The `Copy` trait may not be implemented for this type

Question

I would like to make a fancy universal dictionary for faster prototyping, where the keys are Strings and the values are types wrapped in an AnyType container.

Please be advised that simply adding the normal derive statement above the struct isn't really sufficient, which is why it's not included among the original contents.

My implementation looks like this:

use std::collections::HashMap;
use std::boxed::Box;
use std::rc::Rc;
use std::cell::RefCell;
use std::clone::Clone;

struct AnyType<AT> {
    ___value: Rc<RefCell<Box<AT>>>
}
impl<AT> AnyType<AT> {
    pub fn new(value: AT) -> AnyType<AT> {
        AnyType { 
            ___value: Rc::new(RefCell::from(Box::new(value)))
        }
    }
}

impl<AT> Copy for AnyType<AT> {}
impl<AT> Clone for AnyType<AT> {
    fn clone(&self) -> Self {
        *self
    }
}

struct Dict<AT> {
    ___self: HashMap<String, AnyType<AT>>
}

impl<AT> Dict<AT> {
    pub fn new(keys: Option<Vec<String>>, values: Option<Vec<AnyType<AT>>>) 
        -> Result<Dict<AT>, &'static str>
    {
        // ...

Error:

error[E0204]: the trait `Copy` may not be implemented for this type
  --> src/lib.rs:18:10
   |
8  |     ___value: Rc<RefCell<Box<AT>>>
   |     ------------------------------ this field does not implement `Copy`
...
18 | impl<AT> Copy for AnyType<AT> {}
   |          ^^^^

Playground

Your type can't be Copy because it contains an Rc. Implement Clone explicitly. — ramslök, Apr 28 '20 at 22:31
When your [question has been closed](https://stackoverflow.com/questions/61488616/the-trait-copy-may-not-be-implemented-for-this-type) you should edit it to address the identified problems and then comment asking it to be reopened. If you disagree with it being marked a duplicate, explain why you don't believe the duplicate applies. Please do not post the same question again to get around the closure. — John Kugelman, Apr 28 '20 at 23:44

score 1 · Accepted Answer · edited Apr 28 '20 at 23:39

You are implementing Copy for AnyType but not for AT. Hence the compiler knows that the full type AnyType cannot be copied in all instances. This is due to the fact that you are not specifying that AT must implement Copy as well. You will have to specify a trait bound AT: Copy to tell Rust that AT can also be copied.

A short example:

#[derive(Clone)]
struct A<T> {
    _v: T
}

impl<T> Copy for A<T> {}

// Overloaded functions in Rust!
fn main() {
    let a = A::<i32>{_v: 1};
}

will also lead to error[E0204]: the trait `Copy` may not be implemented for this type.

However, by adding the trait bound Copy to all type parameters, the rust compiler will now accept the code since A can be copied because T can be copied.

#[derive(Clone)]
struct A<T: Copy> {
    _v: T
}

impl<T> Copy for A<T> where T: Copy {}

// Overloaded functions in Rust!
fn main() {
    let a = A::<i32>{_v: 1};
}

How is this an answer to the problem? `Rc` is not `Copy`, no matter what type you put into it. — mcarton, Apr 28 '20 at 23:39

The `Copy` trait may not be implemented for this type

1 Answers1