Function launch 

pub fn launch(
    dest: &SshDest,
    cmd: &str,
    cmd_args: &[String],
    cwd: &str,
    control_dir: Option<&Path>,
) -> Vec<String>

The ssh arguments for a pane. control_dir, on macOS and Linux, is where connection-sharing sockets live (ControlMaster), so a second pane on the same connection reuses the first one’s and asks for nothing.