library/std/src/io/mod.rs

//! 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.
//!
//! ## I/O Safety
//!
//! Rust follows an I/O safety discipline that is comparable to its memory safety discipline. This
//! means that file descriptors can be *exclusively owned*. (Here, "file descriptor" is meant to
//! subsume similar concepts that exist across a wide range of operating systems even if they might
//! use a different name, such as "handle".) An exclusively owned file descriptor is one that no
//! other code is allowed to access in any way, but the owner is allowed to access and even close
//! it any time. A type that owns its file descriptor should usually close it in its `drop`
//! function. Types like [`File`] own their file descriptor. Similarly, file descriptors
//! can be *borrowed*, granting the temporary right to perform operations on this file descriptor.
//! This indicates that the file descriptor will not be closed for the lifetime of the borrow, but
//! it does *not* imply any right to close this file descriptor, since it will likely be owned by
//! someone else.
//!
//! The platform-specific parts of the Rust standard library expose types that reflect these
//! concepts, see [`os::unix`] and [`os::windows`].
//!
//! To uphold I/O safety, it is crucial that no code acts on file descriptors it does not own or
//! borrow, and no code closes file descriptors it does not own. In other words, a safe function
//! that takes a regular integer, treats it as a file descriptor, and acts on it, is *unsound*.
//!
//! Not upholding I/O safety and acting on a file descriptor without proof of ownership can lead to
//! misbehavior and even Undefined Behavior in code that relies on ownership of its file
//! descriptors: a closed file descriptor could be re-allocated, so the original owner of that file
//! descriptor is now working on the wrong file. Some code might even rely on fully encapsulating
//! its file descriptors with no operations being performed by any other part of the program.
//!
//! Note that exclusive ownership of a file descriptor does *not* imply exclusive ownership of the
//! underlying kernel object that the file descriptor references (also called "file description" on
//! some operating systems). File descriptors basically work like [`Arc`]: when you receive an owned
//! file descriptor, you cannot know whether there are any other file descriptors that reference the
//! same kernel object. However, when you create a new kernel object, you know that you are holding
//! the only reference to it. Just be careful not to lend it to anyone, since they can obtain a
//! clone and then you can no longer know what the reference count is! In that sense, [`OwnedFd`] is
//! like `Arc` and [`BorrowedFd<'a>`] is like `&'a Arc` (and similar for the Windows types). In
//! particular, given a `BorrowedFd<'a>`, you are not allowed to close the file descriptor -- just
//! like how, given a `&'a Arc`, you are not allowed to decrement the reference count and
//! potentially free the underlying object. There is no equivalent to `Box` for file descriptors in
//! the standard library (that would be a type that guarantees that the reference count is `1`),
//! however, it would be possible for a crate to define a type with those semantics.
//!
//! [`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
//! [`os::unix`]: ../os/unix/io/index.html
//! [`os::windows`]: ../os/windows/io/index.html
//! [`OwnedFd`]: ../os/fd/struct.OwnedFd.html
//! [`BorrowedFd<'a>`]: ../os/fd/struct.BorrowedFd.html
//! [`Arc`]: crate::sync::Arc

#![stable(feature = "rust1", since = "1.0.0")]

#[cfg(test)]
mod tests;

use crate::cmp;
use crate::fmt;
use crate::mem::take;
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 = "raw_os_error_ty", issue = "107792")]
pub use self::error::RawOsError;
pub(crate) use self::stdio::attempt_print_to_stderr;
#[unstable(feature = "internal_output_capture", issue = "none")]
#[doc(no_inline, hidden)]
pub use self::stdio::set_output_capture;
#[stable(feature = "is_terminal", since = "1.70.0")]
pub use self::stdio::IsTerminal;
#[unstable(feature = "print_internals", issue = "none")]
#[doc(hidden)]
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 core::io::{BorrowedBuf, BorrowedCursor};
pub(crate) use error::const_io_error;

mod buffered;
pub(crate) mod copy;
mod cursor;
mod error;
mod impls;
pub mod prelude;
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
    }
}

