new rule - enforce that loop variables are not used outside the loop

[DetachHead]

the fact that loop variables are still in scope outside of the loop is a very common source of bugs:

for val in values:
    print(val)

... # several lines later

value = 1
print(val) # it's very likely the user intended to use value instead of val

i know there are cases where this is intentional, for example:

for value in values:
    ...
print(f"the last value was {value}")

but imo it's a bad idea to rely on this functionality. what if values was empty? value will be undefined which will cause a runtime error.

in most other languages, loop variables are only in scope within the loop, which is much less error-prone. it would be nice if there was a rule that enforced this:

for val in values:
    ...
print(val) # error: loop variable used outside of loop

[KotlinIsland]

what if values was empty?

you would get an error in bpr

[KotlinIsland]

could this also apply to if statements with walrus:

if match := re.search(...):
  print(match)
print(match)  # error, don't do that

[diceroll123]

This would be Pylint's W0631, which at this time is marked as not implemented in Ruff, in #970

https://pylint.readthedocs.io/en/stable/user_guide/messages/warning/undefined-loop-variable.html

[DetachHead]

oh right, closing in favor of #970