary, the contents of the buffer can be visualized as:
+/// ```not_rust
+/// [             capacity              ]
+/// [ filled |         unfilled         ]
+/// [    initialized    | uninitialized ]
+/// ```
+pub struct ReadBuf<'a> {
+    buf: &'a mut [MaybeUninit<u8>],
+    filled: usize,
+    initialized: usize,
+}
+
+impl Debug for ReadBuf<'_> {
+    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ReadBuf")
+            .field("init", &self.initialized())
+            .field("filled", &self.filled)
+            .field("capacity", &self.capacity())
+            .finish()
+    }
+}
+
+impl<'a> ReadBuf<'a> {
+    /// Creates a new `ReadBuf` from a fully initialized buffer.
+    #[inline]
+    pub fn new(buf: &'a mut [u8]) -> ReadBuf<'a> {
+        let len = buf.len();
+
+        ReadBuf {
+            //SAFETY: initialized data never becoming uninitialized is an invariant of ReadBuf
+            buf: unsafe { (buf as *mut [u8]).as_uninit_slice_mut().unwrap() },
+            filled: 0,
+            initialized: len,
+        }
+    }
+
+    /// Creates a new `ReadBuf` from a fully uninitialized buffer.
+    ///
+    /// Use `assume_init` if part of the buffer is known to be already initialized.
+    #[inline]
+    pub fn uninit(buf: &'a mut [MaybeUninit<u8>]) -> ReadBuf<'a> {
+        ReadBuf { buf, filled: 0, initialized: 0 }
+    }
+
+    /// Returns the total capacity of the buffer.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.buf.len()
+    }
+
+    /// Returns a shared reference to the filled portion of the buffer.
+    #[inline]
+    pub fn filled(&self) -> &[u8] {
+        //SAFETY: We only slice the filled part of the buffer, which is always valid
+        unsafe { MaybeUninit::slice_assume_init_ref(&self.buf[0..self.filled]) }
+    }
+
+    /// Returns a mutable reference to the filled portion of the buffer.
+    #[inline]
+    pub fn filled_mut(&mut self) -> &mut [u8] {
+        //SAFETY: We only slice the filled part of the buffer, which is always valid
+        unsafe { MaybeUninit::slice_assume_init_mut(&mut self.buf[0..self.filled]) }
+    }
+
+    /// Returns a shared reference to the initialized portion of the buffer.
+    ///
+    /// This includes the filled portion.
+    #[inline]
+    pub fn initialized(&self) -> &[u8] {
+        //SAFETY: We only slice the initialized part of the buffer, which is always valid
+        unsafe { MaybeUninit::slice_assume_init_ref(&self.buf[0..self.initialized]) }
+    }
+
+    /// Returns a mutable reference to the initialized portion of the buffer.
+    ///
+    /// This includes the filled portion.
+    #[inline]
+    pub fn initialized_mut(&mut self) -> &mut [u8] {
+        //SAFETY: We only slice the initialized part of the buffer, which is always valid
+        unsafe { MaybeUninit::slice_assume_init_mut(&mut self.buf[0..self.initialized]) }
+    }
+
+    /// Returns a mutable reference to the unfilled part of the buffer without ensuring that it has been fully
+    /// initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller must not de-initialize portions of the buffer that have already been initialized.
+    #[inline]
+    pub unsafe fn unfilled_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+        &mut self.buf[self.filled..]
+    }
+
+    /// Returns a mutable reference to the uninitialized part of the buffer.
+    ///
+    /// It is safe to uninitialize any of these bytes.
+    #[inline]
+    pub fn uninitialized_mut(&mut self) -> &mut [MaybeUninit<u8>] {
+        &mut self.buf[self.initialized..]
+    }
+
+    /// Returns a mutable reference to the unfilled part of the buffer, ensuring it is fully initialized.
+    ///
+    /// Since `ReadBuf` tracks the region of the buffer that has been initialized, this is effectively "free" after
+    /// the first use.
+    #[inline]
+    pub fn initialize_unfilled(&mut self) -> &mut [u8] {
+        // should optimize out the assertion
+        self.initialize_unfilled_to(self.remaining())
+    }
+
+    /// Returns a mutable reference to the first `n` bytes of the unfilled part of the buffer, ensuring it is
+    /// fully initialized.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `self.remaining()` is less than `n`.
+    #[inline]
+    pub fn initialize_unfilled_to(&mut self, n: usize) -> &mut [u8] {
+        assert!(self.remaining() >= n);
+
+        let extra_init = self.initialized - self.filled;
+        // If we don't have enough initialized, do zeroing
+        if n > extra_init {
+            let uninit = n - extra_init;
+            let unfilled = &mut self.uninitialized_mut()[0..uninit];
+
+            for byte in unfilled.iter_mut() {
+                byte.write(0);
+            }
+
+            // SAFETY: we just initialized uninit bytes, and the previous bytes were already init
+            unsafe {
+                self.assume_init(n);
+            }
+        }
+
+        let filled = self.filled;
+
+        &mut self.initialized_mut()[filled..filled + n]
+    }
+
+    /// Returns the number of bytes at the end of the slice that have not yet been filled.
+    #[inline]
+    pub fn remaining(&self) -> usize {
+        self.capacity() - self.filled
+    }
+
+    /// Clears the buffer, resetting the filled region to empty.
+    ///
+    /// The number of initialized bytes is not changed, and the contents of the buffer are not modified.
+    #[inline]
+    pub fn clear(&mut self) -> &mut Self {
+        self.set_filled(0) // The assertion in `set_filled` is optimized out
+    }
+
+    /// Increases the size of the filled region of the buffer.
+    ///
+    /// The number of initialized bytes is not changed.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the filled region of the buffer would become larger than the initialized region.
+    #[inline]
+    pub fn add_filled(&mut self, n: usize) -> &mut Self {
+        self.set_filled(self.filled + n)
+    }
+
+    /// Sets the size of the filled region of the buffer.
+    ///
+    /// The number of initialized bytes is not changed.
+    ///
+    /// Note that this can be used to *shrink* the filled region of the buffer in addition to growing it (for
+    /// example, by a `Read` implementation that compresses data in-place).
