Magic methods in #[pymethods] don't check they have the correct number of arguments

[davidhewitt]

From #1964:

By the way, I discovered that it is possible to omit arguments to the methods, e.g. to define a __richcmp__ without arguments. It seems to be handled all right, just not passing the slot arguments to Rust, but I wonder if that's intentional.

Yeah I kinda knew about this and was thinking it was a minor bug. It probably should be checked and a nice error message emitted. Users missing arguments off is probably an incorrect implementation imo so we should at least force them to be explicit about unused arguments.

The code responsible is:

pyo3/pyo3-macros-backend/src/pymethod.rs

Line 955 in 0f92f28

fn extract_proto_arguments(

method_args are the arguments the user wrote for the method; at the moment the code just iterates it without checking its length. Slightly complicated by the Python arguments but not too bad.

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Anyone who's interested in helping out with this, it should be a reasonably self-contained piece.