内部，AnyHandle 是一个 Arc<RwLock<Box<dyn Any>>>，并且与 Arc 的引用计数行为以及 RwLock 的多读者或单写者线程安全模型相匹配。

use any_handle::{AnyHandle, Any};
struct SomeStruct (i32);

fn main() -> Option<()>{
    // Initialize a handle with an unknown type.
    // If you want to pass in a Box<dyn SomeOtherTrait>, instead of a concrete
    // type, you will have to use `#![feature(trait_upcasting)]`, unfortunately.
    let handle: AnyHandle<dyn Any> = AnyHandle::new(Box::new(SomeStruct(12)));
    // Now we can put it in some sort of generic container...
    
    // ...and when we retrieve it later:
    let mut handle: AnyHandle<SomeStruct> = handle.downcast().ok()?;
    handle.write().do_mut_things_with();
    handle.read().do_things_with();
    Some(())
}