Generated __new__ method does not check args to enforce signature

[matthewlloyd]

Bug Description

Firstly, thank you for creating this amazing crate!

I noticed today that when using #[new] to generate a __new__ method with an empty signature, the generated code doesn't enforce that _args and _kwargs must be empty. As a result, it's possible to call the constructor from Python and provide unused arguments without raising an exception. This could be confusing, for example the developer might expect the kwargs to set field values. I think this is a bug.

Here's part of the code generated by the macros, from cargo expand:

    impl Foo {
        unsafe extern "C" fn __pymethod___new____(
            subtype: *mut _pyo3::ffi::PyTypeObject,
            _args: *mut _pyo3::ffi::PyObject,
            _kwargs: *mut _pyo3::ffi::PyObject,
        ) -> *mut _pyo3::ffi::PyObject {
            use _pyo3::{callback::IntoPyCallbackOutput, pyclass_init::PyObjectInit};
            let gil = _pyo3::GILPool::new();
            let _py = gil.python();
            _pyo3::callback::panic_result_into_callback_output(
                _py,
                ::std::panic::catch_unwind(move || -> _pyo3::PyResult<_> {
                    let result = Foo::new();
                    let initializer: _pyo3::PyClassInitializer<Foo> = result.convert(_py)?;
                    initializer.into_new_object(_py, subtype)
                }),
            )
        }
    }

Steps to Reproduce

#![allow(unused_variables)]
use pyo3::prelude::*;
use pyo3::types::IntoPyDict;

#[pyclass]
struct Foo {}

#[pymethods]
impl Foo {
    #[new]
    pub fn new() -> Self {
        Foo {}
    }
}

pub fn main() -> PyResult<()> {
    pyo3::prepare_freethreaded_python();
    Python::with_gil(|py| {
    	let locals = [("Foo", py.get_type::<Foo>())].into_py_dict(py);
    	py.run("print(Foo()); print(Foo(123)); print(Foo(bar=123))", Some(locals), None)
    })
}

This code prints three objects, without raising any exceptions. The expected behavior would be to print the first and raise an exception on the second (and third of course).

Backtrace

No response

Your operating system and version

WSL2

Your Python version (python --version)

3.11.0

Your Rust version (rustc --version)

rustc 1.60.0-nightly (22e491ac7 2022-01-13)

Your PyO3 version

0.17.3

How did you install python? Did you use a virtualenv?

venv

Additional Info

No response

[birkenfeld]

Indeed, this is due to this check which for empty signatures simply doesn't generate any code, even if it should check for absence of given args.