#![allow(unused_variables)]
fn main() {
    use std::error::Error;
    use std::fmt;
    #[derive(Debug)]
    struct AnError;
    impl fmt::Display for AnError {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(f, "An error")
        }
    }
    impl Error for AnError {}
    unsafe impl Send for AnError {}
    unsafe impl Sync for AnError {}
    let an_error = AnError;
    assert!(0 == size_of_val(& an_error));
    let a_boxed_error = Box::<dyn Error + Send + Sync>::from(an_error);
    assert!(
        size_of::< Box < dyn Error + Send + Sync >> () == size_of_val(& a_boxed_error)
    )
}