Plugin hook for preprocessing annotations

[jtrakk]

I want to add some custom annotations on my variables. For example,

def idiv(left: int, right: int | right != 0) -> int:
    return left // right

Mypy accepts only PEP 484 type hints, so I'd like to provide a plugin to pre-process this annotation and return a PEP 484-compatible hint before Mypy checks it. Could Mypy provide a preprocessing hook for such transformations?

[ilevkivskyi]

This is a reasonable request, but probably not very high priority taking into account how many issues we have now.

The implementation will require adding a new plugin hook somewhere in mypy/fastparse.py.