new diagnostic rule - error in fstring if value does not define a `__str__`

[DetachHead]

usually this results in a useless string representation of an object being displayed to the user (eg. <Foo object at 0xasdfasdf>)

[KotlinIsland]

Must apply to print as well, I suggest we action HelpfulString and UnhelpfulString in basedtyping.

You must also consider __repr__ and None

[beauxq]

To clarify:

if value does not define a __str__

Does this mean if both the __str__ and __repr__ we would get is the same as object.__str__ and object.__repr__?

It wouldn't be nice to have to define this in every subclass of a class where we've defined __str__

    def __str__(self) -> str:
        return super().__str__()

[DetachHead]

the motivation is to prevent non-user friendly strings from being presented to the user. i got the idea from this eslint rule.

users who don't like the rule will be able to disable it if they want

[KotlinIsland]

It wouldn't be nice to have to define this in every subclass of a class where we've defined __str__

it would work like this

class A:
    pass
class B(A):
    def __str__(self):
        return "something"
class C(B):
    pass

f"{object()}"  # warning: reportUnhelpfulString
f"{A()}"  # warning: reportUnhelpfulString
f"{B()}"  # no report
f"{C()}"  # no report

If you inherit from a helpful string, then that counts as a helpful string