[ruff] add fix safety section (RUF017)

[VascoSch92]

The PR add the fix safety section for rule RUF017 (#15584 )

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[dylwil3]

Thanks for this!

Looking at the implementation for this rule, I notice that it is only emitted when the user explicitly provides the start argument for sum as an empty list (either list() or []). The expression sum(lists,[]) would raise a type error if the iterable lists had any non-list items, and the user has no control over the implementation of __add__ or __iadd__ for the primitive list.

However, there is a small difference in implementation between __add__ and __iadd__ for list: the former requires the right summand to be a list, whereas the latter allows for any iterable.

So, unless I'm misunderstanding, I think the only thing the fix could do that would change behavior is turn code that used to raise an exception into code that no longer raises an exception. Namely:

import functools
import operator

ranges = [range(2), range(3)]

# raises a TypeError
sum(ranges, []) 

# evaluates to [0, 1, 0, 1, 2]
functools.reduce(operator.iadd, ranges, [])

So maybe we can reword the fix safety section in light of this?

[VascoSch92]

Hey @dylwil3
thanks for the explanation. It is really subtile the case here, and pretty interesting.

Perhaps something on this direction?

The fix is always marked as unsafe because `sum` uses the `__add__` magic method while
`operator.iadd` uses the `__iadd__` magic method, and these behaves different on lists, i.e., 
the former requires the right summand to be a list, whereas the latter allows for any iterable. 
Therefore, the fix could inadvertently cause code that previously raised an error to silently 
succeed with different results. Moreover, the fix could remove comments from the original code.

Another question: what is the politic with deleted comments? Should this be mentioned in the fix safety section? I think yes but not sure.

[dylwil3]

Thank you! Your wording was good - I added it in with some minor corrections.