如何從特征對象獲取對具體類型的引用？如何獲得Box<B>或&B或&Box<B>從a在此代碼變量：trait A {}struct B;impl A for B {}fn main() {    let mut a: Box<dyn A> = Box::new(B);    let b = a as Box<B>;}此代碼返回錯誤：error[E0605]: non-primitive cast: `std::boxed::Box<dyn A>` as `std::boxed::Box<B>` --> src/main.rs:8:13  |8 |     let b = a as Box<B>;  |             ^^^^^^^^^^^  |  = note: an `as` expression can only be used to convert between primitive types. Consider using the `From` trait