Auto merge of #114590 - ijackson:stdio-stdio-2, r=dtolnay

Allow redirecting subprocess stdout to our stderr etc. (redux)

This is the code from #88561, tidied up, including review suggestions, and with the for-testing-only CI commit removed.  FCP for the API completed in #88561.

I have made a new MR to facilitate review.  The discussion there is very cluttered and the branch is full of changes (in many cases as a result of changes to other Rust stdlib APIs since then).  Assuming this MR is approvedl we should close that one.

### Reviewer doing a de novo review

Just code review these four commits..  FCP discussion starts here: https://github.com/rust-lang/rust/pull/88561#issuecomment-1640527595

Portability tests: you can see that this branch works on Windows too by looking at the CI results in #88561, which has the same code changes as this branch but with an additional "DO NOT MERGE" commit to make the Windows tests run.

### Reviewer doing an incremental review from some version of #88561

Review the new commits since your last review.  I haven't force pushed the branch there.

git diff the two branches (eg `git diff 176886197d6..0842b69c219`).  You'll see that the only difference is in gitlab CI files.  You can also see that *this* MR doesn't touch those files.

library/std/src/process.rs

@@ -1501,6 +1501,66 @@ impl From<fs::File> for Stdio {
     }
 }
 
+#[stable(feature = "stdio_from_stdio", since = "CURRENT_RUSTC_VERSION")]
+impl From<io::Stdout> for Stdio {
+    /// Redirect command stdout/stderr to our stdout
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(exit_status_error)]
+    /// use std::io;
+    /// use std::process::Command;
+    ///
+    /// # fn test() -> Result<(), Box<dyn std::error::Error>> {
+    /// let output = Command::new("whoami")
+    // "whoami" is a command which exists on both Unix and Windows,
+    // and which succeeds, producing some stdout output but no stderr.
+    ///     .stdout(io::stdout())
+    ///     .output()?;
+    /// output.status.exit_ok()?;
+    /// assert!(output.stdout.is_empty());
+    /// # Ok(())
+    /// # }
+    /// #
+    /// # if cfg!(unix) {
+    /// #     test().unwrap();
+    /// # }
+    /// ```
+    fn from(inherit: io::Stdout) -> Stdio {
+        Stdio::from_inner(inherit.into())
+    }
+}
+
+#[stable(feature = "stdio_from_stdio", since = "CURRENT_RUSTC_VERSION")]
+impl From<io::Stderr> for Stdio {
+    /// Redirect command stdout/stderr to our stderr
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// #![feature(exit_status_error)]
+    /// use std::io;
+    /// use std::process::Command;
+    ///
+    /// # fn test() -> Result<(), Box<dyn std::error::Error>> {
+    /// let output = Command::new("whoami")
+    ///     .stdout(io::stderr())
+    ///     .output()?;
+    /// output.status.exit_ok()?;
+    /// assert!(output.stdout.is_empty());
+    /// # Ok(())
+    /// # }
+    /// #
+    /// # if cfg!(unix) {
+    /// #     test().unwrap();
+    /// # }
+    /// ```
+    fn from(inherit: io::Stderr) -> Stdio {
+        Stdio::from_inner(inherit.into())
+    }
+}
+
 /// Describes the result of a process after it has terminated.
 ///
 /// This `struct` is used to represent the exit status or other termination of a child process.