Rollup merge of #133986 - olishmollie:tracking-issue-127154-documentation, r=tgross35

Add documentation for anonymous pipe module

Tracking issue: https://github.com/rust-lang/rust/issues/127154

`@NobodyXu` I've been using this feature lately and thought I might contribute with some documentation. I borrowed liberally from [os_pipe](https://docs.rs/os_pipe/latest/os_pipe/) so thanks to `@oconnor663.`

library/std/src/pipe.rs

@@ -1,20 +1,66 @@
-//! Module for anonymous pipe
+//!  A cross-platform anonymous pipe.
 //!
-//! ```
+//! This module provides support for anonymous OS pipes, like [pipe] on Linux or [CreatePipe] on
+//! Windows.
+//!
+//! # Behavior
+//!
+//! A pipe is a synchronous, unidirectional data channel between two or more processes, like an
+//! interprocess [`mpsc`](crate::sync::mpsc) provided by the OS. In particular:
+//!
+//! * A read on a [`PipeReader`] blocks until the pipe is non-empty.
+//! * A write on a [`PipeWriter`] blocks when the pipe is full.
+//! * When all copies of a [`PipeWriter`] are closed, a read on the corresponding [`PipeReader`]
+//!   returns EOF.
+//! * [`PipeReader`] can be shared, but only one process will consume the data in the pipe.
+//!
+//! # Capacity
+//!
+//! Pipe capacity is platform dependent. To quote the Linux [man page]:
+//!
+//! > Different implementations have different limits for the pipe capacity. Applications should
+//! > not rely on a particular capacity: an application should be designed so that a reading process
+//! > consumes data as soon as it is available, so that a writing process does not remain blocked.
+//!
+//! # Examples
+//!
+//! ```no_run
 //! #![feature(anonymous_pipe)]
-//!
 //! # #[cfg(miri)] fn main() {}
 //! # #[cfg(not(miri))]
 //! # fn main() -> std::io::Result<()> {
-//! let (reader, writer) = std::pipe::pipe()?;
+//! # use std::process::Command;
+//! # use std::io::{Read, Write};
+//! let (ping_rx, mut ping_tx) = std::pipe::pipe()?;
+//! let (mut pong_rx, pong_tx) = std::pipe::pipe()?;
+//!
+//! // Spawn a process that echoes its input.
+//! let mut echo_server = Command::new("cat").stdin(ping_rx).stdout(pong_tx).spawn()?;
+//!
+//! ping_tx.write_all(b"hello")?;
+//! // Close to unblock echo_server's reader.
+//! drop(ping_tx);
+//!
+//! let mut buf = String::new();
+//! // Block until echo_server's writer is closed.
+//! pong_rx.read_to_string(&mut buf)?;
+//! assert_eq!(&buf, "hello");
+//!
+//! echo_server.wait()?;
 //! # Ok(())
 //! # }
 //! ```
-
+//! [pipe]: https://man7.org/linux/man-pages/man2/pipe.2.html
+//! [CreatePipe]: https://learn.microsoft.com/en-us/windows/win32/api/namedpipeapi/nf-namedpipeapi-createpipe
+//! [man page]: https://man7.org/linux/man-pages/man7/pipe.7.html
 use crate::io;
 use crate::sys::anonymous_pipe::{AnonPipe, pipe as pipe_inner};
 
 /// Create anonymous pipe that is close-on-exec and blocking.
+///
+/// # Examples
+///
+/// See the [module-level](crate::pipe) documentation for examples.
 #[unstable(feature = "anonymous_pipe", issue = "127154")]
 #[inline]
 pub fn pipe() -> io::Result<(PipeReader, PipeWriter)> {
@@ -33,6 +79,58 @@ pub struct PipeWriter(pub(crate) AnonPipe);
 
 impl PipeReader {
     /// Create a new [`PipeReader`] instance that shares the same underlying file description.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(anonymous_pipe)]
+    /// # #[cfg(miri)] fn main() {}
+    /// # #[cfg(not(miri))]
+    /// # fn main() -> std::io::Result<()> {
+    /// # use std::fs;
+    /// # use std::io::Write;
+    /// # use std::process::Command;
+    /// const NUM_SLOT: u8 = 2;
+    /// const NUM_PROC: u8 = 5;
+    /// const OUTPUT: &str = "work.txt";
+    ///
+    /// let mut jobs = vec![];
+    /// let (reader, mut writer) = std::pipe::pipe()?;
+    ///
+    /// // Write NUM_SLOT characters the the pipe.
+    /// writer.write_all(&[b'|'; NUM_SLOT as usize])?;
+    ///
+    /// // Spawn several processes that read a character from the pipe, do some work, then
+    /// // write back to the pipe. When the pipe is empty, the processes block, so only
+    /// // NUM_SLOT processes can be working at any given time.
+    /// for _ in 0..NUM_PROC {
+    ///     jobs.push(
+    ///         Command::new("bash")
+    ///             .args(["-c",
+    ///                 &format!(
+    ///                      "read -n 1\n\
+    ///                       echo -n 'x' >> '{OUTPUT}'\n\
+    ///                       echo -n '|'",
+    ///                 ),
+    ///             ])
+    ///             .stdin(reader.try_clone()?)
+    ///             .stdout(writer.try_clone()?)
+    ///             .spawn()?,
+    ///     );
+    /// }
+    ///
+    /// // Wait for all jobs to finish.
+    /// for mut job in jobs {
+    ///     job.wait()?;
+    /// }
+    ///
+    /// // Check our work and clean up.
+    /// let xs = fs::read_to_string(OUTPUT)?;
+    /// fs::remove_file(OUTPUT)?;
+    /// assert_eq!(xs, "x".repeat(NUM_PROC.into()));
+    /// # Ok(())
+    /// # }
+    /// ```
     #[unstable(feature = "anonymous_pipe", issue = "127154")]
     pub fn try_clone(&self) -> io::Result<Self> {
         self.0.try_clone().map(Self)
@@ -41,6 +139,38 @@ impl PipeReader {
 
 impl PipeWriter {
     /// Create a new [`PipeWriter`] instance that shares the same underlying file description.
+    ///
+    /// # Examples
+    ///
+    /// ```no_run
+    /// #![feature(anonymous_pipe)]
+    /// # #[cfg(miri)] fn main() {}
+    /// # #[cfg(not(miri))]
+    /// # fn main() -> std::io::Result<()> {
+    /// # use std::process::Command;
+    /// # use std::io::Read;
+    /// let (mut reader, writer) = std::pipe::pipe()?;
+    ///
+    /// // Spawn a process that writes to stdout and stderr.
+    /// let mut peer = Command::new("bash")
+    ///     .args([
+    ///         "-c",
+    ///         "echo -n foo\n\
+    ///          echo -n bar >&2"
+    ///     ])
+    ///     .stdout(writer.try_clone()?)
+    ///     .stderr(writer)
+    ///     .spawn()?;
+    ///
+    /// // Read and check the result.
+    /// let mut msg = String::new();
+    /// reader.read_to_string(&mut msg)?;
+    /// assert_eq!(&msg, "foobar");
+    ///
+    /// peer.wait()?;
+    /// # Ok(())
+    /// # }
+    /// ```
     #[unstable(feature = "anonymous_pipe", issue = "127154")]
     pub fn try_clone(&self) -> io::Result<Self> {
         self.0.try_clone().map(Self)