error[E0225]: only auto traits can be used as additional traits in a trait object
  --> $DIR/E0225.rs:12:32
   |
LL |     let _: Box<std::io::Read + std::io::Write>;
   |                                ^^^^^^^^^^^^^^ non-auto additional trait

error: aborting due to previous error

If you want more information on this error, try using "rustc --explain E0225"