LSP: Add support for custom file extensions by silverstein · Pull Request #24463 · astral-sh/ruff

silverstein · 2026-04-07T04:56:32Z

Summary

Honor the configured extension mapping when handling formatting requests in ruff server.

This fixes a case where a client sends a formatting request for a document whose file extension is mapped via Ruff configuration, e.g.:

[tool.ruff]
extension = { thing = "markdown" }

[tool.ruff.format]
preview = true

For a file like foo.thing, the CLI already formats correctly as Markdown, but ruff server was still deriving the source type from the raw file path and treating the document like Python during formatting.

Related: astral-sh/ruff-vscode#974

Root cause

The formatting request path was using query.source_type(), which derives the source type from the virtual file path alone.

That bypassed the configured extension mapping already available in settings.formatter.extension.

As a result, a markdown document with a custom mapped extension could be parsed as Python during a formatting request, which leads to warnings like:

Unable to format document: Got unexpected token ` at byte range 9..10

Changes

In full-document formatting, derive the source type from settings.formatter.extension.get_source_type(&file_path).
In range formatting, use the same formatter extension mapping for consistency.
Add a regression test that creates a temporary workspace with extension = { thing = "markdown" }, opens test.thing as a markdown document, and verifies that the formatting request returns edits instead of None.

Test plan

cargo test -p ruff_server custom_extension_mapped_to_markdown
cargo test -p ruff_server
cargo fmt --all --check

AI assistance

I used AI assistance for implementation and drafting, then reviewed the patch manually, reproduced the bug through both the VS Code extension path and a direct LSP request to ruff server, and ran the verification above.

MichaReiser

Thanks for looking into this. I think we need the same handling for linting (and possibly other code paths) too.

MichaReiser · 2026-04-07T16:22:17Z

+            .expect("time went backwards")
+            .as_nanos();
+        std::env::temp_dir().join(format!("ruff-server-{name}-{nanos}"))
+    }


You can add tempdir as a dev-only dependency. It's what we use in all other tests.

silverstein · 2026-04-07T16:45:30Z

Thanks! I broadened the fix so it isn’t formatting-only anymore.

I moved the extension-mapping handling into the shared server request path so it now applies to:

formatting
range formatting
linting
fix-all
code actions
hover

Concretely, DocumentQuery now exposes mapping-aware source-type helpers for lint- and formatter-oriented operations, and the affected request paths use those instead of deriving the type from the raw filename alone.

I also added coverage for the expanded scope:

format_custom_extension_mapped_to_markdown
range_format_custom_extension_mapped_to_markdown_is_unsupported
lint_custom_extension_mapped_to_markdown_emits_no_diagnostics

Reran:

cargo test -p ruff_server
cargo fmt --all --check

The branch is updated now.

silverstein · 2026-04-08T14:46:44Z

Good catch, thank you. I removed the expect-based path instead of trying to justify it.

The updated shape is:

first resolve a mapping-aware source type for the operation (source_type_for_lint / source_type_for_format)
then only construct a Python SourceKind after we’ve already matched on SourceType::Python(...)

That makes the control flow explicit and avoids assuming that SourceKind::from_source_code(...) can never return None or a non-Python kind in the lint/fix path.

I pushed that cleanup in 4e42250cb9 and reran:

cargo test -p ruff_server
cargo fmt --all --check

Still green.

MichaReiser · 2026-04-08T16:25:01Z

Can you confirm that you read our AI policy (https://github.com/astral-sh/.github/blob/main/AI_POLICY.md). I'm asking because I'm getting the impression that I'm chatting with an agent

silverstein · 2026-04-08T17:11:25Z

I hadn't, sorry about that Micha! I did have codex help me clean up my reply so it was clearer. I'm a real person, so happy to clean anything up if you need.

astral-sh-bot · 2026-04-09T08:18:00Z

`ruff-ecosystem` results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Co-authored-by: Micha Reiser <micha@reiser.io>

@charliermarsh

##### [\`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

MichaReiser reviewed Apr 7, 2026

View reviewed changes

MichaReiser added the server Related to the LSP server label Apr 7, 2026

MichaReiser reviewed Apr 8, 2026

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Comment thread crates/ruff_server/src/session/index.rs Outdated

Comment thread crates/ruff_server/src/fix.rs

Comment thread crates/ruff_server/src/server/api/requests/format_range.rs Outdated

silverstein added 5 commits April 8, 2026 11:46

fix(server): honor extension mapping for formatting

cb082dd

test(server): cover range formatting for mapped markdown

a6d8571

fix(server): honor extension mapping across request paths

b2d4e4d

refactor(server): avoid source kind assumptions

1993c8a

test(server): rewrite extension mapping coverage as e2e

1414235

silverstein force-pushed the codex/format-custom-extension-mapping branch from 4e42250 to 1414235 Compare April 8, 2026 18:56

MichaReiser changed the title ~~fix(server): honor extension mapping for formatting~~ LSP: Add support for custom extensions Apr 9, 2026

Cleanup

701b4ad

MichaReiser force-pushed the codex/format-custom-extension-mapping branch from 5e70205 to 701b4ad Compare April 9, 2026 08:29

MichaReiser enabled auto-merge (squash) April 9, 2026 08:30

MichaReiser changed the title ~~LSP: Add support for custom extensions~~ LSP: Add support for custom file extensions Apr 9, 2026

MichaReiser merged commit d8bc700 into astral-sh:main Apr 9, 2026
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BrewTestBot mentioned this pull request Apr 9, 2026

ruff 0.15.10 Homebrew/homebrew-core#276824

Merged

ibraheemdev pushed a commit that referenced this pull request Apr 15, 2026

LSP: Add support for custom extensions (#24463)

575b4ce

Co-authored-by: Micha Reiser <micha@reiser.io>

This was referenced Apr 27, 2026

Always include panic payload in panic diagnostic message #24873

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silverstein commented Apr 7, 2026

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silverstein commented Apr 7, 2026 •

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