+    ///
+    /// # Panics
+    ///
+    /// Panics if the filled region of the buffer would become larger than the initialized region.
+    #[inline]
+    pub fn set_filled(&mut self, n: usize) -> &mut Self {
+        assert!(n <= self.initialized);
+
+        self.filled = n;
+        self
+    }
+
+    /// Asserts that the first `n` unfilled bytes of the buffer are initialized.
+    ///
+    /// `ReadBuf` assumes that bytes are never de-initialized, so this method does nothing when called with fewer
+    /// bytes than are already known to be initialized.
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that the first `n` unfilled bytes of the buffer have already been initialized.
+    #[inline]
+    pub unsafe fn assume_init(&mut self, n: usize) -> &mut Self {
+        self.initialized = cmp::max(self.initialized, self.filled + n);
+        self
+    }
+
+    /// Appends data to the buffer, advancing the written position and possibly also the initialized position.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `self.remaining()` is less than `buf.len()`.
+    #[inline]
+    pub fn append(&mut self, buf: &[u8]) {
+        assert!(self.remaining() >= buf.len());
+
+        // SAFETY: we do not de-initialize any of the elements of the slice
+        unsafe {
+            MaybeUninit::write_slice(&mut self.unfilled_mut()[..buf.len()], buf);
+        }
+
+        // SAFETY: We just added the entire contents of buf to the filled section.
+        unsafe {
+            self.assume_init(buf.len());
+        }
+        self.add_filled(buf.len());
+    }
+
+    /// Returns the amount of bytes that have been filled.
+    #[inline]
+    pub fn filled_len(&self) -> usize {
+        self.filled
+    }
+
+    /// Returns the amount of bytes that have been initialized.
+    #[inline]
+    pub fn initialized_len(&self) -> usize {
+        self.initialized
+    }
+}
diff --git a/library/std/src/io/readbuf/tests.rs b/library/std/src/io/readbuf/tests.rs
new file mode 100644
index 000000000..3b7a5a56d
--- /dev/null
+++ b/library/std/src/io/readbuf/tests.rs
@@ -0,0 +1,181 @@
+use super::ReadBuf;
+use crate::mem::MaybeUninit;
+
+/// Test that ReadBuf has the correct numbers when created with new
+#[test]
+fn new() {
+    let mut buf = [0; 16];
+    let rbuf = ReadBuf::new(&mut buf);
+
+    assert_eq!(rbuf.filled_len(), 0);
+    assert_eq!(rbuf.initialized_len(), 16);
+    assert_eq!(rbuf.capacity(), 16);
+    assert_eq!(rbuf.remaining(), 16);
+}
+
+/// Test that ReadBuf has the correct numbers when created with uninit
+#[test]
+fn uninit() {
+    let mut buf = [MaybeUninit::uninit(); 16];
+    let rbuf = ReadBuf::uninit(&mut buf);
+
+    assert_eq!(rbuf.filled_len(), 0);
+    assert_eq!(rbuf.initialized_len(), 0);
+    assert_eq!(rbuf.capacity(), 16);
+    assert_eq!(rbuf.remaining(), 16);
+}
+
+#[test]
+fn initialize_unfilled() {
+    let mut buf = [MaybeUninit::uninit(); 16];
+    let mut rbuf = ReadBuf::uninit(&mut buf);
+
+    rbuf.initialize_unfilled();
+
+    assert_eq!(rbuf.initialized_len(), 16);
+}
+
+#[test]
+fn initialize_unfilled_to() {
+    let mut buf = [MaybeUninit::uninit(); 16];
+    let mut rbuf = ReadBuf::uninit(&mut buf);
+
+    rbuf.initialize_unfilled_to(8);
+
+    assert_eq!(rbuf.initialized_len(), 8);
+
+    rbuf.initialize_unfilled_to(4);
+
+    assert_eq!(rbuf.initialized_len(), 8);
+
+    rbuf.set_filled(8);
+
+    rbuf.initialize_unfilled_to(6);
+
+    assert_eq!(rbuf.initialized_len(), 14);
+
+    rbuf.initialize_unfilled_to(8);
+
+    assert_eq!(rbuf.initialized_len(), 16);
+}
+
+#[test]
+fn add_filled() {
+    let mut buf = [0; 16];
+    let mut rbuf = ReadBuf::new(&mut buf);
+
+    rbuf.add_filled(1);
+
+    assert_eq!(rbuf.filled_len(), 1);
+    assert_eq!(rbuf.remaining(), 15);
+}
+
+#[test]
+#[should_panic]
+fn add_filled_panic() {
+    let mut buf = [MaybeUninit::uninit(); 16];
+    let mut rbuf = ReadBuf::uninit(&mut buf);
+
+    rbuf.add_filled(1);
+}
+
+#[test]
+fn set_filled() {
+    let mut buf = [0; 16];
+    let mut rbuf = ReadBuf::new(&mut buf);
+
+    rbuf.set_filled(16);
+
+    assert_eq!(rbuf.filled_len(), 16);
+    assert_eq!(rbuf.remaining(), 0);
+
+    rbuf.set_filled(6);
+
+    assert_eq!(rbuf.filled_len(), 6);
+    assert_eq!(rbuf.remaining(), 10);
+}
+
+#[test]
+#[should_panic]
+fn set_filled_panic() {
+    let mut buf = [MaybeUninit::uninit(); 16];
+    let mut rbuf = ReadBuf::uninit(&mut buf);
+
+    rbuf.set_filled(16);
+}
+
+#[test]
+fn clear() {
+    let mut buf = [255; 16];
+    let mut rbuf = ReadBuf::new(&mut buf);
+
+    rbuf.set_filled(16);
+
+    assert_eq!(rbuf.filled_len(), 16);
+    assert_eq!(rbuf.remaining(), 0);
+
+    rbuf.clear();
+
+    assert_eq!(rbuf.filled_len(), 0);
+    assert_eq!(rbuf.remaining(), 16);
+
+    assert_eq!(rbuf.initialized(), [255; 16]);
+}
+
+#[test]
+fn assume_init() {
+    let mut buf = [MaybeUninit::uninit(); 16];
