mod.rs source code [crates/std/src/io/mod.rs]

1	//! Traits, helpers, and type definitions for core I/O functionality.
2	//!
3	//! The `std::io` module contains a number of common things you'll need
4	//! when doing input and output. The most core part of this module is
5	//! the [`Read`] and [`Write`] traits, which provide the
6	//! most general interface for reading and writing input and output.
7	//!
8	//! ## Read and Write
9	//!
10	//! Because they are traits, [`Read`] and [`Write`] are implemented by a number
11	//! of other types, and you can implement them for your types too. As such,
12	//! you'll see a few different types of I/O throughout the documentation in
13	//! this module: [`File`]s, [`TcpStream`]s, and sometimes even [`Vec<T>`]s. For
14	//! example, [`Read`] adds a [`read`][`Read::read`] method, which we can use on
15	//! [`File`]s:
16	//!
17	//! ```no_run
18	//! use std::io;
19	//! use std::io::prelude::*;
20	//! use std::fs::File;
21	//!
22	//! fn main() -> io::Result<()> {
23	//! let mut f = File::open("foo.txt")?;
24	//! let mut buffer = [`0`; `10`];
25	//!
26	//! // read up to 10 bytes
27	//! let n = f.read(&mut buffer)?;
28	//!
29	//! println!("The bytes: {:?}", &buffer[..n]);
30	//! Ok(())
31	//! }
32	//! ```
33	//!
34	//! [`Read`] and [`Write`] are so important, implementors of the two traits have a
35	//! nickname: readers and writers. So you'll sometimes see 'a reader' instead
36	//! of 'a type that implements the [`Read`] trait'. Much easier!
37	//!
38	//! ## Seek and BufRead
39	//!
40	//! Beyond that, there are two important traits that are provided: [`Seek`]
41	//! and [`BufRead`]. Both of these build on top of a reader to control
42	//! how the reading happens. [`Seek`] lets you control where the next byte is
43	//! coming from:
44	//!
45	//! ```no_run
46	//! use std::io;
47	//! use std::io::prelude::*;
48	//! use std::io::SeekFrom;
49	//! use std::fs::File;
50	//!
51	//! fn main() -> io::Result<()> {
52	//! let mut f = File::open("foo.txt")?;
53	//! let mut buffer = [`0`; `10`];
54	//!
55	//! // skip to the last 10 bytes of the file
56	//! f.seek(SeekFrom::End(`-10`))?;
57	//!
58	//! // read up to 10 bytes
59	//! let n = f.read(&mut buffer)?;
60	//!
61	//! println!("The bytes: {:?}", &buffer[..n]);
62	//! Ok(())
63	//! }
64	//! ```
65	//!
66	//! [`BufRead`] uses an internal buffer to provide a number of other ways to read, but
67	//! to show it off, we'll need to talk about buffers in general. Keep reading!
68	//!
69	//! ## BufReader and BufWriter
70	//!
71	//! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
72	//! making near-constant calls to the operating system. To help with this,
73	//! `std::io` comes with two structs, [`BufReader`] and [`BufWriter`], which wrap
74	//! readers and writers. The wrapper uses a buffer, reducing the number of
75	//! calls and providing nicer methods for accessing exactly what you want.
76	//!
77	//! For example, [`BufReader`] works with the [`BufRead`] trait to add extra
78	//! methods to any reader:
79	//!
80	//! ```no_run
81	//! use std::io;
82	//! use std::io::prelude::*;
83	//! use std::io::BufReader;
84	//! use std::fs::File;
85	//!
86	//! fn main() -> io::Result<()> {
87	//! let f = File::open("foo.txt")?;
88	//! let mut reader = BufReader::new(f);
89	//! let mut buffer = String::new();
90	//!
91	//! // read a line into buffer
92	//! reader.read_line(&mut buffer)?;
93	//!
94	//! println!("{buffer}");
95	//! Ok(())
96	//! }
97	//! ```
98	//!
99	//! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
100	//! to [`write`][`Write::write`]:
101	//!
102	//! ```no_run
103	//! use std::io;
104	//! use std::io::prelude::*;
105	//! use std::io::BufWriter;
106	//! use std::fs::File;
107	//!
108	//! fn main() -> io::Result<()> {
109	//! let f = File::create("foo.txt")?;
110	//! {
111	//! let mut writer = BufWriter::new(f);
112	//!
113	//! // write a byte to the buffer
114	//! writer.write(&[`42`])?;
115	//!
116	//! } // the buffer is flushed once writer goes out of scope
117	//!
118	//! Ok(())
119	//! }
120	//! ```
121	//!
122	//! ## Standard input and output
123	//!
124	//! A very common source of input is standard input:
125	//!
126	//! ```no_run
127	//! use std::io;
128	//!
129	//! fn main() -> io::Result<()> {
130	//! let mut input = String::new();
131	//!
132	//! io::stdin().read_line(&mut input)?;
133	//!
134	//! println!("You typed: {}", input.trim());
135	//! Ok(())
136	//! }
137	//! ```
138	//!
139	//! Note that you cannot use the [`?` operator] in functions that do not return
140	//! a [`Result<T, E>`][`Result`]. Instead, you can call [`.unwrap()`]
141	//! or `match` on the return value to catch any possible errors:
142	//!
143	//! ```no_run
144	//! use std::io;
145	//!
146	//! let mut input = String::new();
147	//!
148	//! io::stdin().read_line(&mut input).unwrap();
149	//! ```
150	//!
151	//! And a very common source of output is standard output:
152	//!
153	//! ```no_run
154	//! use std::io;
155	//! use std::io::prelude::*;
156	//!
157	//! fn main() -> io::Result<()> {
158	//! io::stdout().write(&[`42`])?;
159	//! Ok(())
160	//! }
161	//! ```
162	//!
163	//! Of course, using [`io::stdout`] directly is less common than something like
164	//! [`println!`].
165	//!
166	//! ## Iterator types
167	//!
168	//! A large number of the structures provided by `std::io` are for various
169	//! ways of iterating over I/O. For example, [`Lines`] is used to split over
170	//! lines:
171	//!
172	//! ```no_run
173	//! use std::io;
174	//! use std::io::prelude::*;
175	//! use std::io::BufReader;
176	//! use std::fs::File;
177	//!
178	//! fn main() -> io::Result<()> {
179	//! let f = File::open("foo.txt")?;
180	//! let reader = BufReader::new(f);
181	//!
182	//! for line in reader.lines() {
183	//! println!("{}", line?);
184	//! }
185	//! Ok(())
186	//! }
187	//! ```
188	//!
189	//! ## Functions
190	//!
191	//! There are a number of [functions][functions-list] that offer access to various
192	//! features. For example, we can use three of these functions to copy everything
193	//! from standard input to standard output:
194	//!
195	//! ```no_run
196	//! use std::io;
197	//!
198	//! fn main() -> io::Result<()> {
199	//! io::copy(&mut io::stdin(), &mut io::stdout())?;
200	//! Ok(())
201	//! }
202	//! ```
203	//!
204	//! [functions-list]: #functions-1
205	//!
206	//! ## io::Result
207	//!
208	//! Last, but certainly not least, is [`io::Result`]. This type is used
209	//! as the return type of many `std::io` functions that can cause an error, and
210	//! can be returned from your own functions as well. Many of the examples in this
211	//! module use the [`?` operator]:
212	//!
213	//! ```
214	//! use std::io;
215	//!
216	//! fn read_input() -> io::Result<()> {
217	//! let mut input = String::new();
218	//!
219	//! io::stdin().read_line(&mut input)?;
220	//!
221	//! println!("You typed: {}", input.trim());
222	//!
223	//! Ok(())
224	//! }
225	//! ```
226	//!
227	//! The return type of `read_input()`, [`io::Result<()>`][`io::Result`], is a very
228	//! common type for functions which don't have a 'real' return value, but do want to
229	//! return errors if they happen. In this case, the only purpose of this function is
230	//! to read the line and print it, so we use `()`.
231	//!
232	//! ## Platform-specific behavior
233	//!
234	//! Many I/O functions throughout the standard library are documented to indicate
235	//! what various library or syscalls they are delegated to. This is done to help
236	//! applications both understand what's happening under the hood as well as investigate
237	//! any possibly unclear semantics. Note, however, that this is informative, not a binding
238	//! contract. The implementation of many of these functions are subject to change over
239	//! time and may call fewer or more syscalls/library functions.
240	//!
241	//! ## I/O Safety
242	//!
243	//! Rust follows an I/O safety discipline that is comparable to its memory safety discipline. This
244	//! means that file descriptors can be exclusively owned. (Here, "file descriptor" is meant to
245	//! subsume similar concepts that exist across a wide range of operating systems even if they might
246	//! use a different name, such as "handle".) An exclusively owned file descriptor is one that no
247	//! other code is allowed to access in any way, but the owner is allowed to access and even close
248	//! it any time. A type that owns its file descriptor should usually close it in its `drop`
249	//! function. Types like [`File`] own their file descriptor. Similarly, file descriptors
250	//! can be borrowed, granting the temporary right to perform operations on this file descriptor.
251	//! This indicates that the file descriptor will not be closed for the lifetime of the borrow, but
252	//! it does not* imply any right to close this file descriptor, since it will likely be owned by*
253	//! someone else.
254	//!
255	//! The platform-specific parts of the Rust standard library expose types that reflect these
256	//! concepts, see [`os::unix`] and [`os::windows`].
257	//!
258	//! To uphold I/O safety, it is crucial that no code acts on file descriptors it does not own or
259	//! borrow, and no code closes file descriptors it does not own. In other words, a safe function
260	//! that takes a regular integer, treats it as a file descriptor, and acts on it, is unsound.
261	//!
262	//! Not upholding I/O safety and acting on a file descriptor without proof of ownership can lead to
263	//! misbehavior and even Undefined Behavior in code that relies on ownership of its file
264	//! descriptors: a closed file descriptor could be re-allocated, so the original owner of that file
265	//! descriptor is now working on the wrong file. Some code might even rely on fully encapsulating
266	//! its file descriptors with no operations being performed by any other part of the program.
267	//!
268	//! Note that exclusive ownership of a file descriptor does not* imply exclusive ownership of the*
269	//! underlying kernel object that the file descriptor references (also called "open file description" on
270	//! some operating systems). File descriptors basically work like [`Arc`]: when you receive an owned
271	//! file descriptor, you cannot know whether there are any other file descriptors that reference the
272	//! same kernel object. However, when you create a new kernel object, you know that you are holding
273	//! the only reference to it. Just be careful not to lend it to anyone, since they can obtain a
274	//! clone and then you can no longer know what the reference count is! In that sense, [`OwnedFd`] is
275	//! like `Arc` and [`BorrowedFd<'a>`] is like `&'a Arc` (and similar for the Windows types). In
276	//! particular, given a `BorrowedFd<'a>`, you are not allowed to close the file descriptor -- just
277	//! like how, given a `&'a Arc`, you are not allowed to decrement the reference count and
278	//! potentially free the underlying object. There is no equivalent to `Box` for file descriptors in
279	//! the standard library (that would be a type that guarantees that the reference count is `1`),
280	//! however, it would be possible for a crate to define a type with those semantics.
281	//!
282	//! [`File`]: crate::fs::File
283	//! [`TcpStream`]: crate::net::TcpStream
284	//! [`io::stdout`]: stdout
285	//! [`io::Result`]: self::Result
286	//! [`?` operator]: ../../book/appendix-02-operators.html
287	//! [`Result`]: crate::result::Result
288	//! [`.unwrap()`]: crate::result::Result::unwrap
289	//! [`os::unix`]: ../os/unix/io/index.html
290	//! [`os::windows`]: ../os/windows/io/index.html
291	//! [`OwnedFd`]: ../os/fd/struct.OwnedFd.html
292	//! [`BorrowedFd<'a>`]: ../os/fd/struct.BorrowedFd.html
293	//! [`Arc`]: crate::sync::Arc
294
295	#![stable(feature = "rust1", since = "1.0.0")]
296
297	#[cfg(test)]
298	mod tests;
299
300	#[unstable(feature = "read_buf", issue = "78485")]
301	pub use core::io::{BorrowedBuf, BorrowedCursor};
302	use core::slice::memchr;
303
304	#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
305	pub use self::buffered::WriterPanicked;
306	#[unstable(feature = "raw_os_error_ty", issue = "107792")]
307	pub use self::error::RawOsError;
308	#[doc(hidden)]
309	#[unstable(feature = "io_const_error_internals", issue = "none")]
310	pub use self::error::SimpleMessage;
311	#[unstable(feature = "io_const_error", issue = "133448")]
312	pub use self::error::const_error;
313	#[stable(feature = "anonymous_pipe", since = "1.87.0")]
314	pub use self::pipe::{PipeReader, PipeWriter, pipe};
315	#[stable(feature = "is_terminal", since = "1.70.0")]
316	pub use self::stdio::IsTerminal;
317	pub(crate) use self::stdio::attempt_print_to_stderr;
318	#[unstable(feature = "print_internals", issue = "none")]
319	#[doc(hidden)]
320	pub use self::stdio::{_eprint, _print};
321	#[unstable(feature = "internal_output_capture", issue = "none")]
322	#[doc(no_inline, hidden)]
323	pub use self::stdio::{set_output_capture, try_set_output_capture};
324	#[stable(feature = "rust1", since = "1.0.0")]
325	pub use self::{
326	buffered::{BufReader, BufWriter, IntoInnerError, LineWriter},
327	copy::copy,
328	cursor::Cursor,
329	error::{Error, ErrorKind, Result},
330	stdio::{Stderr, StderrLock, Stdin, StdinLock, Stdout, StdoutLock, stderr, stdin, stdout},
331	util::{Empty, Repeat, Sink, empty, repeat, sink},
332	};
333	use crate::mem::take;
334	use crate::ops::{Deref, DerefMut};
335	use crate::{cmp, fmt, slice, str, sys};
336
337	mod buffered;
338	pub(crate) mod copy;
339	mod cursor;
340	mod error;
341	mod impls;
342	mod pipe;
343	pub mod prelude;
344	mod stdio;
345	mod util;
346
347	const DEFAULT_BUF_SIZE: usize = crate::sys::io::DEFAULT_BUF_SIZE;
348
349	pub(crate) use stdio::cleanup;
350
351	struct Guard<'a> {
352	buf: &'a mut Vec<u8>,
353	len: usize,
354	}
355
356	impl Drop for Guard<'_> {
357	fn drop(&mut self) {
358	unsafe {
359	self.buf.set_len(self.len);
360	}
361	}
362	}
363
364	// Several `read_to_string` and `read_line` methods in the standard library will
365	// append data into a `String` buffer, but we need to be pretty careful when
366	// doing this. The implementation will just call `.as_mut_vec()` and then
367	// delegate to a byte-oriented reading method, but we must ensure that when
368	// returning we never leave `buf` in a state such that it contains invalid UTF-8
369	// in its bounds.
370	//
371	// To this end, we use an RAII guard (to protect against panics) which updates
372	// the length of the string when it is dropped. This guard initially truncates
373	// the string to the prior length and only after we've validated that the
374	// new contents are valid UTF-8 do we allow it to set a longer length.
375	//
376	// The unsafety in this function is twofold:
377	//
378	// 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
379	// checks.
380	// 2. We're passing a raw buffer to the function `f`, and it is expected that
381	// the function only appends* bytes to the buffer. We'll get undefined*
382	// behavior if existing bytes are overwritten to have non-UTF-8 data.
