Child Stdin None using Nix PTY - rust

Question

I was trying to play with PTY and some Unix concept using Rust. I ended up with the following code, unfortunately I can't make it work. I really struggle to find out why the stdin. stdout and stderr of the child process end up being None when assigned to the file descriptor of the PTY's slave.
In the Cargo.toml file I have the following dependencies:
[dependencies]
nix = "0.17.0"
libc = "0.2"
From the debugging I have already made, it seems that the None option is coming from the map of ChildStdin/ChildStdout/ChildStderr done in https://github.com/rust-lang/rust/blob/master/src/libstd/process.rs#L195.
I'm not sure it's a Rust issue, and it might be my understanding of how PTYs works, but I have written the same code (as much as possible) in C and it's behaving as expected.
Could you help me figure out what is wrong
use nix::Error;
use std::path::Path;
use std::os::unix::{
io::{AsRawFd, FromRawFd, RawFd}
};
use nix::sys::stat::Mode;
use nix::fcntl::{OFlag, open};
use std::process::{Command, Stdio};
use std::os::unix::process::CommandExt;
use nix::pty::{PtyMaster, grantpt, posix_openpt, ptsname, unlockpt};
use libc::{self, winsize};
#[macro_use]
extern crate nix;
struct Pty {
master: PtyMaster,
slave: RawFd
}
fn construct_internal_pty() -> Result<Pty, Error> {
ioctl_write_ptr_bad!(pty_io_size, nix::libc::TIOCSWINSZ, winsize);
let master_fd = posix_openpt(OFlag::O_RDWR | OFlag::O_NOCTTY)?;
grantpt(&master_fd)?;
unlockpt(&master_fd)?;
let slave_name = unsafe { ptsname(&master_fd) }?;
let slave_fd = open(
Path::new(&slave_name),
OFlag::O_RDWR | OFlag::O_NOCTTY,
Mode::empty())?;
Ok(Pty {
master: master_fd,
slave: slave_fd
})
}
fn main() -> () {
let pty = construct_internal_pty().expect("Unable to construct PTY");
let mut builder = Command::new("/bin/sh");
let slave = pty.slave;
let master = pty.master.as_raw_fd();
builder.env("LOGNAME", "ex0ns");
builder.env("USER", "ex0ns");
builder.env("SHELL", "/bin/sh");
builder.env("HOME", "/home/ex0ns");
builder.current_dir("/home/ex0ns");
// This is not working.... The resulting fd is None
builder.stdin(unsafe { Stdio::from_raw_fd(slave) });
builder.stderr(unsafe { Stdio::from_raw_fd(slave) });
builder.stdout(unsafe { Stdio::from_raw_fd(slave) });
unsafe {
builder.pre_exec(move || {
libc::setsid();
libc::close(slave);
libc::close(master);
libc::signal(libc::SIGCHLD, libc::SIG_DFL);
libc::signal(libc::SIGHUP, libc::SIG_DFL);
libc::signal(libc::SIGINT, libc::SIG_DFL);
libc::signal(libc::SIGQUIT, libc::SIG_DFL);
libc::signal(libc::SIGTERM, libc::SIG_DFL);
libc::signal(libc::SIGALRM, libc::SIG_DFL);
Ok(())
});
}
match builder.spawn() {
Ok(child) => {
println!("{:?}", child);
println!("{:?}", slave);
assert_eq!(child.stdin.is_none(), false);
unsafe { libc::close(slave) }; // close PTY ?
}
Err(error) => {
println!("Unable to spawn child process {}", error);
::std::process::exit(1);
}
}
}
Thank you