Shared via Rust Playground

playground.rs

use std::any::Any;
use std::collections::HashMap;
use std::fmt::Debug;

fn main() {
    let mut map: HashMap<(), Box<Fn() -> Box<Any>>> = HashMap::new();
    map.insert((), Box::new(|| {
        let val: Box<Box<Trait>> = Box::new(Box::new(A));
        val
    }));
    let test = Test {
        map: map,
    };
    
    let a: Option<Box<Box<Trait>>> = test.get::<Box<Trait>>();
    
    println!("{:?}", a);
}

struct Test {
    map: HashMap<(), Box<Fn() -> Box<Any>>>,
}

impl Test {
    fn get<T: Any>(&self) -> Option<Box<T>> {
        let result = self.map.get(&());
        
        match result {
            Some(value) => {
                let returned = value.as_ref()();
                Some(returned.downcast::<T>().unwrap())
            },
            None => None,
        }
    }
}

#[derive(Debug)]
struct A;

trait Trait: Debug {}

impl Trait for A {}