+    let mut rbuf = ReadBuf::uninit(&mut buf);
+
+    unsafe {
+        rbuf.assume_init(8);
+    }
+
+    assert_eq!(rbuf.initialized_len(), 8);
+
+    rbuf.add_filled(4);
+
+    unsafe {
+        rbuf.assume_init(2);
+    }
+
+    assert_eq!(rbuf.initialized_len(), 8);
+
+    unsafe {
+        rbuf.assume_init(8);
+    }
+
+    assert_eq!(rbuf.initialized_len(), 12);
+}
+
+#[test]
+fn append() {
+    let mut buf = [MaybeUninit::new(255); 16];
+    let mut rbuf = ReadBuf::uninit(&mut buf);
+
+    rbuf.append(&[0; 8]);
+
+    assert_eq!(rbuf.initialized_len(), 8);
+    assert_eq!(rbuf.filled_len(), 8);
+    assert_eq!(rbuf.filled(), [0; 8]);
+
+    rbuf.clear();
+
+    rbuf.append(&[1; 16]);
+
+    assert_eq!(rbuf.initialized_len(), 16);
+    assert_eq!(rbuf.filled_len(), 16);
+    assert_eq!(rbuf.filled(), [1; 16]);
+}
+
+#[test]
+fn filled_mut() {
+    let mut buf = [0; 16];
+    let mut rbuf = ReadBuf::new(&mut buf);
+
+    rbuf.add_filled(8);
+
+    let filled = rbuf.filled().to_vec();
+
+    assert_eq!(&*filled, &*rbuf.filled_mut());
+}
diff --git a/library/std/src/io/stdio.rs b/library/std/src/io/stdio.rs
new file mode 100644
index 000000000..4d3736f79
--- /dev/null
+++ b/library/std/src/io/stdio.rs
@@ -0,0 +1,1042 @@
+#![cfg_attr(test, allow(unused))]
+
+#[cfg(test)]
+mod tests;
+
+use crate::io::prelude::*;
+
+use crate::cell::{Cell, RefCell};
+use crate::fmt;
+use crate::io::{self, BufReader, IoSlice, IoSliceMut, LineWriter, Lines};
+use crate::pin::Pin;
+use crate::sync::atomic::{AtomicBool, Ordering};
+use crate::sync::{Arc, Mutex, MutexGuard, OnceLock};
+use crate::sys::stdio;
+use crate::sys_common::remutex::{ReentrantMutex, ReentrantMutexGuard};
+
+type LocalStream = Arc<Mutex<Vec<u8>>>;
+
+thread_local! {
+    /// Used by the test crate to capture the output of the print macros and panics.
+    static OUTPUT_CAPTURE: Cell<Option<LocalStream>> = {
+        Cell::new(None)
+    }
+}
+
+/// Flag to indicate OUTPUT_CAPTURE is used.
+///
+/// If it is None and was never set on any thread, this flag is set to false,
+/// and OUTPUT_CAPTURE can be safely ignored on all threads, saving some time
+/// and memory registering an unused thread local.
+///
+/// Note about memory ordering: This contains information about whether a
+/// thread local variable might be in use. Although this is a global flag, the
+/// memory ordering between threads does not matter: we only want this flag to
+/// have a consistent order between set_output_capture and print_to *within
+/// the same thread*. Within the same thread, things always have a perfectly
+/// consistent order. So Ordering::Relaxed is fine.
+static OUTPUT_CAPTURE_USED: AtomicBool = AtomicBool::new(false);
+
+/// A handle to a raw instance of the standard input stream of this process.
+///
+/// This handle is not synchronized or buffered in any fashion. Constructed via
+/// the `std::io::stdio::stdin_raw` function.
+struct StdinRaw(stdio::Stdin);
+
+/// A handle to a raw instance of the standard output stream of this process.
+///
+/// This handle is not synchronized or buffered in any fashion. Constructed via
+/// the `std::io::stdio::stdout_raw` function.
+struct StdoutRaw(stdio::Stdout);
+
+/// A handle to a raw instance of the standard output stream of this process.
+///
+/// This handle is not synchronized or buffered in any fashion. Constructed via
+/// the `std::io::stdio::stderr_raw` function.
+struct StderrRaw(stdio::Stderr);
+
+/// Constructs a new raw handle to the standard input of this process.
+///
+/// The returned handle does not interact with any other handles created nor
+/// handles returned by `std::io::stdin`. Data buffered by the `std::io::stdin`
+/// handles is **not** available to raw handles returned from this function.
+///
+/// The returned handle has no external synchronization or buffering.
+#[unstable(feature = "libstd_sys_internals", issue = "none")]
+const fn stdin_raw() -> StdinRaw {
+    StdinRaw(stdio::Stdin::new())
+}
+
+/// Constructs a new raw handle to the standard output stream of this process.
+///
+/// The returned handle does not interact with any other handles created nor
+/// handles returned by `std::io::stdout`. Note that data is buffered by the
+/// `std::io::stdout` handles so writes which happen via this raw handle may
+/// appear before previous writes.
+///
+/// The returned handle has no external synchronization or buffering layered on
+/// top.
+#[unstable(feature = "libstd_sys_internals", issue = "none")]
+const fn stdout_raw() -> StdoutRaw {
+    StdoutRaw(stdio::Stdout::new())
+}
+
+/// Constructs a new raw handle to the standard error stream of this process.
+///
+/// The returned handle does not interact with any other handles created nor
+/// handles returned by `std::io::stderr`.
+///
+/// The returned handle has no external synchronization or buffering layered on
+/// top.
