confusing type mismatch error when external crates introduce additional trait implementations

[nagisa]

Consider the following code:

pub enum Error {
    Foo,
}

#[test]
fn it_works() {
    Err(Error::Foo) == Err(Error::Foo);
}

The error message output here is very nice and easy to understand:

error[E0369]: binary operation `==` cannot be applied to type `Result<_, Error>`
 --> src/lib.rs:7:21
  |
7 |     Err(Error::Foo) == Err(Error::Foo);
  |     --------------- ^^ --------------- Result<_, Error>
  |     |
  |     Result<_, Error>
  |
note: an implementation of `PartialEq` might be missing for `Error`
...

Now lets add a dependency on an external crate (inlined here) (playground):

// NB: in the original reproduction this was an external crate.
pub mod rkyv {
    pub enum ArchivedResult<T, E> {
        Ok(T),
        Err(E),
    }

    impl<T: PartialEq<U>, U, E: PartialEq<F>, F> PartialEq<ArchivedResult<T, E>>
    for Result<U, F> {
        fn eq(&self, _: &ArchivedResult<T, E>) -> bool { todo!() }
    }
}

pub enum Error {
    Foo,
}

#[test]
fn it_works() {
    Err(Error::Foo) == Err(Error::Foo);
}

Once that's done the diagnostic regresses significantly, to the point where it made me suspect an incremental or a similarly serious bug somewhere

error[E0308]: mismatched types
  --> src/lib.rs:20:24
   |
20 |     Err(Error::Foo) == Err(Error::Foo);
   |                        ^^^^^^^^^^^^^^^ expected enum `ArchivedResult`, found enum `Result`
   |
   = note: expected enum `ArchivedResult<_, _>`
              found enum `Result<_, Error>`
help: try wrapping the expression in a variant of `ArchivedResult`
   |
20 |     Err(Error::Foo) == rkyv::ArchivedResult::Ok(Err(Error::Foo));
   |                        +++++++++++++++++++++++++               +
20 |     Err(Error::Foo) == rkyv::ArchivedResult::Err(Err(Error::Foo));
   |                        ++++++++++++++++++++++++++               +

I think I would've preferred a plain old “trait is not implemented” error here.

[SNCPlay42]

This looks like #77304.

[nagisa]

Looks like its probably that, yeah, although in my case I've seen the same problem present itself when calling methods directly too.

Duplicate of #77304