Safe conversions of OS streams to Stdio

[SimonSapin]

Writing this issue was triggered by the question How to properly pipe commands in Rust. Currently, the answer is:

.stdin(unsafe { Stdio::from_raw_fd(cmd.stdout.unwrap().as_raw_fd()) })

This is fairly verbose, but more importantly requires unsafe code.

Similar (also unsafe) code can be used to create Stdio from other things like a file or a socket.

For those conversions that are safe⁽¹⁾, the standard library should provide a way to do them without using unsafe. For example, Stdio could implement From<ChildStdout>, From<File>, etc. Or maybe From<T> where T: IntoRawFd / T: IntoRawHandle.

⁽¹⁾ Some combinations seem incorrect, e.g. taking the reading end of a pipe (ChildStdout) and using it for writing (Command::stdout), but can they cause undefined behavior?

[alexcrichton]

We currently avoid trait impls like T: IntoRawFd as they're platform specific (e.g. a TcpSocket as stdin would work on Unix but not on Windows), but specific impls of From<IoObject> sounds fine to me.

[Keruspe]

In rustc internals I see this method: pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)>

If we could get something in these lines availble from outside of rustc, it would maybe allow to do that?

let (in, out) = anon_pipe().unwrap();
Command::new("foo").stdout(in).spawn();
Command::new("bar").stdin(out).spawn();

[mbrubeck]

Fixed by rust-lang/rust#42133.