// Here we must serve many masters with conflicting goals:
//
// - avoid allocating unless necessary
// - avoid overallocating if we know the exact size (#89165)
// - avoid passing large buffers to readers that always initialize the free capacity if they perform short reads (#23815, #23820)
// - pass large buffers to readers that do not initialize the spare capacity. this can amortize per-call overheads
// - and finally pass not-too-small and not-too-large buffers to Windows read APIs because they manage to suffer from both problems
//   at the same time, i.e. small reads suffer from syscall overhead, all reads incur initialization cost
//   proportional to buffer size (#110650)
//
pub(crate) fn default_read_to_end<R: Read   ?Sized>(
    r: &mut R,
    buf: &mut Vec<u8>,
    size_hint: Option<usize>,
) -> Result<usize> {
    let start_len = buf.len();
    let start_cap = buf.capacity();
    // Optionally limit the maximum bytes read on each iteration.
    // This adds an arbitrary fiddle factor to allow for more data than we expect.
    let mut max_read_size = size_hint
        .and_then(|s| s.checked_add(1024)?.checked_next_multiple_of(DEFAULT_BUF_SIZE))
        .unwrap_or(DEFAULT_BUF_SIZE);

    let mut initialized = 0; // Extra initialized bytes from previous loop iteration

    const PROBE_SIZE: usize = 32;

    fn small_probe_read<R: Read   ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
        let mut probe = [0u8; PROBE_SIZE];

        loop {
            match r.read(&mut probe) {
                Ok(n) => {
                    // there is no way to recover from allocation failure here
                    // because the data has already been read.
                    buf.extend_from_slice(&probe[..n]);
                    return Ok(n);
                }
                Err(ref e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            }
        }
    }

    // avoid inflating empty/small vecs before we have determined that there's anything to read
    if (size_hint.is_none() || size_hint == Some(0)) && buf.capacity() - buf.len() < PROBE_SIZE {
        let read = small_probe_read(r, buf)?;

        if read == 0 {
            return Ok(0);
        }
    }

    loop {
        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 read = small_probe_read(r, buf)?;

            if read == 0 {
                return Ok(buf.len() - start_len);
            }
        }

        if buf.len() == buf.capacity() {
            // buf is full, need more space
            buf.try_reserve(PROBE_SIZE).map_err(|_| ErrorKind::OutOfMemory)?;
        }

        let mut spare = buf.spare_capacity_mut();
        let buf_len = cmp::min(spare.len(), max_read_size);
        spare = &mut spare[..buf_len];
        let mut read_buf: BorrowedBuf<'_> = spare.into();

        // SAFETY: These bytes were initialized but not filled in the previous loop
        unsafe {
            read_buf.set_init(initialized);
        }

        let mut cursor = read_buf.unfilled();
        loop {
            match r.read_buf(cursor.reborrow()) {
                Ok(()) => break,
                Err(e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            }
        }

        let unfilled_but_initialized = cursor.init_ref().len();
        let bytes_read = cursor.written();
        let was_fully_initialized = read_buf.init_len() == buf_len;

        if bytes_read == 0 {
            return Ok(buf.len() - start_len);
        }

        // store how much was initialized but not filled
        initialized = unfilled_but_initialized;

        // SAFETY: BorrowedBuf's invariants mean this much memory is initialized.
        unsafe {
            let new_len = bytes_read   buf.len();
            buf.set_len(new_len);
        }

        // Use heuristics to determine the max read size if no initial size hint was provided
        if size_hint.is_none() {
            // The reader is returning short reads but it doesn't call ensure_init().
            // In that case we no longer need to restrict read sizes to avoid
            // initialization costs.
            if !was_fully_initialized {
                max_read_size = usize::MAX;
            }