383	pub(crate) unsafe fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
384	where
385	F: FnOnce(&mut Vec<u8>) -> Result<usize>,
386	{
387	let mut g: Guard<'_> = Guard { len: buf.len(), buf: unsafe { buf.as_mut_vec() } };
388	let ret: Result = f(g.buf);
389
390	// SAFETY: the caller promises to only append data to `buf`
391	let appended: &[u8] = unsafe { g.buf.get_unchecked(index:g.len..) };
392	if str::from_utf8(appended).is_err() {
393	ret.and_then(\|_\| Err(Error::INVALID_UTF8))
394	} else {
395	g.len = g.buf.len();
396	ret
397	}
398	}
399
400	// Here we must serve many masters with conflicting goals:
401	//
402	// - avoid allocating unless necessary
403	// - avoid overallocating if we know the exact size (#89165)
404	// - avoid passing large buffers to readers that always initialize the free capacity if they perform short reads (#23815, #23820)
405	// - pass large buffers to readers that do not initialize the spare capacity. this can amortize per-call overheads
406	// - and finally pass not-too-small and not-too-large buffers to Windows read APIs because they manage to suffer from both problems
407	// at the same time, i.e. small reads suffer from syscall overhead, all reads incur costs proportional to buffer size (#110650)
408	//
409	pub(crate) fn default_read_to_end<R: Read + ?Sized>(
410	r: &mut R,
411	buf: &mut Vec<u8>,
412	size_hint: Option<usize>,
413	) -> Result<usize> {
414	let start_len = buf.len();
415	let start_cap = buf.capacity();
416	// Optionally limit the maximum bytes read on each iteration.
417	// This adds an arbitrary fiddle factor to allow for more data than we expect.
418	let mut max_read_size = size_hint
419	.and_then(\|s\| s.checked_add(`1024`)?.checked_next_multiple_of(DEFAULT_BUF_SIZE))
420	.unwrap_or(DEFAULT_BUF_SIZE);
421
422	let mut initialized = `0`; // Extra initialized bytes from previous loop iteration
423
424	const PROBE_SIZE: usize = `32`;
425
426	fn small_probe_read<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
427	let mut probe = [`0u8`; PROBE_SIZE];
428
429	loop {
430	match r.read(&mut probe) {
431	Ok(n) => {
432	// there is no way to recover from allocation failure here
433	// because the data has already been read.
434	buf.extend_from_slice(&probe[..n]);
435	return Ok(n);
436	}
437	Err(ref e) if e.is_interrupted() => continue,
438	Err(e) => return Err(e),
439	}
440	}
441	}
442
443	// avoid inflating empty/small vecs before we have determined that there's anything to read
444	if (size_hint.is_none() \|\| size_hint == Some(`0`)) && buf.capacity() - buf.len() < PROBE_SIZE {
445	let read = small_probe_read(r, buf)?;
446
447	if read == `0` {
448	return Ok(`0`);
449	}
450	}
451
452	let mut consecutive_short_reads = `0`;
453
454	loop {
455	if buf.len() == buf.capacity() && buf.capacity() == start_cap {
456	// The buffer might be an exact fit. Let's read into a probe buffer
457	// and see if it returns `Ok(0)`. If so, we've avoided an
458	// unnecessary doubling of the capacity. But if not, append the
459	// probe buffer to the primary buffer and let its capacity grow.
460	let read = small_probe_read(r, buf)?;
461
462	if read == `0` {
463	return Ok(buf.len() - start_len);
464	}
465	}
466
467	if buf.len() == buf.capacity() {
468	// buf is full, need more space
469	buf.try_reserve(PROBE_SIZE)?;
470	}
471
472	let mut spare = buf.spare_capacity_mut();
473	let buf_len = cmp::min(spare.len(), max_read_size);
474	spare = &mut spare[..buf_len];
475	let mut read_buf: BorrowedBuf<'_> = spare.into();
476
477	// SAFETY: These bytes were initialized but not filled in the previous loop
478	unsafe {
479	read_buf.set_init(initialized);
480	}
481
482	let mut cursor = read_buf.unfilled();
483	let result = loop {
484	match r.read_buf(cursor.reborrow()) {
485	Err(e) if e.is_interrupted() => continue,
486	// Do not stop now in case of error: we might have received both data
487	// and an error
488	res => break res,
489	}
490	};
491
492	let unfilled_but_initialized = cursor.init_ref().len();
493	let bytes_read = cursor.written();
494	let was_fully_initialized = read_buf.init_len() == buf_len;
495
496	// SAFETY: BorrowedBuf's invariants mean this much memory is initialized.
497	unsafe {
498	let new_len = bytes_read + buf.len();
499	buf.set_len(new_len);
500	}
501
502	// Now that all data is pushed to the vector, we can fail without data loss
503	result?;
504
505	if bytes_read == `0` {
506	return Ok(buf.len() - start_len);
507	}
508
509	if bytes_read < buf_len {
510	consecutive_short_reads += `1`;
511	} else {
512	consecutive_short_reads = `0`;
513	}
514
515	// store how much was initialized but not filled
516	initialized = unfilled_but_initialized;
517
518	// Use heuristics to determine the max read size if no initial size hint was provided
519	if size_hint.is_none() {
520	// The reader is returning short reads but it doesn't call ensure_init().
521	// In that case we no longer need to restrict read sizes to avoid
522	// initialization costs.
523	// When reading from disk we usually don't get any short reads except at EOF.
524	// So we wait for at least 2 short reads before uncapping the read buffer;
525	// this helps with the Windows issue.
526	if !was_fully_initialized && consecutive_short_reads > `1` {
527	max_read_size = usize::MAX;
528	}
529
530	// we have passed a larger buffer than previously and the
531	// reader still hasn't returned a short read
532	if buf_len >= max_read_size && bytes_read == buf_len {
533	max_read_size = max_read_size.saturating_mul(`2`);
534	}
535	}
536	}
537	}
538
539	pub(crate) fn default_read_to_string<R: Read + ?Sized>(
540	r: &mut R,
541	buf: &mut String,
542	size_hint: Option<usize>,
543	) -> Result<usize> {
544	// Note that we do not* call `r.read_to_end()` here. We are passing*
545	// `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
546	// method to fill it up. An arbitrary implementation could overwrite the
547	// entire contents of the vector, not just append to it (which is what
548	// we are expecting).
549	//
550	// To prevent extraneously checking the UTF-8-ness of the entire buffer
551	// we pass it to our hardcoded `default_read_to_end` implementation which
552	// we know is guaranteed to only read data into the end of the buffer.
553	unsafe { append_to_string(buf, \|b: &mut Vec\| default_read_to_end(r, buf:b, size_hint)) }
554	}
555
556	pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
557	where
558	F: FnOnce(&mut [u8]) -> Result<usize>,
559	{
560	let buf: &mut [u8] = bufs.iter_mut().find(\|b\| !b.is_empty()).map_or(&mut [][..], \|b: &mut IoSliceMut<'_>\| &mut **b);
561	read(buf)
562	}
563
564	pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
565	where
566	F: FnOnce(&[u8]) -> Result<usize>,
567	{
568	let buf: &[u8] = bufs.iter().find(\|b\| !b.is_empty()).map_or(&[][..], \|b: &IoSlice<'_>\| &**b);
569	write(buf)
570	}
571
572	pub(crate) fn default_read_exact<R: Read + ?Sized>(this: &mut R, mut buf: &mut [u8]) -> Result<()> {
573	while !buf.is_empty() {
574	match this.read(buf) {
575	Ok(`0`) => break,
576	Ok(n: usize) => {
577	buf = &mut buf[n..];
578	}
579	Err(ref e: &Error) if e.is_interrupted() => {}
580	Err(e: Error) => return Err(e),
581	}
582	}
583	if !buf.is_empty() { Err(Error::READ_EXACT_EOF) } else { Ok(()) }
584	}
585
586	pub(crate) fn default_read_buf<F>(read: F, mut cursor: BorrowedCursor<'_>) -> Result<()>
587	where
588	F: FnOnce(&mut [u8]) -> Result<usize>,
589	{
590	let n: usize = read(cursor.ensure_init().init_mut())?;
591	cursor.advance(n);
592	Ok(())
593	}
594
595	pub(crate) fn default_read_buf_exact<R: Read + ?Sized>(
596	this: &mut R,
597	mut cursor: BorrowedCursor<'_>,
598	) -> Result<()> {
599	while cursor.capacity() > `0` {
600	let prev_written: usize = cursor.written();
601	match this.read_buf(cursor.reborrow()) {
602	Ok(()) => {}
603	Err(e: Error) if e.is_interrupted() => continue,
604	Err(e: Error) => return Err(e),
605	}
606
607	if cursor.written() == prev_written {
608	return Err(Error::READ_EXACT_EOF);
609	}
610	}
611
612	Ok(())
613	}
614
615	pub(crate) fn default_write_fmt<W: Write + ?Sized>(
616	this: &mut W,
617	args: fmt::Arguments<'_>,
618	) -> Result<()> {
619	// Create a shim which translates a `Write` to a `fmt::Write` and saves off
620	// I/O errors, instead of discarding them.
621	struct Adapter<'a, T: ?Sized + 'a> {
622	inner: &'a mut T,
623	error: Result<()>,
624	}
625
626	impl<T: Write + ?Sized> fmt::Write for Adapter<'_, T> {
627	fn write_str(&mut self, s: &str) -> fmt::Result {
628	match self.inner.write_all(s.as_bytes()) {
629	Ok(()) => Ok(()),
630	Err(e) => {
631	self.error = Err(e);
632	Err(fmt::Error)
633	}
634	}
635	}
636	}
637
638	let mut output = Adapter { inner: this, error: Ok(()) };
639	match fmt::write(&mut output, args) {
640	Ok(()) => Ok(()),
641	Err(..) => {
642	// Check whether the error came from the underlying `Write`.
643	if output.error.is_err() {
644	output.error
645	} else {
646	// This shouldn't happen: the underlying stream did not error,
647	// but somehow the formatter still errored?
648	panic!(
649	"a formatting trait implementation returned an error when the underlying stream did not"
650	);
651	}
652	}
653	}
654	}
655
656	/// The `Read` trait allows for reading bytes from a source.
657	///
658	/// Implementors of the `Read` trait are called 'readers'.
659	///
660	/// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
661	/// will attempt to pull bytes from this source into a provided buffer. A
662	/// number of other methods are implemented in terms of [`read()`], giving
663	/// implementors a number of ways to read bytes while only needing to implement
664	/// a single method.
665	///
666	/// Readers are intended to be composable with one another. Many implementors
667	/// throughout [`std::io`] take and provide types which implement the `Read`
668	/// trait.
669	///
670	/// Please note that each call to [`read()`] may involve a system call, and
671	/// therefore, using something that implements [`BufRead`], such as
672	/// [`BufReader`], will be more efficient.
673	///
674	/// Repeated calls to the reader use the same cursor, so for example
675	/// calling `read_to_end` twice on a [`File`] will only return the file's
676	/// contents once. It's recommended to first call `rewind()` in that case.
677	///
678	/// # Examples
679	///
680	/// [`File`]s implement `Read`:
681	///
682	/// ```no_run
683	/// use std::io;
684	/// use std::io::prelude::*;
685	/// use std::fs::File;
686	///
687	/// fn main() -> io::Result<()> {
688	/// let mut f = File::open("foo.txt")?;
689	/// let mut buffer = [`0`; `10`];
690	///
691	/// // read up to 10 bytes
692	/// f.read(&mut buffer)?;
693	///
694	/// let mut buffer = Vec::new();
695	/// // read the whole file
696	/// f.read_to_end(&mut buffer)?;
697	///
698	/// // read into a String, so that you don't need to do the conversion.
699	/// let mut buffer = String::new();
700	/// f.read_to_string(&mut buffer)?;
701	///
702	/// // and more! See the other methods for more details.
703	/// Ok(())
704	/// }
705	/// ```
706	///
707	/// Read from [`&str`] because [`&[u8]`][prim@slice] implements `Read`:
708	///
709	/// ```no_run
710	/// # use std::io;
711	/// use std::io::prelude::*;
712	///
713	/// fn main() -> io::Result<()> {
714	/// let mut b = "This string will be read".as_bytes();
715	/// let mut buffer = [`0`; `10`];
716	///
717	/// // read up to 10 bytes
718	/// b.read(&mut buffer)?;
719	///
720	/// // etc... it works exactly as a File does!
721	/// Ok(())
722	/// }
723	/// ```
724	///
725	/// [`read()`]: Read::read
726	/// [`&str`]: prim@str
727	/// [`std::io`]: self
728	/// [`File`]: crate::fs::File
729	#[stable(feature = "rust1", since = "1.0.0")]
730	#[doc(notable_trait)]
731	#[cfg_attr(not(test), rustc_diagnostic_item = "IoRead")]
732	pub trait Read {
733	/// Pull some bytes from this source into the specified buffer, returning
734	/// how many bytes were read.
735	///
736	/// This function does not provide any guarantees about whether it blocks
737	/// waiting for data, but if an object needs to block for a read and cannot,
738	/// it will typically signal this via an [`Err`] return value.
739	///
740	/// If the return value of this method is [`Ok(n)`], then implementations must
741	/// guarantee that `0 <= n <= buf.len()`. A nonzero `n` value indicates
742	/// that the buffer `buf` has been filled in with `n` bytes of data from this
743	/// source. If `n` is `0`, then it can indicate one of two scenarios:
744	///
745	/// 1. This reader has reached its "end of file" and will likely no longer
746	/// be able to produce bytes. Note that this does not mean that the
747	/// reader will always* no longer be able to produce bytes. As an example,*
748	/// on Linux, this method will call the `recv` syscall for a [`TcpStream`],
749	/// where returning zero indicates the connection was shut down correctly. While
750	/// for [`File`], it is possible to reach the end of file and get zero as result,
751	/// but if more data is appended to the file, future calls to `read` will return
752	/// more data.
753	/// 2. The buffer specified was 0 bytes in length.
754	///
755	/// It is not an error if the returned value `n` is smaller than the buffer size,
756	/// even when the reader is not at the end of the stream yet.
757	/// This may happen for example because fewer bytes are actually available right now
758	/// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
759	///
760	/// As this trait is safe to implement, callers in unsafe code cannot rely on
761	/// `n <= buf.len()` for safety.
762	/// Extra care needs to be taken when `unsafe` functions are used to access the read bytes.
763	/// Callers have to ensure that no unchecked out-of-bounds accesses are possible even if
764	/// `n > buf.len()`.
765	///
766	/// Implementations* of this method can make no assumptions about the contents of `buf` when*
767	/// this function is called. It is recommended that implementations only write data to `buf`
768	/// instead of reading its contents.
769	///
770	/// Correspondingly, however, callers* of this method in unsafe code must not assume*
771	/// any guarantees about how the implementation uses `buf`. The trait is safe to implement,
772	/// so it is possible that the code that's supposed to write to the buffer might also read
773	/// from it. It is your responsibility to make sure that `buf` is initialized
774	/// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
775	/// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
776	///
777	/// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
778	///
779	/// # Errors
780	///
781	/// If this function encounters any form of I/O or other error, an error
782	/// variant will be returned. If an error is returned then it must be
783	/// guaranteed that no bytes were read.
784	///
785	/// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
786	/// operation should be retried if there is nothing else to do.
787	///
788	/// # Examples
789	///
790	/// [`File`]s implement `Read`:
791	///
792	/// [`Ok(n)`]: Ok
793	/// [`File`]: crate::fs::File
794	/// [`TcpStream`]: crate::net::TcpStream
795	///
796	/// ```no_run
797	/// use std::io;
798	/// use std::io::prelude::*;
799	/// use std::fs::File;
800	///
801	/// fn main() -> io::Result<()> {
802	/// let mut f = File::open("foo.txt")?;
803	/// let mut buffer = [`0`; `10`];
804	///
805	/// // read up to 10 bytes
806	/// let n = f.read(&mut buffer[..])?;
807	///
808	/// println!("The bytes: {:?}", &buffer[..n]);
809	/// Ok(())
810	/// }
811	/// ```
812	#[stable(feature = "rust1", since = "1.0.0")]
813	fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
814
815	/// Like `read`, except that it reads into a slice of buffers.
