Error deriving PartialEq in a struct with Box<Trait>

[itaibn]

Code:

pub trait MyTrait {}

impl PartialEq for Box<MyTrait> {
    fn eq(&self, _: &Box<MyTrait>) -> bool {
        false
    }
}

#[derive(PartialEq)]
pub struct MyType(Box<MyTrait>);

fn main() {}

Output:

   Compiling playground v0.0.1 (file:///playground)
error[E0507]: cannot move out of borrowed content
  --> src/main.rs:10:19
   |
10 | pub struct MyType(Box<MyTrait>);
   |                   ^^^^^^^^^^^^^ cannot move out of borrowed content

error: aborting due to 2 previous errors

error: Could not compile `playground`.

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

Three problems:

• The compiler reports that there are two errors, but it only lists one. (This problem does not appear in the current Nightly compiler, only in Beta and Stable.)

• The error message doesn't fit with the span provided. This error is usually associated with a statement or expression but here is given for a struct component.

• I don't see why the derive should not work.

Searching for similar issues, this one may be a duplicate/combination of #39128 and #31740.

[dtolnay]

Thanks! I left a comment in #39128 (originally reported for Arc<Trait>) that the same bug happens for Box<Trait> as well.

[TimNN]

Closing as duplicate of #47823.

Edit: Ugh... no idea what went wrong here... 😕 😳

[dtolnay]

*of #39128.