Multiple types were used as bounds for a closure or trait object.
Erroneous code example:
fn main() {
let _: Box<dyn std::io::Read + std::io::Write>;
}Rust does not currently support this.
Auto traits such as Send and Sync are an exception to this rule: It’s possible to have bounds of one non-builtin trait, plus any number of auto traits. For example, the following compiles correctly:
fn main() {
let _: Box<dyn std::io::Read + Send + Sync>;
}