816	///
817	/// Data is copied to fill each buffer in order, with the final buffer
818	/// written to possibly being only partially filled. This method must
819	/// behave equivalently to a single call to `read` with concatenated
820	/// buffers.
821	///
822	/// The default implementation calls `read` with either the first nonempty
823	/// buffer provided, or an empty one if none exists.
824	#[stable(feature = "iovec", since = "1.36.0")]
825	fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
826	default_read_vectored(\|b\| self.read(b), bufs)
827	}
828
829	/// Determines if this `Read`er has an efficient `read_vectored`
830	/// implementation.
831	///
832	/// If a `Read`er does not override the default `read_vectored`
833	/// implementation, code using it may want to avoid the method all together
834	/// and coalesce writes into a single buffer for higher performance.
835	///
836	/// The default implementation returns `false`.
837	#[unstable(feature = "can_vector", issue = "69941")]
838	fn is_read_vectored(&self) -> bool {
839	`false`
840	}
841
842	/// Reads all bytes until EOF in this source, placing them into `buf`.
843	///
844	/// All bytes read from this source will be appended to the specified buffer
845	/// `buf`. This function will continuously call [`read()`] to append more data to
846	/// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
847	/// non-[`ErrorKind::Interrupted`] kind.
848	///
849	/// If successful, this function will return the total number of bytes read.
850	///
851	/// # Errors
852	///
853	/// If this function encounters an error of the kind
854	/// [`ErrorKind::Interrupted`] then the error is ignored and the operation
855	/// will continue.
856	///
857	/// If any other read error is encountered then this function immediately
858	/// returns. Any bytes which have already been read will be appended to
859	/// `buf`.
860	///
861	/// # Examples
862	///
863	/// [`File`]s implement `Read`:
864	///
865	/// [`read()`]: Read::read
866	/// [`Ok(0)`]: Ok
867	/// [`File`]: crate::fs::File
868	///
869	/// ```no_run
870	/// use std::io;
871	/// use std::io::prelude::*;
872	/// use std::fs::File;
873	///
874	/// fn main() -> io::Result<()> {
875	/// let mut f = File::open("foo.txt")?;
876	/// let mut buffer = Vec::new();
877	///
878	/// // read the whole file
879	/// f.read_to_end(&mut buffer)?;
880	/// Ok(())
881	/// }
882	/// ```
883	///
884	/// (See also the [`std::fs::read`] convenience function for reading from a
885	/// file.)
886	///
887	/// [`std::fs::read`]: crate::fs::read
888	///
889	/// ## Implementing `read_to_end`
890	///
891	/// When implementing the `io::Read` trait, it is recommended to allocate
892	/// memory using [`Vec::try_reserve`]. However, this behavior is not guaranteed
893	/// by all implementations, and `read_to_end` may not handle out-of-memory
894	/// situations gracefully.
895	///
896	/// ```no_run
897	/// # use std::io::{self, BufRead};
898	/// # struct Example { example_datasource: io::Empty } impl Example {
899	/// # fn get_some_data_for_the_example(&self) -> &'static [u8] { &[] }
900	/// fn read_to_end(&mut self, dest_vec: &mut Vec<u8>) -> io::Result<usize> {
901	/// let initial_vec_len = dest_vec.len();
902	/// loop {
903	/// let src_buf = self.example_datasource.fill_buf()?;
904	/// if src_buf.is_empty() {
905	/// break;
906	/// }
907	/// dest_vec.try_reserve(src_buf.len())?;
908	/// dest_vec.extend_from_slice(src_buf);
909	///
910	/// // Any irreversible side effects should happen after `try_reserve` succeeds,
911	/// // to avoid losing data on allocation error.
912	/// let read = src_buf.len();
913	/// self.example_datasource.consume(read);
914	/// }
915	/// Ok(dest_vec.len() - initial_vec_len)
916	/// }
917	/// # }
918	/// ```
919	///
920	/// # Usage Notes
921	///
922	/// `read_to_end` attempts to read a source until EOF, but many sources are continuous streams
923	/// that do not send EOF. In these cases, `read_to_end` will block indefinitely. Standard input
924	/// is one such stream which may be finite if piped, but is typically continuous. For example,
925	/// `cat file \| my-rust-program` will correctly terminate with an `EOF` upon closure of cat.
926	/// Reading user input or running programs that remain open indefinitely will never terminate
927	/// the stream with `EOF` (e.g. `yes \| my-rust-program`).
928	///
929	/// Using `.lines()` with a [`BufReader`] or using [`read`] can provide a better solution
930	///
931	///[`read`]: Read::read
932	///
933	/// [`Vec::try_reserve`]: crate::vec::Vec::try_reserve
934	#[stable(feature = "rust1", since = "1.0.0")]
935	fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
936	default_read_to_end(self, buf, None)
937	}
938
939	/// Reads all bytes until EOF in this source, appending them to `buf`.
940	///
941	/// If successful, this function returns the number of bytes which were read
942	/// and appended to `buf`.
943	///
944	/// # Errors
945	///
946	/// If the data in this stream is not* valid UTF-8 then an error is*
947	/// returned and `buf` is unchanged.
948	///
949	/// See [`read_to_end`] for other error semantics.
950	///
951	/// [`read_to_end`]: Read::read_to_end
952	///
953	/// # Examples
954	///
955	/// [`File`]s implement `Read`:
956	///
957	/// [`File`]: crate::fs::File
958	///
959	/// ```no_run
960	/// use std::io;
961	/// use std::io::prelude::*;
962	/// use std::fs::File;
963	///
964	/// fn main() -> io::Result<()> {
965	/// let mut f = File::open("foo.txt")?;
966	/// let mut buffer = String::new();
967	///
968	/// f.read_to_string(&mut buffer)?;
969	/// Ok(())
970	/// }
971	/// ```
972	///
973	/// (See also the [`std::fs::read_to_string`] convenience function for
974	/// reading from a file.)
975	///
976	/// # Usage Notes
977	///
978	/// `read_to_string` attempts to read a source until EOF, but many sources are continuous streams
979	/// that do not send EOF. In these cases, `read_to_string` will block indefinitely. Standard input
980	/// is one such stream which may be finite if piped, but is typically continuous. For example,
981	/// `cat file \| my-rust-program` will correctly terminate with an `EOF` upon closure of cat.
982	/// Reading user input or running programs that remain open indefinitely will never terminate
983	/// the stream with `EOF` (e.g. `yes \| my-rust-program`).
984	///
985	/// Using `.lines()` with a [`BufReader`] or using [`read`] can provide a better solution
986	///
987	///[`read`]: Read::read
988	///
989	/// [`std::fs::read_to_string`]: crate::fs::read_to_string
990	#[stable(feature = "rust1", since = "1.0.0")]
991	fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
992	default_read_to_string(self, buf, None)
993	}
994
995	/// Reads the exact number of bytes required to fill `buf`.
996	///
997	/// This function reads as many bytes as necessary to completely fill the
998	/// specified buffer `buf`.
999	///
1000	/// Implementations* of this method can make no assumptions about the contents of `buf` when*
1001	/// this function is called. It is recommended that implementations only write data to `buf`
1002	/// instead of reading its contents. The documentation on [`read`] has a more detailed
1003	/// explanation of this subject.
1004	///
1005	/// # Errors
1006	///
1007	/// If this function encounters an error of the kind
1008	/// [`ErrorKind::Interrupted`] then the error is ignored and the operation
1009	/// will continue.
1010	///
1011	/// If this function encounters an "end of file" before completely filling
1012	/// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
1013	/// The contents of `buf` are unspecified in this case.
1014	///
1015	/// If any other read error is encountered then this function immediately
1016	/// returns. The contents of `buf` are unspecified in this case.
1017	///
1018	/// If this function returns an error, it is unspecified how many bytes it
1019	/// has read, but it will never read more than would be necessary to
1020	/// completely fill the buffer.
1021	///
1022	/// # Examples
1023	///
1024	/// [`File`]s implement `Read`:
1025	///
1026	/// [`read`]: Read::read
1027	/// [`File`]: crate::fs::File
1028	///
1029	/// ```no_run
1030	/// use std::io;
1031	/// use std::io::prelude::*;
1032	/// use std::fs::File;
1033	///
1034	/// fn main() -> io::Result<()> {
1035	/// let mut f = File::open("foo.txt")?;
1036	/// let mut buffer = [`0`; `10`];
1037	///
1038	/// // read exactly 10 bytes
1039	/// f.read_exact(&mut buffer)?;
1040	/// Ok(())
1041	/// }
1042	/// ```
1043	#[stable(feature = "read_exact", since = "1.6.0")]
1044	fn read_exact(&mut self, buf: &mut [u8]) -> Result<()> {
1045	default_read_exact(self, buf)
1046	}
1047
1048	/// Pull some bytes from this source into the specified buffer.
1049	///
1050	/// This is equivalent to the [`read`](Read::read) method, except that it is passed a [`BorrowedCursor`] rather than `[u8]` to allow use
1051	/// with uninitialized buffers. The new data will be appended to any existing contents of `buf`.
1052	///
1053	/// The default implementation delegates to `read`.
1054	///
1055	/// This method makes it possible to return both data and an error but it is advised against.
1056	#[unstable(feature = "read_buf", issue = "78485")]
1057	fn read_buf(&mut self, buf: BorrowedCursor<'_>) -> Result<()> {
1058	default_read_buf(\|b\| self.read(b), buf)
1059	}
1060
1061	/// Reads the exact number of bytes required to fill `cursor`.
1062	///
1063	/// This is similar to the [`read_exact`](Read::read_exact) method, except
1064	/// that it is passed a [`BorrowedCursor`] rather than `[u8]` to allow use
1065	/// with uninitialized buffers.
1066	///
1067	/// # Errors
1068	///
1069	/// If this function encounters an error of the kind [`ErrorKind::Interrupted`]
1070	/// then the error is ignored and the operation will continue.
1071	///
1072	/// If this function encounters an "end of file" before completely filling
1073	/// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
1074	///
1075	/// If any other read error is encountered then this function immediately
1076	/// returns.
1077	///
1078	/// If this function returns an error, all bytes read will be appended to `cursor`.
1079	#[unstable(feature = "read_buf", issue = "78485")]
1080	fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()> {
1081	default_read_buf_exact(self, cursor)
1082	}
1083
1084	/// Creates a "by reference" adaptor for this instance of `Read`.
1085	///
1086	/// The returned adapter also implements `Read` and will simply borrow this
1087	/// current reader.
1088	///
1089	/// # Examples
1090	///
1091	/// [`File`]s implement `Read`:
1092	///
1093	/// [`File`]: crate::fs::File
1094	///
1095	/// ```no_run
1096	/// use std::io;
1097	/// use std::io::Read;
1098	/// use std::fs::File;
1099	///
1100	/// fn main() -> io::Result<()> {
1101	/// let mut f = File::open("foo.txt")?;
1102	/// let mut buffer = Vec::new();
1103	/// let mut other_buffer = Vec::new();
1104	///
1105	/// {
1106	/// let reference = f.by_ref();
1107	///
1108	/// // read at most 5 bytes
1109	/// reference.take(`5`).read_to_end(&mut buffer)?;
1110	///
1111	/// } // drop our &mut reference so we can use f again
1112	///
1113	/// // original file still usable, read the rest
1114	/// f.read_to_end(&mut other_buffer)?;
1115	/// Ok(())
1116	/// }
1117	/// ```
1118	#[stable(feature = "rust1", since = "1.0.0")]
1119	fn by_ref(&mut self) -> &mut Self
1120	where
1121	Self: Sized,
1122	{
1123	self
1124	}
1125
1126	/// Transforms this `Read` instance to an [`Iterator`] over its bytes.
1127	///
1128	/// The returned type implements [`Iterator`] where the [`Item`] is
1129	/// <code>[Result]<[u8], [io::Error]></code>.
1130	/// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
1131	/// otherwise. EOF is mapped to returning [`None`] from this iterator.
1132	///
1133	/// The default implementation calls `read` for each byte,
1134	/// which can be very inefficient for data that's not in memory,
1135	/// such as [`File`]. Consider using a [`BufReader`] in such cases.
1136	///
1137	/// # Examples
1138	///
1139	/// [`File`]s implement `Read`:
1140	///
1141	/// [`Item`]: Iterator::Item
1142	/// [`File`]: crate::fs::File "fs::File"
1143	/// [Result]: crate::result::Result "Result"
1144	/// [io::Error]: self::Error "io::Error"
1145	///
1146	/// ```no_run
1147	/// use std::io;
1148	/// use std::io::prelude::*;
1149	/// use std::io::BufReader;
1150	/// use std::fs::File;
1151	///
1152	/// fn main() -> io::Result<()> {
1153	/// let f = BufReader::new(File::open("foo.txt")?);
1154	///
1155	/// for byte in f.bytes() {
1156	/// println!("{}", byte?);
1157	/// }
1158	/// Ok(())
1159	/// }
1160	/// ```
1161	#[stable(feature = "rust1", since = "1.0.0")]
1162	fn bytes(self) -> Bytes<Self>
1163	where
1164	Self: Sized,
1165	{
1166	Bytes { inner: self }
1167	}
1168
1169	/// Creates an adapter which will chain this stream with another.
1170	///
1171	/// The returned `Read` instance will first read all bytes from this object
1172	/// until EOF is encountered. Afterwards the output is equivalent to the
1173	/// output of `next`.
1174	///
1175	/// # Examples
1176	///
1177	/// [`File`]s implement `Read`:
1178	///
1179	/// [`File`]: crate::fs::File
1180	///
1181	/// ```no_run
1182	/// use std::io;
1183	/// use std::io::prelude::*;
1184	/// use std::fs::File;
1185	///
1186	/// fn main() -> io::Result<()> {
1187	/// let f1 = File::open("foo.txt")?;
1188	/// let f2 = File::open("bar.txt")?;
1189	///
1190	/// let mut handle = f1.chain(f2);
1191	/// let mut buffer = String::new();
1192	///
1193	/// // read the value into a String. We could use any Read method here,
1194	/// // this is just one example.
1195	/// handle.read_to_string(&mut buffer)?;
1196	/// Ok(())
1197	/// }
1198	/// ```
1199	#[stable(feature = "rust1", since = "1.0.0")]
1200	fn chain<R: Read>(self, next: R) -> Chain<Self, R>
1201	where
1202	Self: Sized,
1203	{
1204	Chain { first: self, second: next, done_first: `false` }
1205	}
1206
1207	/// Creates an adapter which will read at most `limit` bytes from it.
1208	///
1209	/// This function returns a new instance of `Read` which will read at most
1210	/// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
1211	/// read errors will not count towards the number of bytes read and future
1212	/// calls to [`read()`] may succeed.
1213	///
1214	/// # Examples
1215	///
1216	/// [`File`]s implement `Read`:
1217	///
1218	/// [`File`]: crate::fs::File
1219	/// [`Ok(0)`]: Ok
1220	/// [`read()`]: Read::read
1221	///
1222	/// ```no_run
1223	/// use std::io;
1224	/// use std::io::prelude::*;
1225	/// use std::fs::File;
1226	///
1227	/// fn main() -> io::Result<()> {
1228	/// let f = File::open("foo.txt")?;
1229	/// let mut buffer = [`0`; `5`];
1230	///
1231	/// // read at most five bytes
1232	/// let mut handle = f.take(`5`);
1233	///
1234	/// handle.read(&mut buffer)?;
1235	/// Ok(())
1236	/// }
1237	/// ```
1238	#[stable(feature = "rust1", since = "1.0.0")]
1239	fn take(self, limit: u64) -> Take<Self>
1240	where
1241	Self: Sized,
1242	{
1243	Take { inner: self, len: limit, limit }
1244	}
1245	}
1246
1247	/// Reads all bytes from a [reader][Read] into a new [`String`].
