error[E0225]: only auto traits can be used as additional traits in a trait object --> $DIR/E0225.rs:6:36 | LL | let _: Box; | ------------- ^^^^^^^^^^^^^^ additional non-auto trait | | | first non-auto trait | = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::io::Read + std::io::Write {}` = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit error[E0225]: only auto traits can be used as additional traits in a trait object --> $DIR/E0225.rs:8:20 | LL | trait Foo = std::io::Read + std::io::Write; | ------------- -------------- additional non-auto trait | | | first non-auto trait ... LL | let _: Box; | ^^^ | | | trait alias used in trait object type (additional use) | trait alias used in trait object type (first use) | = help: consider creating a new trait with all of these as super-traits and using that trait here instead: `trait NewTrait: std::io::Read + std::io::Write {}` = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit error: aborting due to 2 previous errors For more information about this error, try `rustc --explain E0225`.