+#[unstable(feature = "libstd_sys_internals", issue = "none")]
+const fn stderr_raw() -> StderrRaw {
+    StderrRaw(stdio::Stderr::new())
+}
+
+impl Read for StdinRaw {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        handle_ebadf(self.0.read(buf), 0)
+    }
+
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        handle_ebadf(self.0.read_vectored(bufs), 0)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        self.0.is_read_vectored()
+    }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        handle_ebadf(self.0.read_to_end(buf), 0)
+    }
+
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        handle_ebadf(self.0.read_to_string(buf), 0)
+    }
+}
+
+impl Write for StdoutRaw {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        handle_ebadf(self.0.write(buf), buf.len())
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let total = bufs.iter().map(|b| b.len()).sum();
+        handle_ebadf(self.0.write_vectored(bufs), total)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.0.is_write_vectored()
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        handle_ebadf(self.0.flush(), ())
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        handle_ebadf(self.0.write_all(buf), ())
+    }
+
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        handle_ebadf(self.0.write_all_vectored(bufs), ())
+    }
+
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
+        handle_ebadf(self.0.write_fmt(fmt), ())
+    }
+}
+
+impl Write for StderrRaw {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        handle_ebadf(self.0.write(buf), buf.len())
+    }
+
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        let total = bufs.iter().map(|b| b.len()).sum();
+        handle_ebadf(self.0.write_vectored(bufs), total)
+    }
+
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.0.is_write_vectored()
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        handle_ebadf(self.0.flush(), ())
+    }
+
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        handle_ebadf(self.0.write_all(buf), ())
+    }
+
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        handle_ebadf(self.0.write_all_vectored(bufs), ())
+    }
+
+    fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> io::Result<()> {
+        handle_ebadf(self.0.write_fmt(fmt), ())
+    }
+}
+
+fn handle_ebadf<T>(r: io::Result<T>, default: T) -> io::Result<T> {
+    match r {
+        Err(ref e) if stdio::is_ebadf(e) => Ok(default),
+        r => r,
+    }
+}
+
+/// A handle to the standard input stream of a process.
+///
+/// Each handle is a shared reference to a global buffer of input data to this
+/// process. A handle can be `lock`'d to gain full access to [`BufRead`] methods
+/// (e.g., `.lines()`). Reads to this handle are otherwise locked with respect
+/// to other reads.
+///
+/// This handle implements the `Read` trait, but beware that concurrent reads
+/// of `Stdin` must be executed with care.
+///
+/// Created by the [`io::stdin`] method.
+///
+/// [`io::stdin`]: stdin
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to read bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::io;
+///
+/// fn main() -> io::Result<()> {
+///     let mut buffer = String::new();
+///     let stdin = io::stdin(); // We get `Stdin` here.
+///     stdin.read_line(&mut buffer)?;
+///     Ok(())
+/// }
+/// ```
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Stdin {
+    inner: &'static Mutex<BufReader<StdinRaw>>,
+}
+
+/// A locked reference to the [`Stdin`] handle.
+///
+/// This handle implements both the [`Read`] and [`BufRead`] traits, and
+/// is constructed via the [`Stdin::lock`] method.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to read bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::io::{self, BufRead};
+///
+/// fn main() -> io::Result<()> {
+///     let mut buffer = String::new();
+///     let stdin = io::stdin(); // We get `Stdin` here.
+///     {
+///         let mut handle = stdin.lock(); // We get `StdinLock` here.
+///         handle.read_line(&mut buffer)?;
+///     } // `StdinLock` is dropped here.
+///     Ok(())
+/// }
+/// ```
+#[must_use = "if unused stdin will immediately unlock"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct StdinLock<'a> {
+    inner: MutexGuard<'a, BufReader<StdinRaw>>,
+}
+
+/// Constructs a new handle to the standard input of the current process.
+///
+/// Each handle returned is a reference to a shared global buffer whose access
+/// is synchronized via a mutex. If you need more explicit control over
+/// locking, see the [`Stdin::lock`] method.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to read bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+///
+/// # Examples
+///
+/// Using implicit synchronization:
+///
+/// ```no_run
+/// use std::io;
+///
+/// fn main() -> io::Result<()> {
+///     let mut buffer = String::new();
+///     io::stdin().read_line(&mut buffer)?;
+///     Ok(())
+/// }
+/// ```
+///
+/// Using explicit synchronization:
+///
+/// ```no_run
+/// use std::io::{self, BufRead};
+///
+/// fn main() -> io::Result<()> {
+///     let mut buffer = String::new();
+///     let stdin = io::stdin();
+///     let mut handle = stdin.lock();
+///
+///     handle.read_line(&mut buffer)?;
+///     Ok(())
+/// }
+/// ```
+#[must_use]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn stdin() -> Stdin {
+    static INSTANCE: OnceLock<Mutex<BufReader<StdinRaw>>> = OnceLock::new();
+    Stdin {
+        inner: INSTANCE.get_or_init(|| {
+            Mutex::new(BufReader::with_capacity(stdio::STDIN_BUF_SIZE, stdin_raw()))
+        }),
+    }
+}
+
+impl Stdin {
+    /// Locks this handle to the standard input stream, returning a readable
+    /// guard.
+    ///
+    /// The lock is released when the returned lock goes out of scope. The
+    /// returned guard also implements the [`Read`] and [`BufRead`] traits for
+    /// accessing the underlying data.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::{self, BufRead};
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut buffer = String::new();
+    ///     let stdin = io::stdin();
+    ///     let mut handle = stdin.lock();
+    ///
+    ///     handle.read_line(&mut buffer)?;
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn lock(&self) -> StdinLock<'static> {
+        // Locks this handle with 'static lifetime. This depends on the
+        // implementation detail that the underlying `Mutex` is static.
+        StdinLock { inner: self.inner.lock().unwrap_or_else(|e| e.into_inner()) }
+    }
+
+    /// Locks this handle and reads a line of input, appending it to the specified buffer.
+    ///
+    /// For detailed semantics of this method, see the documentation on
+    /// [`BufRead::read_line`].
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    ///
+    /// let mut input = String::new();
+    /// match io::stdin().read_line(&mut input) {
+    ///     Ok(n) => {
+    ///         println!("{n} bytes read");
+    ///         println!("{input}");
+    ///     }
+    ///     Err(error) => println!("error: {error}"),
+    /// }
+    /// ```
+    ///
+    /// You can run the example one of two ways:
+    ///
+    /// - Pipe some text to it, e.g., `printf foo | path/to/executable`
+    /// - Give it text interactively by running the executable directly,
+    ///   in which case it will wait for the Enter key to be pressed before
+    ///   continuing
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn read_line(&self, buf: &mut String) -> io::Result<usize> {
+        self.lock().read_line(buf)
+    }
+
+    /// Consumes this handle and returns an iterator over input lines.
+    ///
+    /// For detailed semantics of this method, see the documentation on
+    /// [`BufRead::lines`].
