implicit conversions

Thanks for the great work you do on PyO3.

Pybind11 has a feature called 'implicit conversions'
https://pybind11.readthedocs.io/en/stable/advanced/classes.html#implicit-conversions

This can be quite handy, let me give an example:

#[pyclass]
#[derive(Clone, Copy)]
pub struct Vector3D {
    pub x: f64
    pub y: f64
    pub z: f64
}

#[pymethods]
impl Vector3D {
    #new
    pub fn __new__(v: [f64, 3]) -> Self {
        Self {x: v[0], y: v[1], z: v[2]}
    }
}


#[pyfunction]
fn do_something(vec: Vector3D) -> f64 {
    vec.x * 2
}

>>> from py_rust_module import *

>>> do_something([1,2,3])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: argument 'vec': 'list' object cannot be converted to 'Vector3D'

>>> do_something(Vector3D([1,2,3]))
2

In Pybind11 Vector3D could be declared as implicitly convertible from list and do_something([1,2,3]) would be valid and implicitly casting the list to a Vector3D object.

Is this something that could be done in pyo3? It is quite handy and can save a lot of verbosity...

Start by reviewing PyO3's existing argument-conversion behavior and the linked Pybind11 implicit-conversions documentation. The issue's target behavior is that a Python list can be passed directly to a function expecting a Vector3D; completion should include an agreed design and coverage for that conversion.