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

pub struct DebugAny {
    any: Box<dyn Any>,
    my_debug_impl: fn(&dyn Any, &mut fmt::Formatter) -> fmt::Result
}


impl DebugAny {
    pub fn from_debug<V: Any + fmt::Debug>(v: V) -> Self {
        // I'll just make the damn vtable *myself*!
        let my_debug_impl = |any: &dyn Any, f: &mut fmt::Formatter| {
            let v = any.downcast_ref::<V>().expect("debug vtable busted!");
            v.fmt(f)
        };
        Self {
            any: Box::new(v),
            my_debug_impl,
        }
    }

    pub fn downcast_ref<T: Any>(&self) -> Option<&T> {
        self.any.downcast_ref::<T>()
    }
}

impl fmt::Debug for DebugAny {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        (self.my_debug_impl)(self.any.as_ref(), f)
    }
}

fn main() {
    let foo = "I'm a string!";
    let any = DebugAny::from_debug(foo);
    println!("debug: {:?}", any);
    println!("downcast_ref: {:?}", any.downcast_ref::<&str>())
}