Add const PyClass instances to a module.

[g-bauer]

I am not able to add a const instance of a PyClass to a module. I tried different approaches (see lib.rs below) without success.

Cargo.toml

[package]
version = "0.1.0"
edition = "2018"

# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
[lib]
name = "test_pyo3"
crate-type = ["cdylib"]

[dependencies.pyo3]
git = "https://github.com/PyO3/pyo3"
features = ["extension-module"]

lib.rs

use pyo3::prelude::*;

#[pyclass]
struct Test {}

const T: Test = Test {};

#[pymodule]
fn test_pyo3(py: Python<'_>, m: &PyModule) -> PyResult<()> {
    // original approach
    // the trait `pyo3::conversion::ToPyObject` is not implemented for `Test`
    m.add("T", T)?;
    
    // cannot infer type for type parameter `V` declared on the associated function `add`
    m.add("T", T.into_py(py))?;
    
    // the trait `pyo3::conversion::ToPyObject` is not implemented for `std::result::Result<pyo3::instance::Py<Test>, pyo3::err::PyErr>`  
    m.add("T", Py::new(py, T))?;
    Ok(())
}

Thanks a lot!

[birkenfeld]

m.add("T", Py::new(py, T)?)?; works for the immediate issue.

On master, the into_py() variant also works. (It creates a Py<PyAny> though, not a Py<Test>.)

Plain T doesn't work in both versions.