How to clone a struct with Option<Box<dyn Any>>

[finlaydotb]

I have a struct similar to this

#[derive(Debug, Clone)]
pub struct Token {
    pub lexeme: String,
    pub value: Option<Box<dyn Any>>,
    pub line: u32,
}

Adding the #[derive(Clone)] part leads to the compilation error:

error[E0277]: the trait bound `dyn std::any::Any: Clone` is not satisfied
  --> src/token.rs:85:5
   |
81 | #[derive(Debug, Clone)]
   |                 ----- in this derive macro expansion
...
85 |     pub value: Option<Box<dyn Any>>,
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `dyn std::any::Any`
   |
   = note: required for `Box<dyn std::any::Any>` to implement `Clone`
   = note: 1 redundant requirement hidden
   = note: required for `Option<Box<dyn std::any::Any>>` to implement `Clone`
   = note: this error originates in the derive macro `Clone` (in Nightly builds, run with -Z macro-backtrace for more info)

It seems the issue is Any does not implement Clone which kinda makes sense since Any means any type.

The question now is, with such a data structure, how would one go about using it in cases where it values needs to be clones or copied?

[steffahn]

If you actually want to clone the underlying type in the box, then using the dyn_clone - Rust crate can be an option. To re-gain Any-like functionality on a different trait than Any itself, you can use the downcast - Rust crate. Together this could be something like

/*
[dependencies]
dyn-clone = "1"
downcast = "0.11"
*/

use dyn_clone::{clone_trait_object, DynClone};
use downcast::{Any, downcast};

use std::fmt;

pub trait Value: DynClone + Any {}
clone_trait_object!(Value);
downcast!(dyn Value);

impl<T: Clone + Any> Value for T {}

impl fmt::Debug for dyn Value {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Value").finish_non_exhaustive()
    }
}

#[derive(Debug, Clone)]
pub struct Token {
    pub lexeme: String,
    pub value: Option<Box<dyn Value>>,
    pub line: u32,
}

---

If you only need read-only access to the value anyways, then the alternative of simply using Option<Rc<dyn Any>> could be better, i.e. simpler to code and also more efficient, as cloning an Rc will be cheaper.