未自动转换为引用的框

我正在存储一个 Box 在一个 HashMap

use std::collections::HashMap;

trait A {}

trait B {
    fn get(&self, key: &'static str) -> Option<&A>;
}

struct C {
    map: HashMap<&'static str, Box<A>>,
}

impl B for C {
    fn get(&self, key: &'static str) -> Option<&A> {
        return self.map.get(key)
    }
}

我得到的错误是:

expected trait A, found struct `std::boxed::Box`

Option<&Box<&A>> Option<&A>