Ensure all APIs have safety notes #![deny(clippy::missing_safety_doc)]

[kngwyu]

We have many unsafe APIs but they have poorly documented with their safe precondition, which makes clippy unhappy.

I don't think we need to address all those warnings, but maybe it is variable to add #Safety section to common unsafe APIs!

[davidhewitt]

It would be really cool to try and achieve this in 0.13.

[mejrs]

afaik, there are just two of these left:

• https://github.com/PyO3/pyo3/blob/main/src/pyclass_slots.rs#L17 I'm guessing the safety doc for that is just "pointer must point to a valid PyObject?
• https://github.com/PyO3/pyo3/blob/main/src/type_object.rs#L21 it looks like it's unsafe to implement (which it already is, it's in an unsafe trait), not unsafe to call - this probably doesn't need to be an unsafe function?

[davidhewitt]

Oh nice, I hadn't noticed we were so close!

For 1. I was thinking of tidying up the pyclass_slots stuff so maybe I can sort that at the same time. It has to be a valid python object of the right type.

2. is removed in pyclass: refactor tp_new / tp_dealloc / remove PyCellInner #1657