[ruff] Detect mutable defaults in dataclass field calls (RUF008)

[gopoto]

Summary

Extend RUF008 to catch mutable default values supplied via dataclasses.field and attrs.field calls. Previously, RUF008 only flagged direct assignments like mutable_default: list[int] = [], and ignored cases such as attrs.field(default=[]) within attrs-decorated classes. The rule now inspects calls to field helpers and emits a diagnostic if the default keyword is a mutable literal.

Test fixtures for both dataclasses and attrs classes were updated to include explicit field(default=[]) cases, the attrs fixture now imports attrs so that these calls are resolvable, and snapshots were updated accordingly.

Test Plan

Built the workspace (cargo check) and ran Ruff against the updated fixtures:

cargo run -p ruff -- check crates/ruff_linter/resources/test/fixtures/ruff/RUF008_attrs.py --select RUF008 --preview --no-cache

reports both direct mutable defaults and attrs.field(default=[]) defaults. A minimal reproduction of the original issue:

import attrs
@attrs.define
class B:
    mutable_default: list[int] = []
    mutable_default2: list[int] = attrs.field(default=[])

now yields two RUF008 diagnostics as expected.

Running the hooks (cargo fmt, cargo clippy, etc.) and pre-commit checks pass after formatting, and updated snapshot tests reflect the new diagnostics.

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This PR was generated by an AI system in collaboration with maintainers: @carljm, @ntBre

Fixes #16495

[charliermarsh]

I think this needs to be gated behind preview.

[gopoto]

Good call – I've wrapped the new field(default=[]) check behind checker.settings.preview.is_enabled() and added preview snapshots/tests.

[gopoto]

Added preview gating to the new field(default=[]) detection in RUF008 so the expanded behavior only triggers under --preview. Baseline RUF008 snapshots were reverted to the pre-change expectation with updated line numbers, and new preview__RUF008 snapshots were introduced to cover the field default cases under preview. The test harness was updated to exercise the rule in preview mode.