            // we have passed a larger buffer than previously and the
            // reader still hasn't returned a short read
            if buf_len >= max_read_size && bytes_read == buf_len {
                max_read_size = max_read_size.saturating_mul(2);
            }
        }
    }
}

pub(crate) fn default_read_to_string<R: Read   ?Sized>(
    r: &mut R,
    buf: &mut String,
    size_hint: Option<usize>,
) -> 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, size_hint)) }
}

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) => {
                buf = &mut buf[n..];
            }
            Err(ref e) if e.is_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, mut cursor: BorrowedCursor<'_>) -> Result<()>
where
    F: FnOnce(&mut [u8]) -> Result<usize>,
{
    let n = read(cursor.ensure_init().init_mut())?;
    unsafe {
        // SAFETY: we initialised using `ensure_init` so there is no uninit data to advance to.
        cursor.advance(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.
///
/// Repeated calls to the reader use the same cursor, so for example
/// calling `read_to_end` twice on a [`File`] will only return the file's
/// contents once. It's recommended to first call `rewind()` in that case.
///
/// # 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 in unsafe code 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, so 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 in unsafe code 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
    ///
    /// ## Implementing `read_to_end`
    ///
    /// When implementing the `io::Read` trait, it is recommended to allocate
    /// memory using [`Vec::try_reserve`]. However, this behavior is not guaranteed
    /// by all implementations, and `read_to_end` may not handle out-of-memory
    /// situations gracefully.
    ///
    /// ```no_run
    /// # use std::io::{self, BufRead};
    /// # struct Example { example_datasource: io::Empty } impl Example {
    /// # fn get_some_data_for_the_example(&self) -> &'static [u8] { &[] }
    /// fn read_to_end(&mut self, dest_vec: &mut Vec<u8>) -> io::Result<usize> {
    ///     let initial_vec_len = dest_vec.len();
    ///     loop {
    ///         let src_buf = self.example_datasource.fill_buf()?;
    ///         if src_buf.is_empty() {
    ///             break;
    ///         }
    ///         dest_vec.try_reserve(src_buf.len()).map_err(|_| io::ErrorKind::OutOfMemory)?;
    ///         dest_vec.extend_from_slice(src_buf);
    ///
    ///         // Any irreversible side effects should happen after `try_reserve` succeeds,
    ///         // to avoid losing data on allocation error.
    ///         let read = src_buf.len();
    ///         self.example_datasource.consume(read);
    ///     }
    ///     Ok(dest_vec.len() - initial_vec_len)
    /// }
    /// # }
    /// ```
    ///
    /// [`Vec::try_reserve`]: crate::vec::Vec::try_reserve
    #[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, None)
    }

    /// 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, None)
    }

    /// 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, so 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 [`BorrowedCursor`] 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: BorrowedCursor<'_>) -> Result<()> {
        default_read_buf(|b| self.read(b), buf)
    }

    /// Read the exact number of bytes required to fill `cursor`.
    ///
    /// This is similar to the [`read_exact`](Read::read_exact) method, except
    /// that it is passed a [`BorrowedCursor`] rather than `[u8]` to allow use
    /// with uninitialized buffers.
    ///
    /// # 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`].
    ///
    /// If any other read error is encountered then this function immediately
    /// returns.
    ///
    /// If this function returns an error, all bytes read will be appended to `cursor`.
    #[unstable(feature = "read_buf", issue = "78485")]
    fn read_buf_exact(&mut self, mut cursor: BorrowedCursor<'_>) -> Result<()> {
        while cursor.capacity() > 0 {
            let prev_written = cursor.written();
            match self.read_buf(cursor.reborrow()) {
                Ok(()) => {}
                Err(e) if e.is_interrupted() => continue,
                Err(e) => return Err(e),
            }

