Summary

This PR adds type inference for basic for loop variables to red-knot.

Inferring the type of a loop var involves either the new-style iteration protocol (which invokes __iter__ to get an iterator, and then __next__ on that iterator), or the old-style iteration protocol (which passes incrementally larger ints to a __getitem__ method). If __iter__ is defined on the class, __iter__ always takes precedence, even if __iter__ is e.g. set to None: this is a technique that can be used to make classes that define __getitem__ not-iterable.

Whichever protocol is used to make a class's instances iterable, the dunder methods are always looked up on the class of an instance rather than the instance itself. I.e., the interpreter won't consider foo in this snippet to be iterable, because __iter__ only exists on the instance, not the class:

>>> class Foo: ...
... 
>>> foo = Foo()
>>> foo.__iter__ = lambda: iter(range(42))
>>> list(foo.__iter__())
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41]
>>> for x in foo:
...     pass
... 
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'Foo' object is not iterable

To emulate this runtime behaviour, I added a Type::to_type_of_class method that allows us to go from a type representing <instance of int> to a type representing <the int class itself>.

Test Plan

cargo test