1248	///
1249	/// This is a convenience function for [`Read::read_to_string`]. Using this
1250	/// function avoids having to create a variable first and provides more type
1251	/// safety since you can only get the buffer out if there were no errors. (If you
1252	/// use [`Read::read_to_string`] you have to remember to check whether the read
1253	/// succeeded because otherwise your buffer will be empty or only partially full.)
1254	///
1255	/// # Performance
1256	///
1257	/// The downside of this function's increased ease of use and type safety is
1258	/// that it gives you less control over performance. For example, you can't
1259	/// pre-allocate memory like you can using [`String::with_capacity`] and
1260	/// [`Read::read_to_string`]. Also, you can't re-use the buffer if an error
1261	/// occurs while reading.
1262	///
1263	/// In many cases, this function's performance will be adequate and the ease of use
1264	/// and type safety tradeoffs will be worth it. However, there are cases where you
1265	/// need more control over performance, and in those cases you should definitely use
1266	/// [`Read::read_to_string`] directly.
1267	///
1268	/// Note that in some special cases, such as when reading files, this function will
1269	/// pre-allocate memory based on the size of the input it is reading. In those
1270	/// cases, the performance should be as good as if you had used
1271	/// [`Read::read_to_string`] with a manually pre-allocated buffer.
1272	///
1273	/// # Errors
1274	///
1275	/// This function forces you to handle errors because the output (the `String`)
1276	/// is wrapped in a [`Result`]. See [`Read::read_to_string`] for the errors
1277	/// that can occur. If any error occurs, you will get an [`Err`], so you
1278	/// don't have to worry about your buffer being empty or partially full.
1279	///
1280	/// # Examples
1281	///
1282	/// ```no_run
1283	/// # use std::io;
1284	/// fn main() -> io::Result<()> {
1285	/// let stdin = io::read_to_string(io::stdin())?;
1286	/// println!("Stdin was:");
1287	/// println!("{stdin}");
1288	/// Ok(())
1289	/// }
1290	/// ```
1291	///
1292	/// # Usage Notes
1293	///
1294	/// `read_to_string` attempts to read a source until EOF, but many sources are continuous streams
1295	/// that do not send EOF. In these cases, `read_to_string` will block indefinitely. Standard input
1296	/// is one such stream which may be finite if piped, but is typically continuous. For example,
1297	/// `cat file \| my-rust-program` will correctly terminate with an `EOF` upon closure of cat.
1298	/// Reading user input or running programs that remain open indefinitely will never terminate
1299	/// the stream with `EOF` (e.g. `yes \| my-rust-program`).
1300	///
1301	/// Using `.lines()` with a [`BufReader`] or using [`read`] can provide a better solution
1302	///
1303	///[`read`]: Read::read
1304	///
1305	#[stable(feature = "io_read_to_string", since = "1.65.0")]
1306	pub fn read_to_string<R: Read>(mut reader: R) -> Result<String> {
1307	let mut buf: String = String::new();
1308	reader.read_to_string(&mut buf)?;
1309	Ok(buf)
1310	}
1311
1312	/// A buffer type used with `Read::read_vectored`.
1313	///
1314	/// It is semantically a wrapper around a `&mut [u8]`, but is guaranteed to be
1315	/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1316	/// Windows.
1317	#[stable(feature = "iovec", since = "1.36.0")]
1318	#[repr(transparent)]
1319	pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);
1320
1321	#[stable(feature = "iovec_send_sync", since = "1.44.0")]
1322	unsafe impl<'a> Send for IoSliceMut<'a> {}
1323
1324	#[stable(feature = "iovec_send_sync", since = "1.44.0")]
1325	unsafe impl<'a> Sync for IoSliceMut<'a> {}
1326
1327	#[stable(feature = "iovec", since = "1.36.0")]
1328	impl<'a> fmt::Debug for IoSliceMut<'a> {
1329	fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1330	fmt::Debug::fmt(self.0.as_slice(), f:fmt)
1331	}
1332	}
1333
1334	impl<'a> IoSliceMut<'a> {
1335	/// Creates a new `IoSliceMut` wrapping a byte slice.
1336	///
1337	/// # Panics
1338	///
1339	/// Panics on Windows if the slice is larger than 4GB.
1340	#[stable(feature = "iovec", since = "1.36.0")]
1341	#[inline]
1342	pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
1343	IoSliceMut(sys::io::IoSliceMut::new(buf))
1344	}
1345
1346	/// Advance the internal cursor of the slice.
1347	///
1348	/// Also see [`IoSliceMut::advance_slices`] to advance the cursors of
1349	/// multiple buffers.
1350	///
1351	/// # Panics
1352	///
1353	/// Panics when trying to advance beyond the end of the slice.
1354	///
1355	/// # Examples
1356	///
1357	/// ```
1358	/// use std::io::IoSliceMut;
1359	/// use std::ops::Deref;
1360	///
1361	/// let mut data = [`1`; `8`];
1362	/// let mut buf = IoSliceMut::new(&mut data);
1363	///
1364	/// // Mark 3 bytes as read.
1365	/// buf.advance(`3`);
1366	/// assert_eq!(buf.deref(), [`1`; `5`].as_ref());
1367	/// ```
1368	#[stable(feature = "io_slice_advance", since = "1.81.0")]
1369	#[inline]
1370	pub fn advance(&mut self, n: usize) {
1371	self.0.advance(n)
1372	}
1373
1374	/// Advance a slice of slices.
1375	///
1376	/// Shrinks the slice to remove any `IoSliceMut`s that are fully advanced over.
1377	/// If the cursor ends up in the middle of an `IoSliceMut`, it is modified
1378	/// to start at that cursor.
1379	///
1380	/// For example, if we have a slice of two 8-byte `IoSliceMut`s, and we advance by 10 bytes,
1381	/// the result will only include the second `IoSliceMut`, advanced by 2 bytes.
1382	///
1383	/// # Panics
1384	///
1385	/// Panics when trying to advance beyond the end of the slices.
1386	///
1387	/// # Examples
1388	///
1389	/// ```
1390	/// use std::io::IoSliceMut;
1391	/// use std::ops::Deref;
1392	///
1393	/// let mut buf1 = [`1`; `8`];
1394	/// let mut buf2 = [`2`; `16`];
1395	/// let mut buf3 = [`3`; `8`];
1396	/// let mut bufs = &mut [
1397	/// IoSliceMut::new(&mut buf1),
1398	/// IoSliceMut::new(&mut buf2),
1399	/// IoSliceMut::new(&mut buf3),
1400	/// ][..];
1401	///
1402	/// // Mark 10 bytes as read.
1403	/// IoSliceMut::advance_slices(&mut bufs, `10`);
1404	/// assert_eq!(bufs[`0`].deref(), [`2`; `14`].as_ref());
1405	/// assert_eq!(bufs[`1`].deref(), [`3`; `8`].as_ref());
1406	/// ```
1407	#[stable(feature = "io_slice_advance", since = "1.81.0")]
1408	#[inline]
1409	pub fn advance_slices(bufs: &mut &mut [IoSliceMut<'a>], n: usize) {
1410	// Number of buffers to remove.
1411	let mut remove = `0`;
1412	// Remaining length before reaching n.
1413	let mut left = n;
1414	for buf in bufs.iter() {
1415	if let Some(remainder) = left.checked_sub(buf.len()) {
1416	left = remainder;
1417	remove += `1`;
1418	} else {
1419	break;
1420	}
1421	}
1422
1423	bufs = &mut* take(bufs)[remove..];
1424	if bufs.is_empty() {
1425	assert!(left == `0`, "advancing io slices beyond their length");
1426	} else {
1427	bufs[`0`].advance(left);
1428	}
1429	}
1430
1431	/// Get the underlying bytes as a mutable slice with the original lifetime.
1432	///
1433	/// # Examples
1434	///
1435	/// ```
1436	/// #![feature(io_slice_as_bytes)]
1437	/// use std::io::IoSliceMut;
1438	///
1439	/// let mut data = *b"abcdef";
1440	/// let io_slice = IoSliceMut::new(&mut data);
1441	/// io_slice.into_slice()[`0`] = b'A';
1442	///
1443	/// assert_eq!(&data, b"Abcdef");
1444	/// ```
1445	#[unstable(feature = "io_slice_as_bytes", issue = "132818")]
1446	pub const fn into_slice(self) -> &'a mut [u8] {
1447	self.0.into_slice()
1448	}
1449	}
1450
1451	#[stable(feature = "iovec", since = "1.36.0")]
1452	impl<'a> Deref for IoSliceMut<'a> {
1453	type Target = [u8];
1454
1455	#[inline]
1456	fn deref(&self) -> &[u8] {
1457	self.0.as_slice()
1458	}
1459	}
1460
1461	#[stable(feature = "iovec", since = "1.36.0")]
1462	impl<'a> DerefMut for IoSliceMut<'a> {
1463	#[inline]
1464	fn deref_mut(&mut self) -> &mut [u8] {
1465	self.0.as_mut_slice()
1466	}
1467	}
1468
1469	/// A buffer type used with `Write::write_vectored`.
1470	///
1471	/// It is semantically a wrapper around a `&[u8]`, but is guaranteed to be
1472	/// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1473	/// Windows.
1474	#[stable(feature = "iovec", since = "1.36.0")]
1475	#[derive(Copy, Clone)]
1476	#[repr(transparent)]
1477	pub struct IoSlice<'a>(sys::io::IoSlice<'a>);
1478
1479	#[stable(feature = "iovec_send_sync", since = "1.44.0")]
1480	unsafe impl<'a> Send for IoSlice<'a> {}
1481
1482	#[stable(feature = "iovec_send_sync", since = "1.44.0")]
1483	unsafe impl<'a> Sync for IoSlice<'a> {}
1484
1485	#[stable(feature = "iovec", since = "1.36.0")]
1486	impl<'a> fmt::Debug for IoSlice<'a> {
1487	fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1488	fmt::Debug::fmt(self.0.as_slice(), f:fmt)
1489	}
1490	}
1491
1492	impl<'a> IoSlice<'a> {
1493	/// Creates a new `IoSlice` wrapping a byte slice.
1494	///
1495	/// # Panics
1496	///
1497	/// Panics on Windows if the slice is larger than 4GB.
1498	#[stable(feature = "iovec", since = "1.36.0")]
1499	#[must_use]
1500	#[inline]
1501	pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
1502	IoSlice(sys::io::IoSlice::new(buf))
1503	}
1504
1505	/// Advance the internal cursor of the slice.
1506	///
1507	/// Also see [`IoSlice::advance_slices`] to advance the cursors of multiple
1508	/// buffers.
1509	///
1510	/// # Panics
1511	///
1512	/// Panics when trying to advance beyond the end of the slice.
1513	///
1514	/// # Examples
1515	///
1516	/// ```
1517	/// use std::io::IoSlice;
1518	/// use std::ops::Deref;
1519	///
1520	/// let data = [`1`; `8`];
1521	/// let mut buf = IoSlice::new(&data);
1522	///
1523	/// // Mark 3 bytes as read.
1524	/// buf.advance(`3`);
1525	/// assert_eq!(buf.deref(), [`1`; `5`].as_ref());
1526	/// ```
1527	#[stable(feature = "io_slice_advance", since = "1.81.0")]
1528	#[inline]
1529	pub fn advance(&mut self, n: usize) {
1530	self.0.advance(n)
1531	}
1532
1533	/// Advance a slice of slices.
1534	///
1535	/// Shrinks the slice to remove any `IoSlice`s that are fully advanced over.
1536	/// If the cursor ends up in the middle of an `IoSlice`, it is modified
1537	/// to start at that cursor.
1538	///
1539	/// For example, if we have a slice of two 8-byte `IoSlice`s, and we advance by 10 bytes,
1540	/// the result will only include the second `IoSlice`, advanced by 2 bytes.
1541	///
1542	/// # Panics
1543	///
1544	/// Panics when trying to advance beyond the end of the slices.
1545	///
1546	/// # Examples
1547	///
1548	/// ```
1549	/// use std::io::IoSlice;
1550	/// use std::ops::Deref;
1551	///
1552	/// let buf1 = [`1`; `8`];
1553	/// let buf2 = [`2`; `16`];
1554	/// let buf3 = [`3`; `8`];
1555	/// let mut bufs = &mut [
1556	/// IoSlice::new(&buf1),
1557	/// IoSlice::new(&buf2),
1558	/// IoSlice::new(&buf3),
1559	/// ][..];
1560	///
1561	/// // Mark 10 bytes as written.
1562	/// IoSlice::advance_slices(&mut bufs, `10`);
1563	/// assert_eq!(bufs[`0`].deref(), [`2`; `14`].as_ref());
1564	/// assert_eq!(bufs[`1`].deref(), [`3`; `8`].as_ref());
1565	#[stable(feature = "io_slice_advance", since = "1.81.0")]
1566	#[inline]
1567	pub fn advance_slices(bufs: &mut &mut [IoSlice<'a>], n: usize) {
1568	// Number of buffers to remove.
1569	let mut remove = `0`;
1570	// Remaining length before reaching n. This prevents overflow
1571	// that could happen if the length of slices in `bufs` were instead
1572	// accumulated. Those slice may be aliased and, if they are large
1573	// enough, their added length may overflow a `usize`.
1574	let mut left = n;
1575	for buf in bufs.iter() {
1576	if let Some(remainder) = left.checked_sub(buf.len()) {
1577	left = remainder;
1578	remove += `1`;
1579	} else {
1580	break;
1581	}
1582	}
1583
1584	bufs = &mut* take(bufs)[remove..];
1585	if bufs.is_empty() {
1586	assert!(left == `0`, "advancing io slices beyond their length");
1587	} else {
1588	bufs[`0`].advance(left);
1589	}
1590	}
1591
1592	/// Get the underlying bytes as a slice with the original lifetime.
1593	///
1594	/// This doesn't borrow from `self`, so is less restrictive than calling
1595	/// `.deref()`, which does.
1596	///
1597	/// # Examples
1598	///
1599	/// ```
1600	/// #![feature(io_slice_as_bytes)]
1601	/// use std::io::IoSlice;
1602	///
1603	/// let data = b"abcdef";
1604	///
1605	/// let mut io_slice = IoSlice::new(data);
1606	/// let tail = &io_slice.as_slice()[`3`..];
1607	///
1608	/// // This works because `tail` doesn't borrow `io_slice`
1609	/// io_slice = IoSlice::new(tail);
1610	///
1611	/// assert_eq!(io_slice.as_slice(), b"def");
1612	/// ```
1613	#[unstable(feature = "io_slice_as_bytes", issue = "132818")]
1614	pub const fn as_slice(self) -> &'a [u8] {
1615	self.0.as_slice()
1616	}
1617	}
1618
1619	#[stable(feature = "iovec", since = "1.36.0")]
1620	impl<'a> Deref for IoSlice<'a> {
1621	type Target = [u8];
1622
1623	#[inline]
1624	fn deref(&self) -> &[u8] {
1625	self.0.as_slice()
1626	}
1627	}
1628
1629	/// A trait for objects which are byte-oriented sinks.
1630	///
1631	/// Implementors of the `Write` trait are sometimes called 'writers'.
1632	///
1633	/// Writers are defined by two required methods, [`write`] and [`flush`]:
1634	///
1635	/// The* [`write`] method will attempt to write some data into the object,
1636	/// returning how many bytes were successfully written.