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io;
+    ///
+    /// let lines = io::stdin().lines();
+    /// for line in lines {
+    ///     println!("got a line: {}", line.unwrap());
+    /// }
+    /// ```
+    #[must_use = "`self` will be dropped if the result is not used"]
+    #[stable(feature = "stdin_forwarders", since = "1.62.0")]
+    pub fn lines(self) -> Lines<StdinLock<'static>> {
+        self.lock().lines()
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Stdin {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Stdin").finish_non_exhaustive()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Read for Stdin {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.lock().read(buf)
+    }
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        self.lock().read_vectored(bufs)
+    }
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        self.lock().is_read_vectored()
+    }
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.lock().read_to_end(buf)
+    }
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        self.lock().read_to_string(buf)
+    }
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        self.lock().read_exact(buf)
+    }
+}
+
+// only used by platform-dependent io::copy specializations, i.e. unused on some platforms
+#[cfg(any(target_os = "linux", target_os = "android"))]
+impl StdinLock<'_> {
+    pub(crate) fn as_mut_buf(&mut self) -> &mut BufReader<impl Read> {
+        &mut self.inner
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Read for StdinLock<'_> {
+    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+        self.inner.read(buf)
+    }
+
+    fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> io::Result<usize> {
+        self.inner.read_vectored(bufs)
+    }
+
+    #[inline]
+    fn is_read_vectored(&self) -> bool {
+        self.inner.is_read_vectored()
+    }
+
+    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.inner.read_to_end(buf)
+    }
+
+    fn read_to_string(&mut self, buf: &mut String) -> io::Result<usize> {
+        self.inner.read_to_string(buf)
+    }
+
+    fn read_exact(&mut self, buf: &mut [u8]) -> io::Result<()> {
+        self.inner.read_exact(buf)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl BufRead for StdinLock<'_> {
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        self.inner.fill_buf()
+    }
+
+    fn consume(&mut self, n: usize) {
+        self.inner.consume(n)
+    }
+
+    fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> io::Result<usize> {
+        self.inner.read_until(byte, buf)
+    }
+
+    fn read_line(&mut self, buf: &mut String) -> io::Result<usize> {
+        self.inner.read_line(buf)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for StdinLock<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("StdinLock").finish_non_exhaustive()
+    }
+}
+
+/// A handle to the global standard output stream of the current process.
+///
+/// Each handle shares a global buffer of data to be written to the standard
+/// output stream. Access is also synchronized via a lock and explicit control
+/// over locking is available via the [`lock`] method.
+///
+/// Created by the [`io::stdout`] method.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to write bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+///
+/// [`lock`]: Stdout::lock
+/// [`io::stdout`]: stdout
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Stdout {
+    // FIXME: this should be LineWriter or BufWriter depending on the state of
+    //        stdout (tty or not). Note that if this is not line buffered it
+    //        should also flush-on-panic or some form of flush-on-abort.
+    inner: Pin<&'static ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>>,
+}
+
+/// A locked reference to the [`Stdout`] handle.
+///
+/// This handle implements the [`Write`] trait, and is constructed via
+/// the [`Stdout::lock`] method. See its documentation for more.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to write bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+#[must_use = "if unused stdout will immediately unlock"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct StdoutLock<'a> {
+    inner: ReentrantMutexGuard<'a, RefCell<LineWriter<StdoutRaw>>>,
+}
+
+static STDOUT: OnceLock<ReentrantMutex<RefCell<LineWriter<StdoutRaw>>>> = OnceLock::new();
+
+/// Constructs a new handle to the standard output of the current process.
+///
+/// Each handle returned is a reference to a shared global buffer whose access
+/// is synchronized via a mutex. If you need more explicit control over
+/// locking, see the [`Stdout::lock`] method.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to write bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+///
+/// # Examples
+///
+/// Using implicit synchronization:
+///
+/// ```no_run
+/// use std::io::{self, Write};
+///
+/// fn main() -> io::Result<()> {
+///     io::stdout().write_all(b"hello world")?;
+///
+///     Ok(())
+/// }
+/// ```
+///
+/// Using explicit synchronization:
+///
+/// ```no_run
+/// use std::io::{self, Write};
+///
+/// fn main() -> io::Result<()> {
+///     let stdout = io::stdout();
+///     let mut handle = stdout.lock();
+///
+///     handle.write_all(b"hello world")?;
+///
+///     Ok(())
+/// }
+/// ```
+#[must_use]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn stdout() -> Stdout {
+    Stdout {
+        inner: Pin::static_ref(&STDOUT).get_or_init_pin(
+            || unsafe { ReentrantMutex::new(RefCell::new(LineWriter::new(stdout_raw()))) },
+            |mutex| unsafe { mutex.init() },
+        ),
+    }
+}
+
+pub fn cleanup() {
+    if let Some(instance) = STDOUT.get() {
+        // Flush the data and disable buffering during shutdown
+        // by replacing the line writer by one with zero
+        // buffering capacity.
+        // We use try_lock() instead of lock(), because someone
+        // might have leaked a StdoutLock, which would
+        // otherwise cause a deadlock here.
+        if let Some(lock) = Pin::static_ref(instance).try_lock() {
+            *lock.borrow_mut() = LineWriter::with_capacity(0, stdout_raw());
+        }
+    }
+}
+
+impl Stdout {
+    /// Locks this handle to the standard output stream, returning a writable
+    /// guard.
+    ///
+    /// The lock is released when the returned lock goes out of scope. The
+    /// returned guard also implements the `Write` trait for writing data.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use std::io::{self, Write};
+    ///
+    /// fn main() -> io::Result<()> {
+    ///     let mut stdout = io::stdout().lock();
+    ///
+    ///     stdout.write_all(b"hello world")?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn lock(&self) -> StdoutLock<'static> {
+        // Locks this handle with 'static lifetime. This depends on the
+        // implementation detail that the underlying `ReentrantMutex` is
+        // static.