            if cursor.written() == prev_written {
                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.
    ///
    /// The default implementation calls `read` for each byte,
    /// which can be very inefficient for data that's not in memory,
    /// such as [`File`]. Consider using a [`BufReader`] in such cases.
    ///
    /// # 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::io::BufReader;
    /// use std::fs::File;
    ///
    /// fn main() -> io::Result<()> {
    ///     let f = BufReader::new(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
/// # use std::io;
/// fn main() -> io::Result<()> {
///     let stdin = io::read_to_string(io::stdin())?;
///     println!("Stdin was:");
///     println!("{stdin}");
///     Ok(())
/// }
/// ```
#[stable(feature = "io_read_to_string", since = "1.65.0")]
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;
        // Remaining length before reaching n.
        let mut left = n;
        for buf in bufs.iter() {
            if let Some(remainder) = left.checked_sub(buf.len()) {
                left = remainder;
                remove  = 1;
            } else {
                break;
            }
        }

        *bufs = &mut take(bufs)[remove..];
        if bufs.is_empty() {
            assert!(left == 0, "advancing io slices beyond their length");
        } else {
            bufs[0].advance(left);
        }
    }
}

#[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;
        // Remaining length before reaching n. This prevents overflow
        // that could happen if the length of slices in `bufs` were instead
        // accumulated. Those slice may be aliased and, if they are large
        // enough, their added length may overflow a `usize`.
        let mut left = n;
        for buf in bufs.iter() {
            if let Some(remainder) = left.checked_sub(buf.len()) {
                left = remainder;
                remove  = 1;
            } else {
                break;
            }
        }

        *bufs = &mut take(bufs)[remove..];
        if bufs.is_empty() {
            assert!(left == 0, "advancing io slices beyond their length");
        } else {
            bufs[0].advance(left);
        }
    }
}

#[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. Typically, a call to `write` represents 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 this method consumed `n > 0` bytes of `buf` it must return [`Ok(n)`].
    /// If the return value is `Ok(n)` then `n` must satisfy `n <= buf.len()`.
    /// A return value of `Ok(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.is_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.is_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")]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoSeek")]
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))
    }

    /// Seeks relative to the current position.
    ///
    /// This is equivalent to `self.seek(SeekFrom::Current(offset))` but
    /// doesn't return the new position which can allow some implementations
    /// such as [`BufReader`] to perform more efficient seeks.
    ///
    /// # Example
    ///
    /// ```no_run
    /// #![feature(seek_seek_relative)]
    /// use std::{
    ///     io::{self, Seek},
    ///     fs::File,
    /// };
    ///
    /// fn main() -> io::Result<()> {
    ///     let mut f = File::open("foo.txt")?;
    ///     f.seek_relative(10)?;
    ///     assert_eq!(f.stream_position()?, 10);
    ///     Ok(())
    /// }
    /// ```
    ///
    /// [`BufReader`]: crate::io::BufReader
    #[unstable(feature = "seek_seek_relative", issue = "117374")]
    fn seek_relative(&mut self, offset: i64) -> Result<()> {
        self.seek(SeekFrom::Current(offset))?;
        Ok(())
    }
}

/// 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.is_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);
        }
    }
}

fn skip_until<R: BufRead   ?Sized>(r: &mut R, delim: 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) => (true, i   1),
                None => (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)
    }

    /// Skip all bytes until the delimiter `byte` or EOF is reached.
    ///
    /// This function will read (and discard) bytes from the underlying stream until the
    /// delimiter or EOF is found.
    ///
    /// If successful, this function will return the total number of bytes read,
    /// including the delimiter byte.
    ///
    /// This is useful for efficiently skipping data such as NUL-terminated strings
    /// in binary file formats without buffering.
    ///
    /// 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 some NUL-terminated information
    /// about Ferris from a binary string, skipping the fun fact:
    ///
    /// ```
    /// #![feature(bufread_skip_until)]
    ///
    /// use std::io::{self, BufRead};
    ///
    /// let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0");
    ///
    /// // read name
    /// let mut name = Vec::new();
    /// let num_bytes = cursor.read_until(b'\0', &mut name)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 7);
    /// assert_eq!(name, b"Ferris\0");
    ///
    /// // skip fun fact
    /// let num_bytes = cursor.skip_until(b'\0')
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 30);
    ///
    /// // read animal type
    /// let mut animal = Vec::new();
    /// let num_bytes = cursor.read_until(b'\0', &mut animal)
    ///     .expect("reading from cursor won't fail");
    /// assert_eq!(num_bytes, 11);
    /// assert_eq!(animal, b"Crustacean\0");
    /// ```
    #[unstable(feature = "bufread_skip_until", issue = "111735")]
    fn skip_until(&mut self, byte: u8) -> Result<usize> {
        skip_until(self, byte)
    }

