#24426)

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## 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)

##### [\`v0.15.10\`](https://github.com/astral-sh/ruff/blob/HEAD/CHANGELOG.md#01510)

Released on 2026-04-09.

##### Preview features

- \[`flake8-logging`] Allow closures in except handlers (`LOG004`) ([#24464](astral-sh/ruff#24464))
- \[`flake8-self`] Make `SLF` diagnostics robust to non-self-named variables ([#24281](astral-sh/ruff#24281))
- \[`flake8-simplify`] Make the fix for `collapsible-if` safe in `preview` (`SIM102`) ([#24371](astral-sh/ruff#24371))

##### Bug fixes

- Avoid emitting multi-line f-string elements before Python 3.12 ([#24377](astral-sh/ruff#24377))
- Avoid syntax error from `E502` fixes in f-strings and t-strings ([#24410](astral-sh/ruff#24410))
- Strip form feeds from indent passed to `dedent_to` ([#24381](astral-sh/ruff#24381))
- \[`pyupgrade`] Fix panic caused by handling of octals (`UP012`) ([#24390](astral-sh/ruff#24390))
- Reject multi-line f-string elements before Python 3.12 ([#24355](astral-sh/ruff#24355))

##### Rule changes

- \[`ruff`] Treat f-string interpolation as potential side effect (`RUF019`) ([#24426](astral-sh/ruff#24426))

##### Server

- Add support for custom file extensions ([#24463](astral-sh/ruff#24463))

##### Documentation

- Document adding fixes in CONTRIBUTING.md ([#24393](astral-sh/ruff#24393))
- Fix JSON typo in settings example ([#24517](astral-sh/ruff#24517))

##### Contributors

- [@charliermarsh](https://github.com/charliermarsh)
- [@dylwil3](https://github.com/dylwil3)
- [@silverstein](https://github.com/silverstein)
- [@anishgirianish](https://github.com/anishgirianish)
- [@shizukushq](https://github.com/shizukushq)
- [@zanieb](https://github.com/zanieb)
- [@AlexWaygood](https://github.com/AlexWaygood)
##### [\`v0.15.9\`](https://github.com/astral-sh/ruff/blob/HEAD/CHANGELOG.md#0159)

Released on 2026-04-02.

##### Preview features

- \[`pyflakes`] Flag annotated variable redeclarations as `F811` in preview mode ([#24244](astral-sh/ruff#24244))
- \[`ruff`] Allow dunder-named assignments in non-strict mode for `RUF067` ([#24089](astral-sh/ruff#24089))

##### Bug fixes

- \[`flake8-errmsg`] Avoid shadowing existing `msg` in fix for `EM101` ([#24363](astral-sh/ruff#24363))
- \[`flake8-simplify`] Ignore pre-initialization references in `SIM113` ([#24235](astral-sh/ruff#24235))
- \[`pycodestyle`] Fix `W391` fixes for consecutive empty notebook cells ([#24236](astral-sh/ruff#24236))
- \[`pyupgrade`] Fix `UP008` nested class matching ([#24273](astral-sh/ruff#24273))
- \[`pyupgrade`] Ignore strings with string-only escapes (`UP012`) ([#16058](astral-sh/ruff#16058))
- \[`ruff`] `RUF072`: skip formfeeds on dedent ([#24308](astral-sh/ruff#24308))
- \[`ruff`] Avoid re-using symbol in `RUF024` fix ([#24316](astral-sh/ruff#24316))
- \[`ruff`] Parenthesize expression in `RUF050` fix ([#24234](astral-sh/ruff#24234))
- Disallow starred expressions as values of starred expressions ([#24280](astral-sh/ruff#24280))

##### Rule changes

- \[`flake8-simplify`] Suppress `SIM105` for `except*` before Python 3.12 ([#23869](astral-sh/ruff#23869))
- \[`pyflakes`] Extend `F507` to flag `%`-format strings with zero placeholders ([#24215](astral-sh/ruff#24215))
- \[`pyupgrade`] `UP018` should detect more unnecessarily wrapped literals (UP018) ([#24093](astral-sh/ruff#24093))
- \[`pyupgrade`] Fix `UP008` callable scope handling to support lambdas ([#24274](astral-sh/ruff#24274))
- \[`ruff`] `RUF010`: Mark fix as unsafe when it deletes a comment ([#24270](astral-sh/ruff#24270))

##### Formatter

- Add `nested-string-quote-style` formatting option ([#24312](astral-sh/ruff#24312))

##### Documentation

- \[`flake8-bugbear`] Clarify RUF071 fix safety for non-path string comparisons ([#24149](astral-sh/ruff#24149))
- \[`flake8-type-checking`] Clarify import cycle wording for `TC001`/`TC002`/`TC003` ([#24322](astral-sh/ruff#24322))

##### Other changes

- Avoid rendering fix lines with trailing whitespace after `|` ([#24343](astral-sh/ruff#24343))

##### Contributors

- [@charliermarsh](https://github.com/charliermarsh)
- [@MichaReiser](https://github.com/MichaReiser)
- [@tranhoangtu-it](https://github.com/tranhoangtu-it)
- [@dylwil3](https://github.com/dylwil3)
- [@zsol](https://github.com/zsol)
- [@renovate](https://github.com/renovate)
- [@bitloi](https://github.com/bitloi)
- [@danparizher](https://github.com/danparizher)
- [@chinar-amrutkar](https://github.com/chinar-amrutkar)
- [@second-ed](https://github.com/second-ed)
- [@getehen](https://github.com/getehen)
- [@Redovo1](https://github.com/Redovo1)
- [@matthewlloyd](https://github.com/matthewlloyd)
- [@zanieb](https://github.com/zanieb)
- [@InSyncWithFoo](https://github.com/InSyncWithFoo)
- [@RenzoMXD](https://github.com/RenzoMXD)

Renovate-Branch: renovate/2024.6-ruff-0.15.x
Change-Id: Id4bd542d4f128b509284d9dcda312f2b39c29964
Priv-Id: 28ebcacdeffa50cec7a52eee59091a75ca5e9539