+        StdoutLock { inner: self.inner.lock() }
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Stdout {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Stdout").finish_non_exhaustive()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Write for Stdout {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        (&*self).write(buf)
+    }
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        (&*self).write_vectored(bufs)
+    }
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        io::Write::is_write_vectored(&&*self)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        (&*self).flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        (&*self).write_all(buf)
+    }
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        (&*self).write_all_vectored(bufs)
+    }
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        (&*self).write_fmt(args)
+    }
+}
+
+#[stable(feature = "write_mt", since = "1.48.0")]
+impl Write for &Stdout {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.lock().write(buf)
+    }
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        self.lock().write_vectored(bufs)
+    }
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.lock().is_write_vectored()
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.lock().flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.lock().write_all(buf)
+    }
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        self.lock().write_all_vectored(bufs)
+    }
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        self.lock().write_fmt(args)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Write for StdoutLock<'_> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.inner.borrow_mut().write(buf)
+    }
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        self.inner.borrow_mut().write_vectored(bufs)
+    }
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.inner.borrow_mut().is_write_vectored()
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.borrow_mut().flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.inner.borrow_mut().write_all(buf)
+    }
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        self.inner.borrow_mut().write_all_vectored(bufs)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for StdoutLock<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("StdoutLock").finish_non_exhaustive()
+    }
+}
+
+/// A handle to the standard error stream of a process.
+///
+/// For more information, see the [`io::stderr`] method.
+///
+/// [`io::stderr`]: stderr
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to write bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct Stderr {
+    inner: Pin<&'static ReentrantMutex<RefCell<StderrRaw>>>,
+}
+
+/// A locked reference to the [`Stderr`] handle.
+///
+/// This handle implements the [`Write`] trait and is constructed via
+/// the [`Stderr::lock`] method. See its documentation for more.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to write bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+#[must_use = "if unused stderr will immediately unlock"]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub struct StderrLock<'a> {
+    inner: ReentrantMutexGuard<'a, RefCell<StderrRaw>>,
+}
+
+/// Constructs a new handle to the standard error of the current process.
+///
+/// This handle is not buffered.
+///
+/// ### Note: Windows Portability Considerations
+///
+/// When operating in a console, the Windows implementation of this stream does not support
+/// non-UTF-8 byte sequences. Attempting to write bytes that are not valid UTF-8 will return
+/// an error.
+///
+/// In a process with a detached console, such as one using
+/// `#![windows_subsystem = "windows"]`, or in a child process spawned from such a process,
+/// the contained handle will be null. In such cases, the standard library's `Read` and
+/// `Write` will do nothing and silently succeed. All other I/O operations, via the
+/// standard library or via raw Windows API calls, will fail.
+///
+/// # Examples
+///
+/// Using implicit synchronization:
+///
+/// ```no_run
+/// use std::io::{self, Write};
+///
+/// fn main() -> io::Result<()> {
+///     io::stderr().write_all(b"hello world")?;
+///
+///     Ok(())
+/// }
+/// ```
+///
+/// Using explicit synchronization:
+///
+/// ```no_run
+/// use std::io::{self, Write};
+///
+/// fn main() -> io::Result<()> {
+///     let stderr = io::stderr();
+///     let mut handle = stderr.lock();
+///
+///     handle.write_all(b"hello world")?;
+///
+///     Ok(())
+/// }
+/// ```
+#[must_use]
+#[stable(feature = "rust1", since = "1.0.0")]
+pub fn stderr() -> Stderr {
+    // Note that unlike `stdout()` we don't use `at_exit` here to register a
+    // destructor. Stderr is not buffered , so there's no need to run a
+    // destructor for flushing the buffer
+    static INSTANCE: OnceLock<ReentrantMutex<RefCell<StderrRaw>>> = OnceLock::new();
+
+    Stderr {
+        inner: Pin::static_ref(&INSTANCE).get_or_init_pin(
+            || unsafe { ReentrantMutex::new(RefCell::new(stderr_raw())) },
+            |mutex| unsafe { mutex.init() },
+        ),
+    }
+}
+
+impl Stderr {
+    /// Locks this handle to the standard error stream, returning a writable
+    /// guard.
+    ///
+    /// The lock is released when the returned lock goes out of scope. The
+    /// returned guard also implements the [`Write`] trait for writing data.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use std::io::{self, Write};
+    ///
+    /// fn foo() -> io::Result<()> {
+    ///     let stderr = io::stderr();
+    ///     let mut handle = stderr.lock();
+    ///
+    ///     handle.write_all(b"hello world")?;
+    ///
+    ///     Ok(())
+    /// }
+    /// ```
+    #[stable(feature = "rust1", since = "1.0.0")]
+    pub fn lock(&self) -> StderrLock<'static> {
+        // Locks this handle with 'static lifetime. This depends on the
+        // implementation detail that the underlying `ReentrantMutex` is
+        // static.
+        StderrLock { inner: self.inner.lock() }
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Stderr {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Stderr").finish_non_exhaustive()
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Write for Stderr {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        (&*self).write(buf)
+    }
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        (&*self).write_vectored(bufs)
+    }
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        io::Write::is_write_vectored(&&*self)
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        (&*self).flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        (&*self).write_all(buf)
+    }
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        (&*self).write_all_vectored(bufs)
+    }
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        (&*self).write_fmt(args)
+    }
+}
+
+#[stable(feature = "write_mt", since = "1.48.0")]
+impl Write for &Stderr {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.lock().write(buf)
+    }
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        self.lock().write_vectored(bufs)
+    }
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.lock().is_write_vectored()
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.lock().flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.lock().write_all(buf)
+    }
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        self.lock().write_all_vectored(bufs)
+    }
+    fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> io::Result<()> {
+        self.lock().write_fmt(args)
+    }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Write for StderrLock<'_> {
+    fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
+        self.inner.borrow_mut().write(buf)
+    }
+    fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
+        self.inner.borrow_mut().write_vectored(bufs)
+    }
+    #[inline]
+    fn is_write_vectored(&self) -> bool {
+        self.inner.borrow_mut().is_write_vectored()
+    }
+    fn flush(&mut self) -> io::Result<()> {
+        self.inner.borrow_mut().flush()
+    }
+    fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
+        self.inner.borrow_mut().write_all(buf)
+    }
+    fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> io::Result<()> {
+        self.inner.borrow_mut().write_all_vectored(bufs)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for StderrLock<'_> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("StderrLock").finish_non_exhaustive()
+    }
+}
+
+/// Sets the thread-local output capture buffer and returns the old one.
