Cannot move out of borrowed content on enum containing a boxed trait object when deriving PartialEq

Question

I'm trying to write an enum deriving PartialEq which contains a trait object which does so manually. I used the solution here in order to force implementors of Trait to write an equality method. This fails to compile:

trait Trait {
    fn partial_eq(&self, rhs: &Box<Trait>) -> bool;
}

impl PartialEq for Box<Trait> {
    fn eq(&self, rhs: &Box<Trait>) -> bool {
        self.partial_eq(rhs)
    }
}

#[derive(PartialEq)]
enum Enum {
    Trait(Box<Trait>),
}

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:13:11
   |
13 |     Trait(Box<Trait>),
   |           ^^^^^^^^^^^ cannot move out of borrowed content

This only compiles when I manually impl PartialEq for Enum. Why is this the case?

You probably want to use `&Trait` in your `partial_eq`; `&Box` has unneeded double indirection. — Shepmaster, Mar 17 '18 at 17:35

score 2 · Accepted Answer · answered Mar 17 '18 at 17:42

Expanding the automatically-derived implementation of PartialEq presents a better error message:

impl ::std::cmp::PartialEq for Enum {
    #[inline]
    fn eq(&self, __arg_0: &Enum) -> bool {
        match (&*self, &*__arg_0) {
            (&Enum::Trait(ref __self_0), &Enum::Trait(ref __arg_1_0)) => {
                true && (*__self_0) == (*__arg_1_0)
            }
        }
    }
    #[inline]
    fn ne(&self, __arg_0: &Enum) -> bool {
        match (&*self, &*__arg_0) {
            (&Enum::Trait(ref __self_0), &Enum::Trait(ref __arg_1_0)) => {
                false || (*__self_0) != (*__arg_1_0)
            }
        }
    }
}

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:21:40
   |
21 |                 true && (*__self_0) == (*__arg_1_0)
   |                                        ^^^^^^^^^^^^ cannot move out of borrowed content

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:29:41
   |
29 |                 false || (*__self_0) != (*__arg_1_0)
   |                                         ^^^^^^^^^^^^ cannot move out of borrowed content

This is tracked as Rust issues 31740 and 39128.

You will likely need to implement PartialEq for this type yourself as well.

Cannot move out of borrowed content on enum containing a boxed trait object when deriving PartialEq

1 Answers1