1 You attempted to use multiple types as bounds for a closure or trait object.
2 Rust does not currently support this. A simple example that causes this error:
6 let _: Box<dyn std::io::Read + std::io::Write>;
10 Auto traits such as Send and Sync are an exception to this rule:
11 It's possible to have bounds of one non-builtin trait, plus any number of
12 auto traits. For example, the following compiles correctly:
16 let _: Box<dyn std::io::Read + Send + Sync>;