Summary

Implement mutable-contextvar-default (B039) which was added to flake8-bugbear in PyCQA/flake8-bugbear#476.

This rule is similar to mutable-argument-default (B006) and function-call-in-default-argument (B008), except that it checks the default keyword argument to contextvars.ContextVar.

B039.py:19:26: B039 Do not use mutable data structures for ContextVar defaults
   |
18 | # Bad
19 | ContextVar("cv", default=[])
   |                          ^^ B039
20 | ContextVar("cv", default={})
21 | ContextVar("cv", default=list())
   |
   = help: Replace with `None`; initialize with `.set()` after checking for `None`

In the upstream flake8-plugin, this rule is written expressly as a corollary to B008 and shares much of its logic. Likewise, this implementation reuses the logic of the Ruff implementation of B008, namely

and

Thus, this rule deliberately replicates B006's and B008's heuristics. For example, this rule assumes that all functions are mutable unless otherwise qualified. If improvements are to be made to B039 heuristics, they should probably be made to B006 and B008 as well (whilst trying to match the upstream implementation).

This rule does not have an autofix as it is unknown where the ContextVar next used (and it might not be within the same file).

Closes #12054

Test Plan

cargo nextest run