use std::process::Command;

let bad = Command::new("false").status().unwrap().exit_ok().unwrap_err();
assert_eq!(bad.code(), Some(1));

impl ExitStatus {
    fn exit_ok(self) -> Result<(), ExitStatusError> {..}
}

pub struct ExitStatusError(...); // newtype around a NonZero integer
impl Eq,Copy,Debug,Error,Display for ExitStatusError;
impl Into<ExitStatus> for ExitStatusError;
impl ExitStatusError  {
    fn code(&self) -> Option<i32> {..} } // WIFEXITED WEXITSTATUS
    fn code_nonzero(&self) -> Option<NonZeroi32> {..}
    ...
}
impl ExitStatusExt for ExitStatusError; // .is_signal() etc.

impl Output {
  fn exit_ok(self) -> Result<Self, ExitStatusError>;
}