Issue with binding ownership and scopes in Rust

gistfile1.txt

pub struct Command {
  cmd: String,    
}

impl Command {
  pub fn new(cmd: &str) -> Command {
    Command {
      cmd: String::from(cmd),
    }
  }
}

#[no_mangle]
pub extern "C" fn command_new<'a>(cmd: *const libc::c_char) -> Command {
    let buf_cmd = unsafe { CStr::from_ptr(cmd).to_bytes() };
    let str_cmd = String::from_utf8(buf_cmd.to_vec()).unwrap();
    Command::new(str_cmd)
}