1637	///
1638	/// The* [`flush`] method is useful for adapters and explicit buffers
1639	/// themselves for ensuring that all buffered data has been pushed out to the
1640	/// 'true sink'.
1641	///
1642	/// Writers are intended to be composable with one another. Many implementors
1643	/// throughout [`std::io`] take and provide types which implement the `Write`
1644	/// trait.
1645	///
1646	/// [`write`]: Write::write
1647	/// [`flush`]: Write::flush
1648	/// [`std::io`]: self
1649	///
1650	/// # Examples
1651	///
1652	/// ```no_run
1653	/// use std::io::prelude::*;
1654	/// use std::fs::File;
1655	///
1656	/// fn main() -> std::io::Result<()> {
1657	/// let data = b"some bytes";
1658	///
1659	/// let mut pos = `0`;
1660	/// let mut buffer = File::create("foo.txt")?;
1661	///
1662	/// while pos < data.len() {
1663	/// let bytes_written = buffer.write(&data[pos..])?;
1664	/// pos += bytes_written;
1665	/// }
1666	/// Ok(())
1667	/// }
1668	/// ```
1669	///
1670	/// The trait also provides convenience methods like [`write_all`], which calls
1671	/// `write` in a loop until its entire input has been written.
1672	///
1673	/// [`write_all`]: Write::write_all
1674	#[stable(feature = "rust1", since = "1.0.0")]
1675	#[doc(notable_trait)]
1676	#[cfg_attr(not(test), rustc_diagnostic_item = "IoWrite")]
1677	pub trait Write {
1678	/// Writes a buffer into this writer, returning how many bytes were written.
1679	///
1680	/// This function will attempt to write the entire contents of `buf`, but
1681	/// the entire write might not succeed, or the write may also generate an
1682	/// error. Typically, a call to `write` represents one attempt to write to
1683	/// any wrapped object.
1684	///
1685	/// Calls to `write` are not guaranteed to block waiting for data to be
1686	/// written, and a write which would otherwise block can be indicated through
1687	/// an [`Err`] variant.
1688	///
1689	/// If this method consumed `n > 0` bytes of `buf` it must return [`Ok(n)`].
1690	/// If the return value is `Ok(n)` then `n` must satisfy `n <= buf.len()`.
1691	/// A return value of `Ok(0)` typically means that the underlying object is
1692	/// no longer able to accept bytes and will likely not be able to in the
1693	/// future as well, or that the buffer provided is empty.
1694	///
1695	/// # Errors
1696	///
1697	/// Each call to `write` may generate an I/O error indicating that the
1698	/// operation could not be completed. If an error is returned then no bytes
1699	/// in the buffer were written to this writer.
1700	///
1701	/// It is not* considered an error if the entire buffer could not be*
1702	/// written to this writer.
1703	///
1704	/// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
1705	/// write operation should be retried if there is nothing else to do.
1706	///
1707	/// # Examples
1708	///
1709	/// ```no_run
1710	/// use std::io::prelude::*;
1711	/// use std::fs::File;
1712	///
1713	/// fn main() -> std::io::Result<()> {
1714	/// let mut buffer = File::create("foo.txt")?;
1715	///
1716	/// // Writes some prefix of the byte string, not necessarily all of it.
1717	/// buffer.write(b"some bytes")?;
1718	/// Ok(())
1719	/// }
1720	/// ```
1721	///
1722	/// [`Ok(n)`]: Ok
1723	#[stable(feature = "rust1", since = "1.0.0")]
1724	fn write(&mut self, buf: &[u8]) -> Result<usize>;
1725
1726	/// Like [`write`], except that it writes from a slice of buffers.
1727	///
1728	/// Data is copied from each buffer in order, with the final buffer
1729	/// read from possibly being only partially consumed. This method must
1730	/// behave as a call to [`write`] with the buffers concatenated would.
1731	///
1732	/// The default implementation calls [`write`] with either the first nonempty
1733	/// buffer provided, or an empty one if none exists.
1734	///
1735	/// # Examples
1736	///
1737	/// ```no_run
1738	/// use std::io::IoSlice;
1739	/// use std::io::prelude::*;
1740	/// use std::fs::File;
1741	///
1742	/// fn main() -> std::io::Result<()> {
1743	/// let data1 = [`1`; `8`];
1744	/// let data2 = [`15`; `8`];
1745	/// let io_slice1 = IoSlice::new(&data1);
1746	/// let io_slice2 = IoSlice::new(&data2);
1747	///
1748	/// let mut buffer = File::create("foo.txt")?;
1749	///
1750	/// // Writes some prefix of the byte string, not necessarily all of it.
1751	/// buffer.write_vectored(&[io_slice1, io_slice2])?;
1752	/// Ok(())
1753	/// }
1754	/// ```
1755	///
1756	/// [`write`]: Write::write
1757	#[stable(feature = "iovec", since = "1.36.0")]
1758	fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
1759	default_write_vectored(\|b\| self.write(b), bufs)
1760	}
1761
1762	/// Determines if this `Write`r has an efficient [`write_vectored`]
1763	/// implementation.
1764	///
1765	/// If a `Write`r does not override the default [`write_vectored`]
1766	/// implementation, code using it may want to avoid the method all together
1767	/// and coalesce writes into a single buffer for higher performance.
1768	///
1769	/// The default implementation returns `false`.
1770	///
1771	/// [`write_vectored`]: Write::write_vectored
1772	#[unstable(feature = "can_vector", issue = "69941")]
1773	fn is_write_vectored(&self) -> bool {
1774	`false`
1775	}
1776
1777	/// Flushes this output stream, ensuring that all intermediately buffered
1778	/// contents reach their destination.
1779	///
1780	/// # Errors
1781	///
1782	/// It is considered an error if not all bytes could be written due to
1783	/// I/O errors or EOF being reached.
1784	///
1785	/// # Examples
1786	///
1787	/// ```no_run
1788	/// use std::io::prelude::*;
1789	/// use std::io::BufWriter;
1790	/// use std::fs::File;
1791	///
1792	/// fn main() -> std::io::Result<()> {
1793	/// let mut buffer = BufWriter::new(File::create("foo.txt")?);
1794	///
1795	/// buffer.write_all(b"some bytes")?;
1796	/// buffer.flush()?;
1797	/// Ok(())
1798	/// }
1799	/// ```
1800	#[stable(feature = "rust1", since = "1.0.0")]
1801	fn flush(&mut self) -> Result<()>;
1802
1803	/// Attempts to write an entire buffer into this writer.
1804	///
1805	/// This method will continuously call [`write`] until there is no more data
1806	/// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
1807	/// returned. This method will not return until the entire buffer has been
1808	/// successfully written or such an error occurs. The first error that is
1809	/// not of [`ErrorKind::Interrupted`] kind generated from this method will be
1810	/// returned.
1811	///
1812	/// If the buffer contains no data, this will never call [`write`].
1813	///
1814	/// # Errors
1815	///
1816	/// This function will return the first error of
1817	/// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
1818	///
1819	/// [`write`]: Write::write
1820	///
1821	/// # Examples
1822	///
1823	/// ```no_run
1824	/// use std::io::prelude::*;
1825	/// use std::fs::File;
1826	///
1827	/// fn main() -> std::io::Result<()> {
1828	/// let mut buffer = File::create("foo.txt")?;
1829	///
1830	/// buffer.write_all(b"some bytes")?;
1831	/// Ok(())
1832	/// }
1833	/// ```
1834	#[stable(feature = "rust1", since = "1.0.0")]
1835	fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
1836	while !buf.is_empty() {
1837	match self.write(buf) {
1838	Ok(`0`) => {
1839	return Err(Error::WRITE_ALL_EOF);
1840	}
1841	Ok(n) => buf = &buf[n..],
1842	Err(ref e) if e.is_interrupted() => {}
1843	Err(e) => return Err(e),
1844	}
1845	}
1846	Ok(())
1847	}
1848
1849	/// Attempts to write multiple buffers into this writer.
1850	///
1851	/// This method will continuously call [`write_vectored`] until there is no
1852	/// more data to be written or an error of non-[`ErrorKind::Interrupted`]
1853	/// kind is returned. This method will not return until all buffers have
1854	/// been successfully written or such an error occurs. The first error that
1855	/// is not of [`ErrorKind::Interrupted`] kind generated from this method
1856	/// will be returned.
1857	///
1858	/// If the buffer contains no data, this will never call [`write_vectored`].
1859	///
1860	/// # Notes
1861	///
1862	/// Unlike [`write_vectored`], this takes a mutable* reference to*
1863	/// a slice of [`IoSlice`]s, not an immutable one. That's because we need to
1864	/// modify the slice to keep track of the bytes already written.
1865	///
1866	/// Once this function returns, the contents of `bufs` are unspecified, as
1867	/// this depends on how many calls to [`write_vectored`] were necessary. It is
1868	/// best to understand this function as taking ownership of `bufs` and to
1869	/// not use `bufs` afterwards. The underlying buffers, to which the
1870	/// [`IoSlice`]s point (but not the [`IoSlice`]s themselves), are unchanged and
1871	/// can be reused.
1872	///
1873	/// [`write_vectored`]: Write::write_vectored
1874	///
1875	/// # Examples
1876	///
1877	/// ```
1878	/// #![feature(write_all_vectored)]
1879	/// # fn main() -> std::io::Result<()> {
1880	///
1881	/// use std::io::{Write, IoSlice};
1882	///
1883	/// let mut writer = Vec::new();
1884	/// let bufs = &mut [
1885	/// IoSlice::new(&[`1`]),
1886	/// IoSlice::new(&[`2`, `3`]),
1887	/// IoSlice::new(&[`4`, `5`, `6`]),
1888	/// ];
1889	///
1890	/// writer.write_all_vectored(bufs)?;
1891	/// // Note: the contents of `bufs` is now undefined, see the Notes section.
1892	///
1893	/// assert_eq!(writer, &[`1`, `2`, `3`, `4`, `5`, `6`]);
1894	/// # Ok(()) }
1895	/// ```
1896	#[unstable(feature = "write_all_vectored", issue = "70436")]
1897	fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
1898	// Guarantee that bufs is empty if it contains no data,
1899	// to avoid calling write_vectored if there is no data to be written.
1900	IoSlice::advance_slices(&mut bufs, `0`);
1901	while !bufs.is_empty() {
1902	match self.write_vectored(bufs) {
1903	Ok(`0`) => {
1904	return Err(Error::WRITE_ALL_EOF);
1905	}
1906	Ok(n) => IoSlice::advance_slices(&mut bufs, n),
1907	Err(ref e) if e.is_interrupted() => {}
1908	Err(e) => return Err(e),
1909	}
1910	}
1911	Ok(())
1912	}
1913
1914	/// Writes a formatted string into this writer, returning any error
1915	/// encountered.
1916	///
1917	/// This method is primarily used to interface with the
1918	/// [`format_args!()`] macro, and it is rare that this should
1919	/// explicitly be called. The [`write!()`] macro should be favored to
1920	/// invoke this method instead.
1921	///
1922	/// This function internally uses the [`write_all`] method on
1923	/// this trait and hence will continuously write data so long as no errors
1924	/// are received. This also means that partial writes are not indicated in
1925	/// this signature.
1926	///
1927	/// [`write_all`]: Write::write_all
1928	///
1929	/// # Errors
1930	///
1931	/// This function will return any I/O error reported while formatting.
1932	///
1933	/// # Examples
1934	///
1935	/// ```no_run
1936	/// use std::io::prelude::*;
1937	/// use std::fs::File;
1938	///
1939	/// fn main() -> std::io::Result<()> {
1940	/// let mut buffer = File::create("foo.txt")?;
1941	///
1942	/// // this call
1943	/// write!(buffer, "{:.*}", `2`, `1.234567`)?;
1944	/// // turns into this:
1945	/// buffer.write_fmt(format_args!("{:.*}", `2`, `1.234567`))?;
1946	/// Ok(())
1947	/// }
1948	/// ```
1949	#[stable(feature = "rust1", since = "1.0.0")]
1950	fn write_fmt(&mut self, args: fmt::Arguments<'_>) -> Result<()> {
1951	if let Some(s) = args.as_statically_known_str() {
1952	self.write_all(s.as_bytes())
1953	} else {
1954	default_write_fmt(self, args)
1955	}
1956	}
1957
1958	/// Creates a "by reference" adapter for this instance of `Write`.
1959	///
1960	/// The returned adapter also implements `Write` and will simply borrow this
1961	/// current writer.
1962	///
1963	/// # Examples
1964	///
1965	/// ```no_run
1966	/// use std::io::Write;
1967	/// use std::fs::File;
1968	///
1969	/// fn main() -> std::io::Result<()> {
1970	/// let mut buffer = File::create("foo.txt")?;
1971	///
1972	/// let reference = buffer.by_ref();
1973	///
1974	/// // we can use reference just like our original buffer
1975	/// reference.write_all(b"some bytes")?;
1976	/// Ok(())
1977	/// }
1978	/// ```
1979	#[stable(feature = "rust1", since = "1.0.0")]
1980	fn by_ref(&mut self) -> &mut Self
1981	where
1982	Self: Sized,
1983	{
1984	self
1985	}
1986	}
1987
1988	/// The `Seek` trait provides a cursor which can be moved within a stream of
1989	/// bytes.
1990	///
1991	/// The stream typically has a fixed size, allowing seeking relative to either
1992	/// end or the current offset.
1993	///
1994	/// # Examples
1995	///
1996	/// [`File`]s implement `Seek`:
1997	///
1998	/// [`File`]: crate::fs::File
1999	///
2000	/// ```no_run
2001	/// use std::io;
2002	/// use std::io::prelude::*;
2003	/// use std::fs::File;
2004	/// use std::io::SeekFrom;
2005	///
2006	/// fn main() -> io::Result<()> {
2007	/// let mut f = File::open("foo.txt")?;
2008	///
2009	/// // move the cursor 42 bytes from the start of the file
2010	/// f.seek(SeekFrom::Start(`42`))?;
2011	/// Ok(())
2012	/// }
2013	/// ```
2014	#[stable(feature = "rust1", since = "1.0.0")]
2015	#[cfg_attr(not(test), rustc_diagnostic_item = "IoSeek")]
2016	pub trait Seek {
2017	/// Seek to an offset, in bytes, in a stream.
2018	///
2019	/// A seek beyond the end of a stream is allowed, but behavior is defined
2020	/// by the implementation.
2021	///
2022	/// If the seek operation completed successfully,
2023	/// this method returns the new position from the start of the stream.
2024	/// That position can be used later with [`SeekFrom::Start`].
2025	///
2026	/// # Errors
2027	///
2028	/// Seeking can fail, for example because it might involve flushing a buffer.
2029	///
2030	/// Seeking to a negative offset is considered an error.
2031	#[stable(feature = "rust1", since = "1.0.0")]
2032	fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
2033
2034	/// Rewind to the beginning of a stream.
2035	///
2036	/// This is a convenience method, equivalent to `seek(SeekFrom::Start(0))`.
2037	///
2038	/// # Errors
2039	///
2040	/// Rewinding can fail, for example because it might involve flushing a buffer.
2041	///
2042	/// # Example
2043	///
2044	/// ```no_run
2045	/// use std::io::{Read, Seek, Write};
2046	/// use std::fs::OpenOptions;
2047	///
2048	/// let mut f = OpenOptions::new()
2049	/// .write(`true`)
2050	/// .read(`true`)
2051	/// .create(`true`)
2052	/// .open("foo.txt")?;
2053	///
2054	/// let hello = "Hello!`\n`";
2055	/// write!(f, "{hello}")?;
2056	/// f.rewind()?;
2057	///
2058	/// let mut buf = String::new();
2059	/// f.read_to_string(&mut buf)?;
2060	/// assert_eq!(&buf, hello);
2061	/// # std::io::Result::Ok(())
2062	/// ```
2063	#[stable(feature = "seek_rewind", since = "1.55.0")]
2064	fn rewind(&mut self) -> Result<()> {
2065	self.seek(SeekFrom::Start(`0`))?;
2066	Ok(())
2067	}
2068
2069	/// Returns the length of this stream (in bytes).
