Using vtable dispatch to work with owned type behind Box<dyn> trait

playground.rs

use std::any::{Any, TypeId};
use std::fmt::Debug;

pub trait Resource: Any + Debug {
    // this returns correct function for the Self to work on Box so that it can be casted and unboxed
    fn combine_fn(&self) -> fn(left: Box<dyn Resource>, right: Box<dyn Resource>) -> Box<dyn Resource>;
}

#[derive(Debug)]
struct File(u32);

impl File {
    fn combine(left: File, right: File) -> File {
        File(left.0 + right.0)
    }
}

// taken from Box::downcast as I cannot cast Box<dyn Resource> to Box<dyn Any>
// https://github.com/rust-lang/rfcs/issues/2765#issuecomment-703950666
pub fn downcast<T: Resource>(res: Box<dyn Resource>) -> Result<Box<T>, Box<dyn Resource>> {
    if <dyn Resource>::type_id(&*res) == TypeId::of::<T>() {
        unsafe {
            let raw: *mut (dyn Resource) = Box::into_raw(res);
            Ok(Box::from_raw(raw as *mut T))
        }
    } else {
        Err(res)
    }
}

impl Resource for File {
    fn combine_fn(&self) -> fn(left: Box<dyn Resource>, right: Box<dyn Resource>) -> Box<dyn Resource> {
        fn dyn_combine(left: Box<dyn Resource>, right: Box<dyn Resource>) -> Box<dyn Resource>  {
            let left: File = *downcast::<File>(left).unwrap();
            let right: File = *downcast::<File>(right).unwrap();
            Box::new(File::combine(left, right))
        }
        dyn_combine
    }
}


fn main() {
    let a = File(1);
    let b = File(10);

    let a: Box<dyn Resource> = Box::new(a);
    let b: Box<dyn Resource> = Box::new(b);
    
    let c = a.combine_fn()(a,b);
    
    dbg![c];
}