Rollup merge of rust-lang#55200 - octronics:gh51430, r=kennytm

Documents `From` implementations for `Stdio`

This PR solves part of rust-lang#51430 by adding a basic summary and an example to each `impl From` inside `process` module (`ChildStdin`, `ChildStdout`, `ChildStderr`, `File`).

It does not document if the conversions allocate memory and how expensive they are.

src/libstd/process.rs

@@ -1016,27 +1016,115 @@ impl fmt::Debug for Stdio {
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<ChildStdin> for Stdio {
+    /// Converts a `ChildStdin` into a `Stdio`
+    ///
+    /// # Examples
+    ///
+    /// `ChildStdin` will be converted to `Stdio` using `Stdio::from` under the hood.
+    ///
+    /// ```rust
+    /// use std::process::{Command, Stdio};
+    ///
+    /// let reverse = Command::new("rev")
+    ///     .stdin(Stdio::piped())
+    ///     .spawn()
+    ///     .expect("failed reverse command");
+    ///
+    /// let _echo = Command::new("echo")
+    ///     .arg("Hello, world!")
+    ///     .stdout(reverse.stdin.unwrap()) // Converted into a Stdio here
+    ///     .output()
+    ///     .expect("failed echo command");
+    ///
+    /// // "!dlrow ,olleH" echoed to console
+    /// ```
     fn from(child: ChildStdin) -> Stdio {
         Stdio::from_inner(child.into_inner().into())
     }
 }
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<ChildStdout> for Stdio {
+    /// Converts a `ChildStdout` into a `Stdio`
+    ///
+    /// # Examples
+    ///
+    /// `ChildStdout` will be converted to `Stdio` using `Stdio::from` under the hood.
+    ///
+    /// ```rust
+    /// use std::process::{Command, Stdio};
+    ///
+    /// let hello = Command::new("echo")
+    ///     .arg("Hello, world!")
+    ///     .stdout(Stdio::piped())
+    ///     .spawn()
+    ///     .expect("failed echo command");
+    ///
+    /// let reverse = Command::new("rev")
+    ///     .stdin(hello.stdout.unwrap())  // Converted into a Stdio here
+    ///     .output()
+    ///     .expect("failed reverse command");
+    ///
+    /// assert_eq!(reverse.stdout, b"!dlrow ,olleH\n");
+    /// ```
     fn from(child: ChildStdout) -> Stdio {
         Stdio::from_inner(child.into_inner().into())
     }
 }
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<ChildStderr> for Stdio {
+    /// Converts a `ChildStderr` into a `Stdio`
+    ///
+    /// # Examples
+    ///
+    /// ```rust,no_run
+    /// use std::process::{Command, Stdio};
+    ///
+    /// let reverse = Command::new("rev")
+    ///     .arg("non_existing_file.txt")
+    ///     .stderr(Stdio::piped())
+    ///     .spawn()
+    ///     .expect("failed reverse command");
+    ///
+    /// let cat = Command::new("cat")
+    ///     .arg("-")
+    ///     .stdin(reverse.stderr.unwrap()) // Converted into a Stdio here
+    ///     .output()
+    ///     .expect("failed echo command");
+    ///
+    /// assert_eq!(
+    ///     String::from_utf8_lossy(&cat.stdout),
+    ///     "rev: cannot open non_existing_file.txt: No such file or directory\n"
+    /// );
+    /// ```
     fn from(child: ChildStderr) -> Stdio {
         Stdio::from_inner(child.into_inner().into())
     }
 }
 
 #[stable(feature = "stdio_from", since = "1.20.0")]
 impl From<fs::File> for Stdio {
+    /// Converts a `File` into a `Stdio`
+    ///
+    /// # Examples
+    ///
+    /// `File` will be converted to `Stdio` using `Stdio::from` under the hood.
+    ///
+    /// ```rust,no_run
+    /// use std::fs::File;
+    /// use std::process::Command;
+    ///
+    /// // With the `foo.txt` file containing `Hello, world!"
+    /// let file = File::open("foo.txt").unwrap();
+    ///
+    /// let reverse = Command::new("rev")
+    ///     .stdin(file)  // Implicit File convertion into a Stdio
+    ///     .output()
+    ///     .expect("failed reverse command");
+    ///
+    /// assert_eq!(reverse.stdout, b"!dlrow ,olleH");
+    /// ```
     fn from(file: fs::File) -> Stdio {
         Stdio::from_inner(file.into_inner().into())
     }