+#[unstable(
+    feature = "internal_output_capture",
+    reason = "this function is meant for use in the test crate \
+        and may disappear in the future",
+    issue = "none"
+)]
+#[doc(hidden)]
+pub fn set_output_capture(sink: Option<LocalStream>) -> Option<LocalStream> {
+    if sink.is_none() && !OUTPUT_CAPTURE_USED.load(Ordering::Relaxed) {
+        // OUTPUT_CAPTURE is definitely None since OUTPUT_CAPTURE_USED is false.
+        return None;
+    }
+    OUTPUT_CAPTURE_USED.store(true, Ordering::Relaxed);
+    OUTPUT_CAPTURE.with(move |slot| slot.replace(sink))
+}
+
+/// Write `args` to the capture buffer if enabled and possible, or `global_s`
+/// otherwise. `label` identifies the stream in a panic message.
+///
+/// This function is used to print error messages, so it takes extra
+/// care to avoid causing a panic when `local_s` is unusable.
+/// For instance, if the TLS key for the local stream is
+/// already destroyed, or if the local stream is locked by another
+/// thread, it will just fall back to the global stream.
+///
+/// However, if the actual I/O causes an error, this function does panic.
+fn print_to<T>(args: fmt::Arguments<'_>, global_s: fn() -> T, label: &str)
+where
+    T: Write,
+{
+    if OUTPUT_CAPTURE_USED.load(Ordering::Relaxed)
+        && OUTPUT_CAPTURE.try_with(|s| {
+            // Note that we completely remove a local sink to write to in case
+            // our printing recursively panics/prints, so the recursive
+            // panic/print goes to the global sink instead of our local sink.
+            s.take().map(|w| {
+                let _ = w.lock().unwrap_or_else(|e| e.into_inner()).write_fmt(args);
+                s.set(Some(w));
+            })
+        }) == Ok(Some(()))
+    {
+        // Successfully wrote to capture buffer.
+        return;
+    }
+
+    if let Err(e) = global_s().write_fmt(args) {
+        panic!("failed printing to {label}: {e}");
+    }
+}
+
+#[unstable(
+    feature = "print_internals",
+    reason = "implementation detail which may disappear or be replaced at any time",
+    issue = "none"
+)]
+#[doc(hidden)]
+#[cfg(not(test))]
+pub fn _print(args: fmt::Arguments<'_>) {
+    print_to(args, stdout, "stdout");
+}
+
+#[unstable(
+    feature = "print_internals",
+    reason = "implementation detail which may disappear or be replaced at any time",
+    issue = "none"
+)]
+#[doc(hidden)]
+#[cfg(not(test))]
+pub fn _eprint(args: fmt::Arguments<'_>) {
+    print_to(args, stderr, "stderr");
+}
+
+#[cfg(test)]
+pub use realstd::io::{_eprint, _print};
diff --git a/library/std/src/io/stdio/tests.rs b/library/std/src/io/stdio/tests.rs
new file mode 100644
index 000000000..f89fd27ce
--- /dev/null
+++ b/library/std/src/io/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().lock());
+}
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn test_lock_stdin() {
+    test_lock(stdin, || stdin().lock());
+}
+#[test]
+#[cfg_attr(target_os = "emscripten", ignore)]
+fn test_lock_stdout() {
+    test_lock(stdout, || stdout().lock());
+}
+
+// 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]
+    );
+}
diff --git a/library/std/src/io/tests.rs b/library/std/src/io/tests.rs
new file mode 100644
index 000000000..f357f33ec
--- /dev/null
+++ b/library/std/src/io/tests.rs
@@ -0,0 +1,623 @@
+use super::{repeat, Cursor, ReadBuf, SeekFrom};
+use crate::cmp::{self, min};
+use crate::io::{self, IoSlice, IoSliceMut};
+use crate::io::{BufRead, BufReader, Read, Seek, Write};
+use crate::mem::MaybeUninit;
+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 read_buf_exact() {
+    let mut buf = [0; 4];
+    let mut buf = ReadBuf::new(&mut buf);
+
+    let mut c = Cursor::new(&b""[..]);
+    assert_eq!(c.read_buf_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+
+    let mut c = Cursor::new(&b"123456789"[..]);
+    c.read_buf_exact(&mut buf).unwrap();
+    assert_eq!(buf.filled(), b"1234");
+
+    buf.clear();
+
+    c.read_buf_exact(&mut buf).unwrap();
+    assert_eq!(buf.filled(), b"5678");
+
+    buf.clear();
+
+    assert_eq!(c.read_buf_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
+}
+
+#[test]
+fn take_eof() {
+    struct R;
+
+    impl Read for R {
+        fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
+            Err(io::const_io_error!(io::ErrorKind::Other, ""))
+        }
+    }
+    impl BufRead for R {
+        fn fill_buf(&mut self) -> io::Result<&[u8]> {
+            Err(io::const_io_error!(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::default_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() takes care not to over-allocate when a buffer is the
+    // exact size needed.
+    let mut vec1 = Vec::with_capacity(input.len());
+    ExampleSliceReader { slice: input }.read_to_end(&mut vec1)?;
+    assert_eq!(vec1.len(), input.len());
+    assert_eq!(vec1.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]
+#[should_panic]
+fn io_slice_mut_advance_slices_empty_slice() {
+    let mut empty_bufs = &mut [][..];
+    IoSliceMut::advance_slices(&mut empty_bufs, 1);
+}
+
+#[test]
+#[should_panic]
+fn io_slice_mut_advance_slices_beyond_total_length() {
+    let mut buf1 = [1; 8];
+    let mut bufs = &mut [IoSliceMut::new(&mut buf1)][..];
+
+    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]
+#[should_panic]
+fn io_slice_advance_slices_empty_slice() {
+    let mut empty_bufs = &mut [][..];
+    IoSlice::advance_slices(&mut empty_bufs, 1);
+}
+
+#[test]
+#[should_panic]
+fn io_slice_advance_slices_beyond_total_length() {
+    let buf1 = [1; 8];
+    let mut bufs = &mut [IoSlice::new(&buf1)][..];
+
+    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);
+        }
+    }
+}
+
+// Issue 94981
+#[test]
+#[should_panic = "number of read bytes exceeds limit"]
+fn test_take_wrong_length() {
+    struct LieAboutSize(bool);
+
+    impl Read for LieAboutSize {
+        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
+            // Lie about the read size at first time of read.
