Rust error E0225: only auto traits can be used as additional traits in a trait object

What rustc prints

error[E0225]: only auto traits can be used as additional traits in a trait object
 --> main.rs:2:36
  |
2 |     let _: Box<dyn std::io::Read + std::io::Write>;
  |                    -------------   ^^^^^^^^^^^^^^ additional non-auto trait
  |                    |
  |                    first non-auto trait
  |
  = help: consider creating a new trait with all of these as supertraits 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 <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>

Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.

fn main() {
    let _: Box<dyn std::io::Read + std::io::Write>;
}

fn main() {
    let _: Box<dyn std::io::Read + Send + Sync>;
}

Related errors

• E0308mismatched types
• E0277the trait bound is not satisfied
• E0599no method named X found in the current scope
• E0433failed to resolve
• E0223ambiguous associated type
• E0224at least one trait is required for an object type
• E0271type mismatch resolving `<i8 as Trait>::AssociatedType == u32`
• E0275overflow evaluating the requirement `Bar<Bar<Bar<Bar<Bar<Bar<Bar<_>>>>>>>: Foo`

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