Cannot use &PyClass as argument type in protocol methods

[davidhewitt]

🌍 Environment

• Your PyO3 version: 0.10.1
• Have you tried using latest PyO3 master (replace version = "0.x.y" with git = "https://github.com/PyO3/pyo3")?: Yes

💥 Reproducing

Create a custom object #[pyclass], let's call it MyClass.

Try to use &MyClass as a function argument in a #[pyproto] block:

use pyo3::prelude::*;
use pyo3::basic::{CompareOp, PyObjectProtocol};

#[pyclass]
pub struct MyClass {}

#[pyproto]
impl PyObjectProtocol for MyClass {
    fn __richcmp__(&self, other: &MyClass, op: CompareOp) -> PyResult<()> {
        unimplemented!()
    }
}

This will fail to compile:

error[E0277]: the trait bound `&MyClass: pyo3::pyclass::PyClass` is not satisfied
   --> src\lib.rs:7:1
    |
7   | #[pyproto]
    | ^^^^^^^^^^ the trait `pyo3::pyclass::PyClass` is not implemented for `&MyClass`
    |
   ::: C:\Users\david\dev\pyo3\src\class\basic.rs:140:17
    |
140 |     type Other: FromPyObject<'p>;
    |                 ---------------- required by this bound in `pyo3::class::basic::PyObjectRichcmpProtocol`
    |
    = help: the following implementations were found:
              <MyClass as pyo3::pyclass::PyClass>
    = note: required because of the requirements on the impl of `pyo3::conversion::FromPyObject<'p>` for `&MyClass`
    = note: this error originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)

I expect this broke on 0.9.0 when we changed so that &PyClass implements derive_utils::ExtractExt instead of FromPyObject.

cc @kngwyu - I guess that when you refactor protocol methods to remove specialization this problem might automatically be fixed at the same time. It could be interesting to add a test for it then.

[kngwyu]

I don't think it's possible. Users can use &PyRef instead.
To enable this, we need a more complex trait that can represent T: FromPyObject or U: FromPyObject, U: Deref<Target = T>.

I guess that when you refactor protocol methods to remove specialization this problem might automatically be fixed at the same time.

I don't think they're related.

[davidhewitt]

Ok cool, thanks for the feedback 👍