Derived PartialEq for Box<dyn Trait> is incorrect.

[davidli2010]

pub trait T {}

impl PartialEq for dyn T {
    fn eq(&self, _other: &Self) -> bool {
        todo!()
    }
}

#[derive(PartialEq)]
pub struct A {
    child: Box<dyn T>,
}

impl T for A {}

// impl PartialEq for A {
//     fn eq(&self, other: &Self) -> bool {
//         *self.child == *other.child
//     }
// }

fn main() {}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0507]: cannot move out of `*__self_1_0` which is behind a shared reference
  --> src/main.rs:11:5
   |
11 |     child: Box<dyn T>,
   |     ^^^^^^^^^^^^^^^^^ move occurs because `*__self_1_0` has type `std::boxed::Box<dyn T>`, which does not implement the `Copy` trait
   |
   = note: this error originates in a derive macro (in Nightly builds, run with -Z macro-backtrace for more info)

error: aborting due to previous error

For more information about this error, try `rustc --explain E0507`.
error: could not compile `playground`.

To learn more, run the command again with --verbose.

[davidli2010]

If I remove #[derive(PartialEq)] and impl it manually:

impl PartialEq for A {
    fn eq(&self, other: &Self) -> bool {
        *self.child == *other.child
    }
}

It works.

[sfackler]

Duplicate of #39128.