[ruff] Treat f-string interpolation as potential side effect in RUF019

[anishgirianish]

Summary

Fixes #12953

F-string interpolation can call __format__/__str__/__repr__, which may have side effects. RUF019 was applying a safe auto-fix that collapsed two __str__ calls into one, changing behavior.

Added a tri-state SideEffect enum (No/Maybe/Yes) and a side_effect() function that reuses the existing any_over_expr traversal via a new FnMut variant (any_over_expr_mut), following the approach suggested by @ntBre in the other closed pr tagged in the issue

In RUF019, SideEffect::Maybe (non-literal f-string interpolation) now produces an unsafe fix instead of a safe one. Literal interpolations like f"{1}" remain safe.

Test Plan

• Added f-string fixture cases to RUF019.py (non-literal → unsafe, literal → safe, no interpolation → safe).
• cargo nextest run -p ruff_linter
• Ecosystem check (stable + preview)

[astral-sh-bot]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[MichaReiser]

I think this is somewhat pedantic but it's probably fine for RUF019. I left a few smaller nit comments on how we can improve the code.

[anishgirianish]

@MichaReiser Thank you so much for the review! Addressed all feedback, refactored to SideEffect::from_expr with a match, renamed variants to Absent/Possible/Present, added Named as a side effect, and updated the fixture with a class that has a side-effectful str. I really appreciate the suggestions. Ready for re-review whenever you have a moment.

[MichaReiser]

This is great. Thank you. A few more smaller nits.

[anishgirianish]

@MichaReiser thank you so much for the review. Addressed all three feedback, removed Option from from_expr, made both matches exhaustive, added const fn. Ready for another look whenever you get a chance.

thank you