2070	///
2071	/// The default implementation uses up to three seek operations. If this
2072	/// method returns successfully, the seek position is unchanged (i.e. the
2073	/// position before calling this method is the same as afterwards).
2074	/// However, if this method returns an error, the seek position is
2075	/// unspecified.
2076	///
2077	/// If you need to obtain the length of many* streams and you don't care*
2078	/// about the seek position afterwards, you can reduce the number of seek
2079	/// operations by simply calling `seek(SeekFrom::End(0))` and using its
2080	/// return value (it is also the stream length).
2081	///
2082	/// Note that length of a stream can change over time (for example, when
2083	/// data is appended to a file). So calling this method multiple times does
2084	/// not necessarily return the same length each time.
2085	///
2086	/// # Example
2087	///
2088	/// ```no_run
2089	/// #![feature(seek_stream_len)]
2090	/// use std::{
2091	/// io::{self, Seek},
2092	/// fs::File,
2093	/// };
2094	///
2095	/// fn main() -> io::Result<()> {
2096	/// let mut f = File::open("foo.txt")?;
2097	///
2098	/// let len = f.stream_len()?;
2099	/// println!("The file is currently {len} bytes long");
2100	/// Ok(())
2101	/// }
2102	/// ```
2103	#[unstable(feature = "seek_stream_len", issue = "59359")]
2104	fn stream_len(&mut self) -> Result<u64> {
2105	stream_len_default(self)
2106	}
2107
2108	/// Returns the current seek position from the start of the stream.
2109	///
2110	/// This is equivalent to `self.seek(SeekFrom::Current(0))`.
2111	///
2112	/// # Example
2113	///
2114	/// ```no_run
2115	/// use std::{
2116	/// io::{self, BufRead, BufReader, Seek},
2117	/// fs::File,
2118	/// };
2119	///
2120	/// fn main() -> io::Result<()> {
2121	/// let mut f = BufReader::new(File::open("foo.txt")?);
2122	///
2123	/// let before = f.stream_position()?;
2124	/// f.read_line(&mut String::new())?;
2125	/// let after = f.stream_position()?;
2126	///
2127	/// println!("The first line was {} bytes long", after - before);
2128	/// Ok(())
2129	/// }
2130	/// ```
2131	#[stable(feature = "seek_convenience", since = "1.51.0")]
2132	fn stream_position(&mut self) -> Result<u64> {
2133	self.seek(SeekFrom::Current(`0`))
2134	}
2135
2136	/// Seeks relative to the current position.
2137	///
2138	/// This is equivalent to `self.seek(SeekFrom::Current(offset))` but
2139	/// doesn't return the new position which can allow some implementations
2140	/// such as [`BufReader`] to perform more efficient seeks.
2141	///
2142	/// # Example
2143	///
2144	/// ```no_run
2145	/// use std::{
2146	/// io::{self, Seek},
2147	/// fs::File,
2148	/// };
2149	///
2150	/// fn main() -> io::Result<()> {
2151	/// let mut f = File::open("foo.txt")?;
2152	/// f.seek_relative(`10`)?;
2153	/// assert_eq!(f.stream_position()?, `10`);
2154	/// Ok(())
2155	/// }
2156	/// ```
2157	///
2158	/// [`BufReader`]: crate::io::BufReader
2159	#[stable(feature = "seek_seek_relative", since = "1.80.0")]
2160	fn seek_relative(&mut self, offset: i64) -> Result<()> {
2161	self.seek(SeekFrom::Current(offset))?;
2162	Ok(())
2163	}
2164	}
2165
2166	pub(crate) fn stream_len_default<T: Seek + ?Sized>(self_: &mut T) -> Result<u64> {
2167	let old_pos: u64 = self_.stream_position()?;
2168	let len: u64 = self_.seek(pos:SeekFrom::End(`0`))?;
2169
2170	// Avoid seeking a third time when we were already at the end of the
2171	// stream. The branch is usually way cheaper than a seek operation.
2172	if old_pos != len {
2173	self_.seek(pos:SeekFrom::Start(old_pos))?;
2174	}
2175
2176	Ok(len)
2177	}
2178
2179	/// Enumeration of possible methods to seek within an I/O object.
2180	///
2181	/// It is used by the [`Seek`] trait.
2182	#[derive(Copy, PartialEq, Eq, Clone, Debug)]
2183	#[stable(feature = "rust1", since = "1.0.0")]
2184	#[cfg_attr(not(test), rustc_diagnostic_item = "SeekFrom")]
2185	pub enum SeekFrom {
2186	/// Sets the offset to the provided number of bytes.
2187	#[stable(feature = "rust1", since = "1.0.0")]
2188	Start(#[stable(feature = "rust1", since = "1.0.0")] u64),
2189
2190	/// Sets the offset to the size of this object plus the specified number of
2191	/// bytes.
2192	///
2193	/// It is possible to seek beyond the end of an object, but it's an error to
2194	/// seek before byte 0.
2195	#[stable(feature = "rust1", since = "1.0.0")]
2196	End(#[stable(feature = "rust1", since = "1.0.0")] i64),
2197
2198	/// Sets the offset to the current position plus the specified number of
2199	/// bytes.
2200	///
2201	/// It is possible to seek beyond the end of an object, but it's an error to
2202	/// seek before byte 0.
2203	#[stable(feature = "rust1", since = "1.0.0")]
2204	Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
2205	}
2206
2207	fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
2208	let mut read = `0`;
2209	loop {
2210	let (done, used) = {
2211	let available = match r.fill_buf() {
2212	Ok(n) => n,
2213	Err(ref e) if e.is_interrupted() => continue,
2214	Err(e) => return Err(e),
2215	};
2216	match memchr::memchr(delim, available) {
2217	Some(i) => {
2218	buf.extend_from_slice(&available[..=i]);
2219	(`true`, i + `1`)
2220	}
2221	None => {
2222	buf.extend_from_slice(available);
2223	(`false`, available.len())
2224	}
2225	}
2226	};
2227	r.consume(used);
2228	read += used;
2229	if done \|\| used == `0` {
2230	return Ok(read);
2231	}
2232	}
2233	}
2234
2235	fn skip_until<R: BufRead + ?Sized>(r: &mut R, delim: u8) -> Result<usize> {
2236	let mut read: usize = `0`;
2237	loop {
2238	let (done: bool, used: usize) = {
2239	let available: &[u8] = match r.fill_buf() {
2240	Ok(n: &[u8]) => n,
2241	Err(ref e: &Error) if e.kind() == ErrorKind::Interrupted => continue,
2242	Err(e: Error) => return Err(e),
2243	};
2244	match memchr::memchr(x:delim, text:available) {
2245	Some(i: usize) => (`true`, i + `1`),
2246	None => (`false`, available.len()),
2247	}
2248	};
2249	r.consume(amount:used);
2250	read += used;
2251	if done \|\| used == `0` {
2252	return Ok(read);
2253	}
2254	}
2255	}
2256
2257	/// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
2258	/// to perform extra ways of reading.
2259	///
2260	/// For example, reading line-by-line is inefficient without using a buffer, so
2261	/// if you want to read by line, you'll need `BufRead`, which includes a
2262	/// [`read_line`] method as well as a [`lines`] iterator.
2263	///
2264	/// # Examples
2265	///
2266	/// A locked standard input implements `BufRead`:
2267	///
2268	/// ```no_run
2269	/// use std::io;
2270	/// use std::io::prelude::*;
2271	///
2272	/// let stdin = io::stdin();
2273	/// for line in stdin.lock().lines() {
2274	/// println!("{}", line?);
2275	/// }
2276	/// # std::io::Result::Ok(())
2277	/// ```
2278	///
2279	/// If you have something that implements [`Read`], you can use the [`BufReader`
2280	/// type][`BufReader`] to turn it into a `BufRead`.
2281	///
2282	/// For example, [`File`] implements [`Read`], but not `BufRead`.
2283	/// [`BufReader`] to the rescue!
2284	///
2285	/// [`File`]: crate::fs::File
2286	/// [`read_line`]: BufRead::read_line
2287	/// [`lines`]: BufRead::lines
2288	///
2289	/// ```no_run
2290	/// use std::io::{self, BufReader};
2291	/// use std::io::prelude::*;
2292	/// use std::fs::File;
2293	///
2294	/// fn main() -> io::Result<()> {
2295	/// let f = File::open("foo.txt")?;
2296	/// let f = BufReader::new(f);
2297	///
2298	/// for line in f.lines() {
2299	/// let line = line?;
2300	/// println!("{line}");
2301	/// }
2302	///
2303	/// Ok(())
2304	/// }
2305	/// ```
2306	#[stable(feature = "rust1", since = "1.0.0")]
2307	#[cfg_attr(not(test), rustc_diagnostic_item = "IoBufRead")]
2308	pub trait BufRead: Read {
2309	/// Returns the contents of the internal buffer, filling it with more data, via `Read` methods, if empty.
2310	///
2311	/// This is a lower-level method and is meant to be used together with [`consume`],
2312	/// which can be used to mark bytes that should not be returned by subsequent calls to `read`.
2313	///
2314	/// [`consume`]: BufRead::consume
2315	///
2316	/// Returns an empty buffer when the stream has reached EOF.
2317	///
2318	/// # Errors
2319	///
2320	/// This function will return an I/O error if a `Read` method was called, but returned an error.
2321	///
2322	/// # Examples
2323	///
2324	/// A locked standard input implements `BufRead`:
2325	///
2326	/// ```no_run
2327	/// use std::io;
2328	/// use std::io::prelude::*;
2329	///
2330	/// let stdin = io::stdin();
2331	/// let mut stdin = stdin.lock();
2332	///
2333	/// let buffer = stdin.fill_buf()?;
2334	///
2335	/// // work with buffer
2336	/// println!("{buffer:?}");
2337	///
2338	/// // mark the bytes we worked with as read
2339	/// let length = buffer.len();
2340	/// stdin.consume(length);
2341	/// # std::io::Result::Ok(())
2342	/// ```
2343	#[stable(feature = "rust1", since = "1.0.0")]
2344	fn fill_buf(&mut self) -> Result<&[u8]>;
2345
2346	/// Marks the given `amount` of additional bytes from the internal buffer as having been read.
2347	/// Subsequent calls to `read` only return bytes that have not been marked as read.
2348	///
2349	/// This is a lower-level method and is meant to be used together with [`fill_buf`],
2350	/// which can be used to fill the internal buffer via `Read` methods.
2351	///
2352	/// It is a logic error if `amount` exceeds the number of unread bytes in the internal buffer, which is returned by [`fill_buf`].
2353	///
2354	/// # Examples
2355	///
2356	/// Since `consume()` is meant to be used with [`fill_buf`],
2357	/// that method's example includes an example of `consume()`.
2358	///
2359	/// [`fill_buf`]: BufRead::fill_buf
2360	#[stable(feature = "rust1", since = "1.0.0")]
2361	fn consume(&mut self, amount: usize);
2362
2363	/// Checks if there is any data left to be `read`.
2364	///
2365	/// This function may fill the buffer to check for data,
2366	/// so this function returns `Result<bool>`, not `bool`.
2367	///
2368	/// The default implementation calls `fill_buf` and checks that the
2369	/// returned slice is empty (which means that there is no data left,
2370	/// since EOF is reached).
2371	///
2372	/// # Errors
2373	///
2374	/// This function will return an I/O error if a `Read` method was called, but returned an error.
2375	///
2376	/// Examples
2377	///
2378	/// ```
2379	/// #![feature(buf_read_has_data_left)]
2380	/// use std::io;
2381	/// use std::io::prelude::*;
2382	///
2383	/// let stdin = io::stdin();
2384	/// let mut stdin = stdin.lock();
2385	///
2386	/// while stdin.has_data_left()? {
2387	/// let mut line = String::new();
2388	/// stdin.read_line(&mut line)?;
2389	/// // work with line
2390	/// println!("{line:?}");
2391	/// }
2392	/// # std::io::Result::Ok(())
2393	/// ```
2394	#[unstable(feature = "buf_read_has_data_left", reason = "recently added", issue = "86423")]
2395	fn has_data_left(&mut self) -> Result<bool> {
2396	self.fill_buf().map(\|b\| !b.is_empty())
2397	}
2398
2399	/// Reads all bytes into `buf` until the delimiter `byte` or EOF is reached.
2400	///
2401	/// This function will read bytes from the underlying stream until the
2402	/// delimiter or EOF is found. Once found, all bytes up to, and including,
2403	/// the delimiter (if found) will be appended to `buf`.
2404	///
2405	/// If successful, this function will return the total number of bytes read.
2406	///
2407	/// This function is blocking and should be used carefully: it is possible for
2408	/// an attacker to continuously send bytes without ever sending the delimiter
2409	/// or EOF.
2410	///
2411	/// # Errors
2412	///
2413	/// This function will ignore all instances of [`ErrorKind::Interrupted`] and
2414	/// will otherwise return any errors returned by [`fill_buf`].
2415	///
2416	/// If an I/O error is encountered then all bytes read so far will be
2417	/// present in `buf` and its length will have been adjusted appropriately.
2418	///
2419	/// [`fill_buf`]: BufRead::fill_buf
2420	///
2421	/// # Examples
2422	///
2423	/// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2424	/// this example, we use [`Cursor`] to read all the bytes in a byte slice
2425	/// in hyphen delimited segments:
2426	///
2427	/// ```
2428	/// use std::io::{self, BufRead};
2429	///
2430	/// let mut cursor = io::Cursor::new(b"lorem-ipsum");
2431	/// let mut buf = vec![];
2432	///
2433	/// // cursor is at 'l'
2434	/// let num_bytes = cursor.read_until(b'-', &mut buf)
2435	/// .expect("reading from cursor won't fail");
2436	/// assert_eq!(num_bytes, `6`);
2437	/// assert_eq!(buf, b"lorem-");
2438	/// buf.clear();
2439	///
2440	/// // cursor is at 'i'
2441	/// let num_bytes = cursor.read_until(b'-', &mut buf)
2442	/// .expect("reading from cursor won't fail");
2443	/// assert_eq!(num_bytes, `5`);
2444	/// assert_eq!(buf, b"ipsum");
2445	/// buf.clear();
2446	///
2447	/// // cursor is at EOF
2448	/// let num_bytes = cursor.read_until(b'-', &mut buf)
2449	/// .expect("reading from cursor won't fail");
2450	/// assert_eq!(num_bytes, `0`);
2451	/// assert_eq!(buf, b"");
2452	/// ```
2453	#[stable(feature = "rust1", since = "1.0.0")]
2454	fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
2455	read_until(self, byte, buf)
2456	}
2457
2458	/// Skips all bytes until the delimiter `byte` or EOF is reached.
2459	///
2460	/// This function will read (and discard) bytes from the underlying stream until the
2461	/// delimiter or EOF is found.
2462	///
2463	/// If successful, this function will return the total number of bytes read,
2464	/// including the delimiter byte.
2465	///
2466	/// This is useful for efficiently skipping data such as NUL-terminated strings
2467	/// in binary file formats without buffering.
2468	///
2469	/// This function is blocking and should be used carefully: it is possible for
2470	/// an attacker to continuously send bytes without ever sending the delimiter
2471	/// or EOF.
