#[derive(PartialEq)] doesn't compile when fields are Box<Something> with PartialEq for Box<Something>

[stepancheg]

trait Foo {
}

// our custom implementation of PartialEq
impl PartialEq for Box<Foo> {
    fn eq(&self, _other: &Self) -> bool {
        unimplemented!()
    }
}


#[derive(PartialEq)]
struct Bar {
    foo: Box<Foo>, // <!-- compilation error here
}


// this version works properly

struct Baz {
    foo: Box<Foo>,
}

// this version works
impl PartialEq for Baz {
    fn eq(&self, other: &Self) -> bool {
        PartialEq::eq(&self.foo, &other.foo)
    }
}

Error is:

error[E0507]: cannot move out of borrowed content
  --> src/main.rs:13:5
   |
13 |     foo: Box<Foo>,
   |     ^^^^^^^^^^^^^ cannot move out of borrowed content

Play: https://play.rust-lang.org/?gist=b2dcd5013fee7a19b46241390a347572&version=stable&mode=debug

[Mark-Simulacrum]

Duplicate of #39128.