message: we don't need to flush stderr
diff: @@ -392,13 +392,16 @@ pub trait EvalContextExt<'a, 'mir, 'tcx: 'a+'mir>: crate::MiriEvalContextExt<'a,
  let buf_cont = this.memory().read_bytes(buf, Size::from_bytes(n))?;
  // We need to flush to make sure this actually appears on the screen
  let res = if fd == 1 {
  + // Stdout is buffered, flush to make sure it appears on the screen.
  + // This is the write() syscall of the interpreted program, we want it
  + // to correspond to a write() syscall on the host -- there is no good
  + // in adding extra buffering here.
  let res = io::stdout().write(buf_cont);
  io::stdout().flush().unwrap();
  res
  } else {
  - let res = io::stderr().write(buf_cont);
  - io::stderr().flush().unwrap();
  - res
  + // No need to flush, stderr is not buffered.
  + io::stderr().write(buf_cont)
  };
  match res {
  Ok(n) => n as i64,
  