Improve diagnostic for invalid function passed to from_py_with

[davidhewitt]

Just looking at #4748, and the compile error on invalid from_py_with function.

I wonder if we can somehow wrap the function up in a trait and then use diagnostic::on_unimplemented to give a more user-friendly error message for that case? 🤔

[mejrs]

The macro currently emits something like

        let ret = function(::pyo3::impl_::extract_argument::from_py_with(
            unsafe {
                ::pyo3::impl_::extract_argument::unwrap_required_argument(output[0usize].as_deref())
            },
            "bytes",
            from_py_with_0 as fn(_) -> _,
        )?);

If you use a type annotation rather than an as cast, like

        let from_py_with_0: fn(_) -> _ = from_py_with_0;
        let ret = function(::pyo3::impl_::extract_argument::from_py_with(
            unsafe {
                ::pyo3::impl_::extract_argument::unwrap_required_argument(output[0usize].as_deref())
            },
            "bytes",
            from_py_with_0,
        )?);

then you get an error that at least says

    = note: expected fn pointer `fn(&pyo3::Bound<'_, PyAny>) -> Result<_, PyErr>`
               found fn pointer `fn(&pyo3::Bound<'_, PyBytes>) -> Vec<u8>`

I wonder if we can somehow wrap the function up in a trait and then use diagnostic::on_unimplemented

I'm not a fan of this approach, it seems like a better fix for this sort of thing is to make changes in the compiler itself to make this kind of error message better.

[davidhewitt]

👍 true, given it's a diagnostic it would be good enough to get an improvement into latest stable.