    /// Read all bytes until a newline (the `0xA` byte) is reached, and append
    /// them to the provided `String` buffer.
    ///
    /// Previous content of the buffer will be preserved. To avoid appending to
    /// the buffer, you need to [`clear`] it first.
    ///
    /// 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. You can use [`take`] to limit the maximum number of bytes read.
    ///
    /// [`Ok(0)`]: Ok
    /// [`clear`]: String::clear
    /// [`take`]: crate::io::Read::take
    ///
    /// # 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, mut buf: BorrowedCursor<'_>) -> Result<()> {
        // Don't call into inner reader at all at EOF because it may still block
        if self.limit == 0 {
            return Ok(());
        }

        if self.limit <= buf.capacity() 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.init_ref().len());

            // SAFETY: no uninit data is written to ibuf
            let ibuf = unsafe { &mut buf.as_mut()[..limit] };

            let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();

            // SAFETY: extra_init bytes of ibuf are known to be initialized
            unsafe {
                sliced_buf.set_init(extra_init);
            }

            let mut cursor = sliced_buf.unfilled();
            self.inner.read_buf(cursor.reborrow())?;

            let new_init = cursor.init_ref().len();
            let filled = sliced_buf.len();

            // cursor / sliced_buf / ibuf must drop here

            unsafe {
                // SAFETY: filled bytes have been filled and therefore initialized
                buf.advance(filled);
                // SAFETY: new_init bytes of buf's unfilled buffer have been initialized
                buf.set_init(new_init);
            }

            self.limit -= filled as u64;
        } else {
            let written = buf.written();
            self.inner.read_buf(buf.reborrow())?;
            self.limit -= (buf.written() - written) 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>;

    // Not `#[inline]`. This function gets inlined even without it, but having
    // the inline annotation can result in worse code generation. See #116785.
    fn next(&mut self) -> Option<Result<u8>> {
        SpecReadByte::spec_read_byte(&mut self.inner)
    }

    #[inline]
    fn size_hint(&self) -> (usize, Option<usize>) {
        SizeHint::size_hint(&self.inner)
    }
}

/// For the specialization of `Bytes::next`.
trait SpecReadByte {
    fn spec_read_byte(&mut self) -> Option<Result<u8>>;
}

impl<R> SpecReadByte for R
where
    Self: Read,
{
    #[inline]
    default fn spec_read_byte(&mut self) -> Option<Result<u8>> {
        inlined_slow_read_byte(self)
    }
}

/// Read a single byte in a slow, generic way. This is used by the default
/// `spec_read_byte`.
#[inline]
fn inlined_slow_read_byte<R: Read>(reader: &mut R) -> Option<Result<u8>> {
    let mut byte = 0;
    loop {
        return match reader.read(slice::from_mut(&mut byte)) {
            Ok(0) => None,
            Ok(..) => Some(Ok(byte)),
            Err(ref e) if e.is_interrupted() => continue,
            Err(e) => Some(Err(e)),
        };
    }
}

// Used by `BufReader::spec_read_byte`, for which the `inline(ever)` is
// important.
#[inline(never)]
fn uninlined_slow_read_byte<R: Read>(reader: &mut R) -> Option<Result<u8>> {
    inlined_slow_read_byte(reader)
}

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: ?Sized> 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)]
#[cfg_attr(not(test), rustc_diagnostic_item = "IoLines")]
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)),
        }
    }
}