+            if core::mem::take(&mut self.0) { Ok(buf.len() + 1) } else { Ok(buf.len()) }
+        }
+    }
+
+    let mut buffer = vec![0; 4];
+    let mut reader = LieAboutSize(true).take(4);
+    // Primed the `Limit` by lying about the read size.
+    let _ = reader.read(&mut buffer[..]);
+}
+
+#[bench]
+fn bench_take_read(b: &mut test::Bencher) {
+    b.iter(|| {
+        let mut buf = [0; 64];
+
+        [255; 128].take(64).read(&mut buf).unwrap();
+    });
+}
+
+#[bench]
+fn bench_take_read_buf(b: &mut test::Bencher) {
+    b.iter(|| {
+        let mut buf = [MaybeUninit::uninit(); 64];
+
+        let mut rbuf = ReadBuf::uninit(&mut buf);
+
+        [255; 128].take(64).read_buf(&mut rbuf).unwrap();
+    });
+}
diff --git a/library/std/src/io/util.rs b/library/std/src/io/util.rs
new file mode 100644
index 000000000..c1300cd67
--- /dev/null
+++ b/library/std/src/io/util.rs
@@ -0,0 +1,270 @@
+#![allow(missing_copy_implementations)]
+
+#[cfg(test)]
+mod tests;
+
+use crate::fmt;
+use crate::io::{
+    self, BufRead, IoSlice, IoSliceMut, Read, ReadBuf, Seek, SeekFrom, SizeHint, Write,
+};
+
+/// A reader which is always at EOF.
+///
+/// This struct is generally created by calling [`empty()`]. Please see
+/// the documentation of [`empty()`] for more details.
+#[stable(feature = "rust1", since = "1.0.0")]
+#[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 <code>[Ok]\(0)</code>.
+///
+/// # 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());
+/// ```
+#[must_use]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
+pub const fn empty() -> Empty {
+    Empty
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+impl Read for Empty {
+    #[inline]
+    fn read(&mut self, _buf: &mut [u8]) -> io::Result<usize> {
+        Ok(0)
+    }
+
+    #[inline]
+    fn read_buf(&mut self, _buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        Ok(())
+    }
+}
+#[stable(feature = "rust1", since = "1.0.0")]
+impl BufRead for Empty {
+    #[inline]
+    fn fill_buf(&mut self) -> io::Result<&[u8]> {
+        Ok(&[])
+    }
+    #[inline]
+    fn consume(&mut self, _n: usize) {}
+}
+
+#[stable(feature = "empty_seek", since = "1.51.0")]
+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)
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+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.
+#[stable(feature = "rust1", since = "1.0.0")]
+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]);
+/// ```
+#[must_use]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
+pub const fn repeat(byte: u8) -> Repeat {
+    Repeat { byte }
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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())
+    }
+
+    fn read_buf(&mut self, buf: &mut ReadBuf<'_>) -> io::Result<()> {
+        // SAFETY: No uninit bytes are being written
+        for slot in unsafe { buf.unfilled_mut() } {
+            slot.write(self.byte);
+        }
+
+        let remaining = buf.remaining();
+
+        // SAFETY: the entire unfilled portion of buf has been initialized
+        unsafe {
+            buf.assume_init(remaining);
+        }
+
+        buf.add_filled(remaining);
+
+        Ok(())
+    }
+
+    #[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
+    }
+}
+
+impl SizeHint for Repeat {
+    #[inline]
+    fn lower_bound(&self) -> usize {
+        usize::MAX
+    }
+
+    #[inline]
+    fn upper_bound(&self) -> Option<usize> {
+        None
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+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.
+#[stable(feature = "rust1", since = "1.0.0")]
+#[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);
+/// ```
+#[must_use]
+#[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_const_unstable(feature = "const_io_structs", issue = "78812")]
+pub const fn sink() -> Sink {
+    Sink
+}
+
+#[stable(feature = "rust1", since = "1.0.0")]
+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(())
+    }
+}
+
+#[stable(feature = "write_mt", since = "1.48.0")]
+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(())
+    }
+}
+
+#[stable(feature = "std_debug", since = "1.16.0")]
+impl fmt::Debug for Sink {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Sink").finish_non_exhaustive()
+    }
+}
diff --git a/library/std/src/io/util/tests.rs b/library/std/src/io/util/tests.rs
new file mode 100644
index 000000000..08972a59a
--- /dev/null
+++ b/library/std/src/io/util/tests.rs
@@ -0,0 +1,147 @@
+use crate::cmp::{max, min};
+use crate::io::prelude::*;
+use crate::io::{
+    copy, empty, repeat, sink, BufWriter, Empty, ReadBuf, Repeat, Result, SeekFrom, Sink,
+    DEFAULT_BUF_SIZE,
+};
+
+use crate::mem::MaybeUninit;
+
+#[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);
+
+    let mut buf = [];
+    let mut buf = ReadBuf::uninit(&mut buf);
+    e.read_buf(&mut buf).unwrap();
+    assert_eq!(buf.filled_len(), 0);
+    assert_eq!(buf.initialized_len(), 0);
+
+    let mut buf = [MaybeUninit::uninit()];
+    let mut buf = ReadBuf::uninit(&mut buf);
+    e.read_buf(&mut buf).unwrap();
+    assert_eq!(buf.filled_len(), 0);
+    assert_eq!(buf.initialized_len(), 0);
+
+    let mut buf = [MaybeUninit::uninit(); 1024];
+    let mut buf = ReadBuf::uninit(&mut buf);
+    e.read_buf(&mut buf).unwrap();
+    assert_eq!(buf.filled_len(), 0);
+    assert_eq!(buf.initialized_len(), 0);
+
+    let mut buf = [MaybeUninit::uninit(); 1024];
+    let mut buf = ReadBuf::uninit(&mut buf);
+    e.by_ref().read_buf(&mut buf).unwrap();
+    assert_eq!(buf.filled_len(), 0);
+    assert_eq!(buf.initialized_len(), 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();
+}