请教一下如何在trait的fn中将self作为trait object？

代码如下

trait A {
    fn plus(&self) -> &dyn A {
        self
    }
}

错误：

error[E0277]: the size for values of type `Self` cannot be known at compilation time
 --> src/lib.rs:3:9
  |
2 |     fn plus(&self) -> &dyn A {
  |                             - help: consider further restricting `Self`: `where Self: std::marker::Sized`
3 |         self
  |         ^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `Self`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: required for the cast to the object type `dyn A`

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