Why there is no downcast method

[BORSXN]

use std::{any::Any, collections::HashMap};

fn main() {
    println!("Hello, world!");
    let s = SHM::new();
    s.get(&Screen::Login).unwrap().downcast_ref::<Login>();
}

trait Page: Any {}
struct Login;
impl Page for Login {}
type SHM = HashMap<Screen, Box<dyn Page>>;
#[derive(Debug, Hash, PartialEq, Eq)]
enum Screen {
    Login,
}

    Checking tdowncast v0.1.0 (E:\Coding\Code\Rust\tdowncast)
error[E0599]: no method named `downcast_ref` found for reference `&Box<dyn Page>` in the current scope
 --> src\main.rs:6:36
  |
6 |     s.get(&Screen::Login).unwrap().downcast_ref::<Login>();
  |                                    ^^^^^^^^^^^^
  |
help: there is a method `as_ref` with a similar name
  |
6 -     s.get(&Screen::Login).unwrap().downcast_ref::<Login>();
6 +     s.get(&Screen::Login).unwrap().as_ref::<Login>();
  |

For more information about this error, try `rustc --explain E0599`.

[bettertree]

You need to implement Page for Screen.

After Screen implements Page, take a look at this

Downcast_ref() not working on a trait that *does* inherit from Any

I have a trait that is defined like this: pub trait MyTrait: Any + Send { ... } Application will implement this trait: struct MyImplementation; impl MyTrait for MyImplementation { ... } ...and pass its implementation to my library: fn register(&mut self, callbacks: impl MyTrait) { ... } The instance is stored in a Box<dyn MyTrait> inside of my library: struct MyLibrary { registered: Box<dyn MyTrait>, } Now, if I later return the Box<dyn MyTrait> back to the application, how I can c…

[quinedot]

Ignoring dyn compatibility, if it were a method of the Any trait, you would be calling downcast_ref on Box<dyn Page>, which could not succeed for a target other than itself.