2472	///
2473	/// # Errors
2474	///
2475	/// This function will ignore all instances of [`ErrorKind::Interrupted`] and
2476	/// will otherwise return any errors returned by [`fill_buf`].
2477	///
2478	/// If an I/O error is encountered then all bytes read so far will be
2479	/// present in `buf` and its length will have been adjusted appropriately.
2480	///
2481	/// [`fill_buf`]: BufRead::fill_buf
2482	///
2483	/// # Examples
2484	///
2485	/// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2486	/// this example, we use [`Cursor`] to read some NUL-terminated information
2487	/// about Ferris from a binary string, skipping the fun fact:
2488	///
2489	/// ```
2490	/// use std::io::{self, BufRead};
2491	///
2492	/// let mut cursor = io::Cursor::new(b"Ferris`\0`Likes long walks on the beach`\0`Crustacean`\0`");
2493	///
2494	/// // read name
2495	/// let mut name = Vec::new();
2496	/// let num_bytes = cursor.read_until(b'`\0`', &mut name)
2497	/// .expect("reading from cursor won't fail");
2498	/// assert_eq!(num_bytes, `7`);
2499	/// assert_eq!(name, b"Ferris`\0`");
2500	///
2501	/// // skip fun fact
2502	/// let num_bytes = cursor.skip_until(b'`\0`')
2503	/// .expect("reading from cursor won't fail");
2504	/// assert_eq!(num_bytes, `30`);
2505	///
2506	/// // read animal type
2507	/// let mut animal = Vec::new();
2508	/// let num_bytes = cursor.read_until(b'`\0`', &mut animal)
2509	/// .expect("reading from cursor won't fail");
2510	/// assert_eq!(num_bytes, `11`);
2511	/// assert_eq!(animal, b"Crustacean`\0`");
2512	/// ```
2513	#[stable(feature = "bufread_skip_until", since = "1.83.0")]
2514	fn skip_until(&mut self, byte: u8) -> Result<usize> {
2515	skip_until(self, byte)
2516	}
2517
2518	/// Reads all bytes until a newline (the `0xA` byte) is reached, and append
2519	/// them to the provided `String` buffer.
2520	///
2521	/// Previous content of the buffer will be preserved. To avoid appending to
2522	/// the buffer, you need to [`clear`] it first.
2523	///
2524	/// This function will read bytes from the underlying stream until the
2525	/// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
2526	/// up to, and including, the delimiter (if found) will be appended to
2527	/// `buf`.
2528	///
2529	/// If successful, this function will return the total number of bytes read.
2530	///
2531	/// If this function returns [`Ok(0)`], the stream has reached EOF.
2532	///
2533	/// This function is blocking and should be used carefully: it is possible for
2534	/// an attacker to continuously send bytes without ever sending a newline
2535	/// or EOF. You can use [`take`] to limit the maximum number of bytes read.
2536	///
2537	/// [`Ok(0)`]: Ok
2538	/// [`clear`]: String::clear
2539	/// [`take`]: crate::io::Read::take
2540	///
2541	/// # Errors
2542	///
2543	/// This function has the same error semantics as [`read_until`] and will
2544	/// also return an error if the read bytes are not valid UTF-8. If an I/O
2545	/// error is encountered then `buf` may contain some bytes already read in
2546	/// the event that all data read so far was valid UTF-8.
2547	///
2548	/// [`read_until`]: BufRead::read_until
2549	///
2550	/// # Examples
2551	///
2552	/// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2553	/// this example, we use [`Cursor`] to read all the lines in a byte slice:
2554	///
2555	/// ```
2556	/// use std::io::{self, BufRead};
2557	///
2558	/// let mut cursor = io::Cursor::new(b"foo`\n`bar");
2559	/// let mut buf = String::new();
2560	///
2561	/// // cursor is at 'f'
2562	/// let num_bytes = cursor.read_line(&mut buf)
2563	/// .expect("reading from cursor won't fail");
2564	/// assert_eq!(num_bytes, `4`);
2565	/// assert_eq!(buf, "foo`\n`");
2566	/// buf.clear();
2567	///
2568	/// // cursor is at 'b'
2569	/// let num_bytes = cursor.read_line(&mut buf)
2570	/// .expect("reading from cursor won't fail");
2571	/// assert_eq!(num_bytes, `3`);
2572	/// assert_eq!(buf, "bar");
2573	/// buf.clear();
2574	///
2575	/// // cursor is at EOF
2576	/// let num_bytes = cursor.read_line(&mut buf)
2577	/// .expect("reading from cursor won't fail");
2578	/// assert_eq!(num_bytes, `0`);
2579	/// assert_eq!(buf, "");
2580	/// ```
2581	#[stable(feature = "rust1", since = "1.0.0")]
2582	fn read_line(&mut self, buf: &mut String) -> Result<usize> {
2583	// Note that we are not calling the `.read_until` method here, but
2584	// rather our hardcoded implementation. For more details as to why, see
2585	// the comments in `default_read_to_string`.
2586	unsafe { append_to_string(buf, \|b\| read_until(self, b'`\n`', b)) }
2587	}
2588
2589	/// Returns an iterator over the contents of this reader split on the byte
2590	/// `byte`.
2591	///
2592	/// The iterator returned from this function will return instances of
2593	/// <code>[io::Result]<[Vec]\<u8>></code>. Each vector returned will not* have*
2594	/// the delimiter byte at the end.
2595	///
2596	/// This function will yield errors whenever [`read_until`] would have
2597	/// also yielded an error.
2598	///
2599	/// [io::Result]: self::Result "io::Result"
2600	/// [`read_until`]: BufRead::read_until
2601	///
2602	/// # Examples
2603	///
2604	/// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2605	/// this example, we use [`Cursor`] to iterate over all hyphen delimited
2606	/// segments in a byte slice
2607	///
2608	/// ```
2609	/// use std::io::{self, BufRead};
2610	///
2611	/// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
2612	///
2613	/// let mut split_iter = cursor.split(b'-').map(\|l\| l.unwrap());
2614	/// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
2615	/// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
2616	/// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
2617	/// assert_eq!(split_iter.next(), None);
2618	/// ```
2619	#[stable(feature = "rust1", since = "1.0.0")]
2620	fn split(self, byte: u8) -> Split<Self>
2621	where
2622	Self: Sized,
2623	{
2624	Split { buf: self, delim: byte }
2625	}
2626
2627	/// Returns an iterator over the lines of this reader.
2628	///
2629	/// The iterator returned from this function will yield instances of
2630	/// <code>[io::Result]<[String]></code>. Each string returned will not* have a newline*
2631	/// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
2632	///
2633	/// [io::Result]: self::Result "io::Result"
2634	///
2635	/// # Examples
2636	///
2637	/// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2638	/// this example, we use [`Cursor`] to iterate over all the lines in a byte
2639	/// slice.
2640	///
2641	/// ```
2642	/// use std::io::{self, BufRead};
2643	///
2644	/// let cursor = io::Cursor::new(b"lorem`\n`ipsum`\r\n`dolor");
2645	///
2646	/// let mut lines_iter = cursor.lines().map(\|l\| l.unwrap());
2647	/// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
2648	/// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
2649	/// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
2650	/// assert_eq!(lines_iter.next(), None);
2651	/// ```
2652	///
2653	/// # Errors
2654	///
2655	/// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
2656	#[stable(feature = "rust1", since = "1.0.0")]
2657	fn lines(self) -> Lines<Self>
2658	where
2659	Self: Sized,
2660	{
2661	Lines { buf: self }
2662	}
2663	}
2664
2665	/// Adapter to chain together two readers.
2666	///
2667	/// This struct is generally created by calling [`chain`] on a reader.
2668	/// Please see the documentation of [`chain`] for more details.
2669	///
2670	/// [`chain`]: Read::chain
2671	#[stable(feature = "rust1", since = "1.0.0")]
2672	#[derive(Debug)]
2673	pub struct Chain<T, U> {
2674	first: T,
2675	second: U,
2676	done_first: bool,
2677	}
2678
2679	impl<T, U> Chain<T, U> {
2680	/// Consumes the `Chain`, returning the wrapped readers.
2681	///
2682	/// # Examples
2683	///
2684	/// ```no_run
2685	/// use std::io;
2686	/// use std::io::prelude::*;
2687	/// use std::fs::File;
2688	///
2689	/// fn main() -> io::Result<()> {
2690	/// let mut foo_file = File::open("foo.txt")?;
2691	/// let mut bar_file = File::open("bar.txt")?;
2692	///
2693	/// let chain = foo_file.chain(bar_file);
2694	/// let (foo_file, bar_file) = chain.into_inner();
2695	/// Ok(())
2696	/// }
2697	/// ```
2698	#[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2699	pub fn into_inner(self) -> (T, U) {
2700	(self.first, self.second)
2701	}
2702
2703	/// Gets references to the underlying readers in this `Chain`.
2704	///
2705	/// Care should be taken to avoid modifying the internal I/O state of the
2706	/// underlying readers as doing so may corrupt the internal state of this
2707	/// `Chain`.
2708	///
2709	/// # Examples
2710	///
2711	/// ```no_run
2712	/// use std::io;
2713	/// use std::io::prelude::*;
2714	/// use std::fs::File;
2715	///
2716	/// fn main() -> io::Result<()> {
2717	/// let mut foo_file = File::open("foo.txt")?;
2718	/// let mut bar_file = File::open("bar.txt")?;
2719	///
2720	/// let chain = foo_file.chain(bar_file);
2721	/// let (foo_file, bar_file) = chain.get_ref();
2722	/// Ok(())
2723	/// }
2724	/// ```
2725	#[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2726	pub fn get_ref(&self) -> (&T, &U) {
2727	(&self.first, &self.second)
2728	}
2729
2730	/// Gets mutable references to the underlying readers in this `Chain`.
2731	///
2732	/// Care should be taken to avoid modifying the internal I/O state of the
2733	/// underlying readers as doing so may corrupt the internal state of this
2734	/// `Chain`.
2735	///
2736	/// # Examples
2737	///
2738	/// ```no_run
2739	/// use std::io;
2740	/// use std::io::prelude::*;
2741	/// use std::fs::File;
2742	///
2743	/// fn main() -> io::Result<()> {
2744	/// let mut foo_file = File::open("foo.txt")?;
2745	/// let mut bar_file = File::open("bar.txt")?;
2746	///
2747	/// let mut chain = foo_file.chain(bar_file);
2748	/// let (foo_file, bar_file) = chain.get_mut();
2749	/// Ok(())
2750	/// }
2751	/// ```
2752	#[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2753	pub fn get_mut(&mut self) -> (&mut T, &mut U) {
2754	(&mut self.first, &mut self.second)
2755	}
2756	}
2757
2758	#[stable(feature = "rust1", since = "1.0.0")]
2759	impl<T: Read, U: Read> Read for Chain<T, U> {
2760	fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2761	if !self.done_first {
2762	match self.first.read(buf)? {
2763	`0` if !buf.is_empty() => self.done_first = `true`,
2764	n => return Ok(n),
2765	}
2766	}
2767	self.second.read(buf)
2768	}
2769
2770	fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
2771	if !self.done_first {
2772	match self.first.read_vectored(bufs)? {
2773	`0` if bufs.iter().any(\|b\| !b.is_empty()) => self.done_first = `true`,
2774	n => return Ok(n),
2775	}
2776	}
2777	self.second.read_vectored(bufs)
2778	}
2779
2780	#[inline]
2781	fn is_read_vectored(&self) -> bool {
2782	self.first.is_read_vectored() \|\| self.second.is_read_vectored()
2783	}
2784
2785	fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
2786	let mut read = `0`;
2787	if !self.done_first {
2788	read += self.first.read_to_end(buf)?;
2789	self.done_first = `true`;
2790	}
2791	read += self.second.read_to_end(buf)?;
2792	Ok(read)
2793	}
2794
2795	// We don't override `read_to_string` here because an UTF-8 sequence could
2796	// be split between the two parts of the chain
2797
2798	fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
2799	if buf.capacity() == `0` {
2800	return Ok(());
2801	}
2802
2803	if !self.done_first {
2804	let old_len = buf.written();
2805	self.first.read_buf(buf.reborrow())?;
2806
2807	if buf.written() != old_len {
2808	return Ok(());
2809	} else {
2810	self.done_first = `true`;
2811	}
2812	}
2813	self.second.read_buf(buf)
2814	}
2815	}
2816
2817	#[stable(feature = "chain_bufread", since = "1.9.0")]
2818	impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
2819	fn fill_buf(&mut self) -> Result<&[u8]> {
2820	if !self.done_first {
2821	match self.first.fill_buf()? {
2822	buf if buf.is_empty() => self.done_first = `true`,
2823	buf => return Ok(buf),
2824	}
2825	}
2826	self.second.fill_buf()
2827	}
2828
2829	fn consume(&mut self, amt: usize) {
2830	if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
2831	}
2832
2833	fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
2834	let mut read = `0`;
2835	if !self.done_first {
2836	let n = self.first.read_until(byte, buf)?;
2837	read += n;
2838
2839	match buf.last() {
2840	Some(b) if b == byte && n != `0` => return* Ok(read),
2841	_ => self.done_first = `true`,
2842	}
2843	}
2844	read += self.second.read_until(byte, buf)?;
2845	Ok(read)
2846	}
2847
2848	// We don't override `read_line` here because an UTF-8 sequence could be
2849	// split between the two parts of the chain
2850	}
2851
2852	impl<T, U> SizeHint for Chain<T, U> {
2853	#[inline]
2854	fn lower_bound(&self) -> usize {
2855	SizeHint::lower_bound(&self.first) + SizeHint::lower_bound(&self.second)
2856	}
2857
2858	#[inline]
2859	fn upper_bound(&self) -> Option<usize> {
2860	match (SizeHint::upper_bound(&self.first), SizeHint::upper_bound(&self.second)) {
2861	(Some(first: usize), Some(second: usize)) => first.checked_add(second),
2862	_ => None,
2863	}
2864	}
2865	}
2866
2867	/// Reader adapter which limits the bytes read from an underlying reader.
2868	///
2869	/// This struct is generally created by calling [`take`] on a reader.
2870	/// Please see the documentation of [`take`] for more details.
2871	///
2872	/// [`take`]: Read::take
2873	#[stable(feature = "rust1", since = "1.0.0")]
2874	#[derive(Debug)]
2875	pub struct Take<T> {
2876	inner: T,
2877	len: u64,
2878	limit: u64,
2879	}
2880
2881	impl<T> Take<T> {
2882	/// Returns the number of bytes that can be read before this instance will
2883	/// return EOF.
2884	///
2885	/// # Note
2886	///
2887	/// This instance may reach `EOF` after reading fewer bytes than indicated by
2888	/// this method if the underlying [`Read`] instance reaches EOF.
2889	///
2890	/// # Examples
2891	///
2892	/// ```no_run
2893	/// use std::io;
2894	/// use std::io::prelude::*;
2895	/// use std::fs::File;
2896	///
2897	/// fn main() -> io::Result<()> {
2898	/// let f = File::open("foo.txt")?;
2899	///
2900	/// // read at most five bytes
2901	/// let handle = f.take(`5`);
2902	///
2903	/// println!("limit: {}", handle.limit());
2904	/// Ok(())
2905	/// }
2906	/// ```
2907	#[stable(feature = "rust1", since = "1.0.0")]
2908	pub fn limit(&self) -> u64 {
2909	self.limit
2910	}
2911
2912	/// Returns the number of bytes read so far.
2913	#[unstable(feature = "seek_io_take_position", issue = "97227")]
2914	pub fn position(&self) -> u64 {
2915	self.len - self.limit
2916	}
2917
2918	/// Sets the number of bytes that can be read before this instance will
2919	/// return EOF. This is the same as constructing a new `Take` instance, so
2920	/// the amount of bytes read and the previous limit value don't matter when
2921	/// calling this method.
2922	///
2923	/// # Examples
2924	///
2925	/// ```no_run
2926	/// use std::io;
2927	/// use std::io::prelude::*;
2928	/// use std::fs::File;
2929	///
2930	/// fn main() -> io::Result<()> {
2931	/// let f = File::open("foo.txt")?;
2932	///
2933	/// // read at most five bytes
2934	/// let mut handle = f.take(`5`);
2935	/// handle.set_limit(`10`);
2936	///
2937	/// assert_eq!(handle.limit(), `10`);
2938	/// Ok(())
2939	/// }
2940	/// ```
2941	#[stable(feature = "take_set_limit", since = "1.27.0")]
2942	pub fn set_limit(&mut self, limit: u64) {
2943	self.len = limit;
2944	self.limit = limit;
2945	}
2946
2947	/// Consumes the `Take`, returning the wrapped reader.
2948	///
2949	/// # Examples
2950	///
2951	/// ```no_run
2952	/// use std::io;
2953	/// use std::io::prelude::*;
2954	/// use std::fs::File;
2955	///
2956	/// fn main() -> io::Result<()> {
2957	/// let mut file = File::open("foo.txt")?;
2958	///
2959	/// let mut buffer = [`0`; `5`];
2960	/// let mut handle = file.take(`5`);
2961	/// handle.read(&mut buffer)?;
2962	///
2963	/// let file = handle.into_inner();
2964	/// Ok(())
2965	/// }
2966	/// ```
2967	#[stable(feature = "io_take_into_inner", since = "1.15.0")]
2968	pub fn into_inner(self) -> T {
2969	self.inner
2970	}
2971
2972	/// Gets a reference to the underlying reader.
2973	///
2974	/// Care should be taken to avoid modifying the internal I/O state of the
2975	/// underlying reader as doing so may corrupt the internal limit of this
2976	/// `Take`.
2977	///
2978	/// # Examples
2979	///
2980	/// ```no_run
2981	/// use std::io;
2982	/// use std::io::prelude::*;
2983	/// use std::fs::File;
2984	///
2985	/// fn main() -> io::Result<()> {
2986	/// let mut file = File::open("foo.txt")?;
2987	///
2988	/// let mut buffer = [`0`; `5`];
2989	/// let mut handle = file.take(`5`);
2990	/// handle.read(&mut buffer)?;
2991	///
2992	/// let file = handle.get_ref();
2993	/// Ok(())
2994	/// }
2995	/// ```
2996	#[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2997	pub fn get_ref(&self) -> &T {
2998	&self.inner
2999	}
3000
3001	/// Gets a mutable reference to the underlying reader.
3002	///
3003	/// Care should be taken to avoid modifying the internal I/O state of the
3004	/// underlying reader as doing so may corrupt the internal limit of this
3005	/// `Take`.
3006	///
3007	/// # Examples
3008	///
3009	/// ```no_run
3010	/// use std::io;
3011	/// use std::io::prelude::*;
3012	/// use std::fs::File;
3013	///
3014	/// fn main() -> io::Result<()> {
3015	/// let mut file = File::open("foo.txt")?;
3016	///
3017	/// let mut buffer = [`0`; `5`];
3018	/// let mut handle = file.take(`5`);
3019	/// handle.read(&mut buffer)?;
3020	///
3021	/// let file = handle.get_mut();
3022	/// Ok(())
3023	/// }
3024	/// ```
3025	#[stable(feature = "more_io_inner_methods", since = "1.20.0")]
3026	pub fn get_mut(&mut self) -> &mut T {
3027	&mut self.inner
3028	}
3029	}
3030
3031	#[stable(feature = "rust1", since = "1.0.0")]
3032	impl<T: Read> Read for Take<T> {
3033	fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
3034	// Don't call into inner reader at all at EOF because it may still block
3035	if self.limit == `0` {
3036	return Ok(`0`);
3037	}
3038
3039	let max = cmp::min(buf.len() as u64, self.limit) as usize;
3040	let n = self.inner.read(&mut buf[..max])?;
3041	assert!(n as u64 <= self.limit, "number of read bytes exceeds limit");
3042	self.limit -= n as u64;
3043	Ok(n)
3044	}
3045
3046	fn read_buf(&mut self, mut buf: BorrowedCursor<'_>) -> Result<()> {
3047	// Don't call into inner reader at all at EOF because it may still block
3048	if self.limit == `0` {
3049	return Ok(());
3050	}
3051
3052	if self.limit < buf.capacity() as u64 {
3053	// The condition above guarantees that `self.limit` fits in `usize`.
3054	let limit = self.limit as usize;
3055
3056	let extra_init = cmp::min(limit, buf.init_ref().len());
3057
3058	// SAFETY: no uninit data is written to ibuf
3059	let ibuf = unsafe { &mut buf.as_mut()[..limit] };
3060
3061	let mut sliced_buf: BorrowedBuf<'_> = ibuf.into();
3062
3063	// SAFETY: extra_init bytes of ibuf are known to be initialized
3064	unsafe {
3065	sliced_buf.set_init(extra_init);
3066	}
3067
3068	let mut cursor = sliced_buf.unfilled();
3069	let result = self.inner.read_buf(cursor.reborrow());
3070
3071	let new_init = cursor.init_ref().len();
3072	let filled = sliced_buf.len();
3073
3074	// cursor / sliced_buf / ibuf must drop here
3075
3076	unsafe {
3077	// SAFETY: filled bytes have been filled and therefore initialized
3078	buf.advance_unchecked(filled);
3079	// SAFETY: new_init bytes of buf's unfilled buffer have been initialized
3080	buf.set_init(new_init);
3081	}
3082
3083	self.limit -= filled as u64;
3084
3085	result
3086	} else {
3087	let written = buf.written();
3088	let result = self.inner.read_buf(buf.reborrow());
3089	self.limit -= (buf.written() - written) as u64;
3090	result
3091	}
3092	}
3093	}
3094
3095	#[stable(feature = "rust1", since = "1.0.0")]
3096	impl<T: BufRead> BufRead for Take<T> {
3097	fn fill_buf(&mut self) -> Result<&[u8]> {
3098	// Don't call into inner reader at all at EOF because it may still block
3099	if self.limit == `0` {
3100	return Ok(&[]);
3101	}
3102
3103	let buf: &[u8] = self.inner.fill_buf()?;
3104	let cap: usize = cmp::min(v1:buf.len() as u64, self.limit) as usize;
3105	Ok(&buf[..cap])
3106	}
3107
3108	fn consume(&mut self, amt: usize) {
3109	// Don't let callers reset the limit by passing an overlarge value
3110	let amt: usize = cmp::min(v1:amt as u64, self.limit) as usize;
3111	self.limit -= amt as u64;
3112	self.inner.consume(amount:amt);
3113	}
3114	}
3115
3116	impl<T> SizeHint for Take<T> {
3117	#[inline]
3118	fn lower_bound(&self) -> usize {
3119	cmp::min(v1:SizeHint::lower_bound(&self.inner) as u64, self.limit) as usize
3120	}
3121
3122	#[inline]
3123	fn upper_bound(&self) -> Option<usize> {
3124	match SizeHint::upper_bound(&self.inner) {
3125	Some(upper_bound: usize) => Some(cmp::min(v1:upper_bound as u64, self.limit) as usize),
3126	None => self.limit.try_into().ok(),
3127	}
3128	}
3129	}
3130
3131	#[stable(feature = "seek_io_take", since = "CURRENT_RUSTC_VERSION")]
3132	impl<T: Seek> Seek for Take<T> {
3133	fn seek(&mut self, pos: SeekFrom) -> Result<u64> {
3134	let new_position = match pos {
3135	SeekFrom::Start(v) => Some(v),
3136	SeekFrom::Current(v) => self.position().checked_add_signed(v),
3137	SeekFrom::End(v) => self.len.checked_add_signed(v),
3138	};
3139	let new_position = match new_position {
3140	Some(v) if v <= self.len => v,
3141	_ => return Err(ErrorKind::InvalidInput.into()),
3142	};
3143	while new_position != self.position() {
3144	if let Some(offset) = new_position.checked_signed_diff(self.position()) {
3145	self.inner.seek_relative(offset)?;
3146	self.limit = self.limit.wrapping_sub(offset as u64);
3147	break;
3148	}
3149	let offset = if new_position > self.position() { i64::MAX } else { i64::MIN };
3150	self.inner.seek_relative(offset)?;
3151	self.limit = self.limit.wrapping_sub(offset as u64);
3152	}
3153	Ok(new_position)
3154	}
3155
3156	fn stream_len(&mut self) -> Result<u64> {
3157	Ok(self.len)
3158	}
3159
3160	fn stream_position(&mut self) -> Result<u64> {
3161	Ok(self.position())
3162	}
3163
3164	fn seek_relative(&mut self, offset: i64) -> Result<()> {
3165	if !self.position().checked_add_signed(offset).is_some_and(\|p\| p <= self.len) {
3166	return Err(ErrorKind::InvalidInput.into());
3167	}
3168	self.inner.seek_relative(offset)?;
3169	self.limit = self.limit.wrapping_sub(offset as u64);
3170	Ok(())
3171	}
3172	}
3173
3174	/// An iterator over `u8` values of a reader.
3175	///
3176	/// This struct is generally created by calling [`bytes`] on a reader.
3177	/// Please see the documentation of [`bytes`] for more details.
3178	///
3179	/// [`bytes`]: Read::bytes
3180	#[stable(feature = "rust1", since = "1.0.0")]
3181	#[derive(Debug)]
3182	pub struct Bytes<R> {
3183	inner: R,
3184	}
3185
3186	#[stable(feature = "rust1", since = "1.0.0")]
3187	impl<R: Read> Iterator for Bytes<R> {
3188	type Item = Result<u8>;
3189
3190	// Not `#[inline]`. This function gets inlined even without it, but having
3191	// the inline annotation can result in worse code generation. See #116785.
3192	fn next(&mut self) -> Option<Result<u8>> {
3193	SpecReadByte::spec_read_byte(&mut self.inner)
3194	}
3195
3196	#[inline]
3197	fn size_hint(&self) -> (usize, Option<usize>) {
3198	SizeHint::size_hint(&self.inner)
3199	}
3200	}
3201
3202	/// For the specialization of `Bytes::next`.
3203	trait SpecReadByte {
3204	fn spec_read_byte(&mut self) -> Option<Result<u8>>;
3205	}
3206
3207	impl<R> SpecReadByte for R
3208	where
3209	Self: Read,
3210	{
3211	#[inline]
3212	default fn spec_read_byte(&mut self) -> Option<Result<u8>> {
3213	inlined_slow_read_byte(self)
3214	}
3215	}
3216
3217	/// Reads a single byte in a slow, generic way. This is used by the default
3218	/// `spec_read_byte`.
3219	#[inline]
3220	fn inlined_slow_read_byte<R: Read>(reader: &mut R) -> Option<Result<u8>> {
3221	let mut byte: u8 = `0`;
3222	loop {
3223	return match reader.read(buf:slice::from_mut(&mut byte)) {
3224	Ok(`0`) => None,
3225	Ok(..) => Some(Ok(byte)),
3226	Err(ref e: &Error) if e.is_interrupted() => continue,
3227	Err(e: Error) => Some(Err(e)),
3228	};
3229	}
3230	}
3231
3232	// Used by `BufReader::spec_read_byte`, for which the `inline(ever)` is
3233	// important.
3234	#[inline(never)]
3235	fn uninlined_slow_read_byte<R: Read>(reader: &mut R) -> Option<Result<u8>> {
3236	inlined_slow_read_byte(reader)
3237	}
3238
3239	trait SizeHint {
3240	fn lower_bound(&self) -> usize;
3241
3242	fn upper_bound(&self) -> Option<usize>;
3243
3244	fn size_hint(&self) -> (usize, Option<usize>) {
3245	(self.lower_bound(), self.upper_bound())
3246	}
3247	}
3248
3249	impl<T: ?Sized> SizeHint for T {
3250	#[inline]
3251	default fn lower_bound(&self) -> usize {
3252	`0`
3253	}
3254
3255	#[inline]
3256	default fn upper_bound(&self) -> Option<usize> {
3257	None
3258	}
3259	}
3260
3261	impl<T> SizeHint for &mut T {
3262	#[inline]
3263	fn lower_bound(&self) -> usize {
3264	SizeHint::lower_bound(*self)
3265	}
3266
3267	#[inline]
3268	fn upper_bound(&self) -> Option<usize> {
3269	SizeHint::upper_bound(*self)
3270	}
3271	}
3272
3273	impl<T> SizeHint for Box<T> {
3274	#[inline]
3275	fn lower_bound(&self) -> usize {
3276	SizeHint::lower_bound(&**self)
3277	}
3278
3279	#[inline]
3280	fn upper_bound(&self) -> Option<usize> {
3281	SizeHint::upper_bound(&**self)
3282	}
3283	}
3284
3285	impl SizeHint for &[u8] {
3286	#[inline]
3287	fn lower_bound(&self) -> usize {
3288	self.len()
3289	}
3290
3291	#[inline]
3292	fn upper_bound(&self) -> Option<usize> {
3293	Some(self.len())
3294	}
3295	}
3296
3297	/// An iterator over the contents of an instance of `BufRead` split on a
3298	/// particular byte.
3299	///
3300	/// This struct is generally created by calling [`split`] on a `BufRead`.
3301	/// Please see the documentation of [`split`] for more details.
3302	///
3303	/// [`split`]: BufRead::split
3304	#[stable(feature = "rust1", since = "1.0.0")]
3305	#[derive(Debug)]
3306	pub struct Split<B> {
3307	buf: B,
3308	delim: u8,
3309	}
3310
3311	#[stable(feature = "rust1", since = "1.0.0")]
3312	impl<B: BufRead> Iterator for Split<B> {
3313	type Item = Result<Vec<u8>>;
3314
3315	fn next(&mut self) -> Option<Result<Vec<u8>>> {
3316	let mut buf: Vec = Vec::new();
3317	match self.buf.read_until(self.delim, &mut buf) {
3318	Ok(`0`) => None,
3319	Ok(_n: usize) => {
3320	if buf[buf.len() - `1`] == self.delim {
3321	buf.pop();
3322	}
3323	Some(Ok(buf))
3324	}
3325	Err(e: Error) => Some(Err(e)),
3326	}
3327	}
3328	}
3329
3330	/// An iterator over the lines of an instance of `BufRead`.
3331	///
3332	/// This struct is generally created by calling [`lines`] on a `BufRead`.
3333	/// Please see the documentation of [`lines`] for more details.
3334	///
3335	/// [`lines`]: BufRead::lines
3336	#[stable(feature = "rust1", since = "1.0.0")]
3337	#[derive(Debug)]
3338	#[cfg_attr(not(test), rustc_diagnostic_item = "IoLines")]
3339	pub struct Lines<B> {
3340	buf: B,
3341	}
3342
3343	#[stable(feature = "rust1", since = "1.0.0")]
3344	impl<B: BufRead> Iterator for Lines<B> {
3345	type Item = Result<String>;
3346
3347	fn next(&mut self) -> Option<Result<String>> {
3348	let mut buf: String = String::new();
3349	match self.buf.read_line(&mut buf) {
3350	Ok(`0`) => None,
3351	Ok(_n: usize) => {
3352	if buf.ends_with('`\n`') {
3353	buf.pop();
3354	if buf.ends_with('`\r`') {
3355	buf.pop();
3356	}
3357	}
3358	Some(Ok(buf))
3359	}
3360	Err(e: Error) => Some(Err(e)),
3361	}
3362	}
3363	}
3364