Add --extension support to the formatter (#9483)

## Summary

We added `--extension` to `ruff check`, but it's equally applicable to
`ruff format`.

Closes https://github.com/astral-sh/ruff/issues/9482.

Resolves https://github.com/astral-sh/ruff/discussions/9481.

## Test Plan

`cargo test`

crates/ruff_cli/src/args.rs

@@ -290,6 +290,10 @@ pub struct CheckCommand {
     /// The name of the file when passing it through stdin.
     #[arg(long, help_heading = "Miscellaneous")]
     pub stdin_filename: Option<PathBuf>,
+    /// List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]). For
+    /// example, to treat `.ipy` files as IPython notebooks, use `--extension ipy:ipynb`.
+    #[arg(long, value_delimiter = ',')]
+    pub extension: Option<Vec<ExtensionPair>>,
     /// Exit with status code "0", even upon detecting lint violations.
     #[arg(
         short,
@@ -352,9 +356,6 @@ pub struct CheckCommand {
         conflicts_with = "watch",
     )]
     pub show_settings: bool,
-    /// List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]).
-    #[arg(long, value_delimiter = ',', hide = true)]
-    pub extension: Option<Vec<ExtensionPair>>,
     /// Dev-only argument to show fixes
     #[arg(long, hide = true)]
     pub ecosystem_ci: bool,
@@ -423,6 +424,10 @@ pub struct FormatCommand {
     /// The name of the file when passing it through stdin.
     #[arg(long, help_heading = "Miscellaneous")]
     pub stdin_filename: Option<PathBuf>,
+    /// List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]). For
+    /// example, to treat `.ipy` files as IPython notebooks, use `--extension ipy:ipynb`.
+    #[arg(long, value_delimiter = ',')]
+    pub extension: Option<Vec<ExtensionPair>>,
     /// The minimum Python version that should be supported.
     #[arg(long, value_enum)]
     pub target_version: Option<PythonVersion>,
@@ -571,6 +576,7 @@ impl FormatCommand {
                 force_exclude: resolve_bool_arg(self.force_exclude, self.no_force_exclude),
                 target_version: self.target_version,
                 cache_dir: self.cache_dir,
+                extension: self.extension,
 
                 // Unsupported on the formatter CLI, but required on `Overrides`.
                 ..CliOverrides::default()
@@ -739,7 +745,7 @@ impl ConfigurationTransformer for CliOverrides {
             config.target_version = Some(*target_version);
         }
         if let Some(extension) = &self.extension {
-            config.lint.extension = Some(extension.clone().into_iter().collect());
+            config.extension = Some(extension.iter().cloned().collect());
         }
 
         config

crates/ruff_cli/src/commands/format.rs

@@ -106,13 +106,19 @@ pub(crate) fn format(
             match entry {
                 Ok(resolved_file) => {
                     let path = resolved_file.path();
-                    let SourceType::Python(source_type) = SourceType::from(&path) else {
+                    let settings = resolver.resolve(path);
+
+                    let source_type = match settings.formatter.extension.get(path) {
+                        None => match SourceType::from(path) {
+                            SourceType::Python(source_type) => source_type,
+                            SourceType::Toml(_) => {
                                 // Ignore any non-Python files.
                                 return None;
+                            }
+                        },
+                        Some(language) => PySourceType::from(language),
                     };
 
-                    let settings = resolver.resolve(path);
-
                     // Ignore files that are excluded from formatting
                     if (settings.file_resolver.force_exclude || !resolved_file.is_root())
                         && match_exclusion(

crates/ruff_cli/src/commands/format_stdin.rs

@@ -53,16 +53,23 @@ pub(crate) fn format_stdin(cli: &FormatArguments, overrides: &CliOverrides) -> R
     }
 
     let path = cli.stdin_filename.as_deref();
+    let settings = &resolver.base_settings().formatter;
 
-    let SourceType::Python(source_type) = path.map(SourceType::from).unwrap_or_default() else {
+    let source_type = match path.and_then(|path| settings.extension.get(path)) {
+        None => match path.map(SourceType::from).unwrap_or_default() {
+            SourceType::Python(source_type) => source_type,
+            SourceType::Toml(_) => {
                 if mode.is_write() {
                     parrot_stdin()?;
                 }
                 return Ok(ExitStatus::Success);
+            }
+        },
+        Some(language) => PySourceType::from(language),
     };
 
     // Format the file.
-    match format_source_code(path, &resolver.base_settings().formatter, source_type, mode) {
+    match format_source_code(path, settings, source_type, mode) {
         Ok(result) => match mode {
             FormatMode::Write => Ok(ExitStatus::Success),
             FormatMode::Check | FormatMode::Diff => {

crates/ruff_cli/src/diagnostics.rs

@@ -17,7 +17,7 @@ use ruff_linter::logging::DisplayParseError;
 use ruff_linter::message::Message;
 use ruff_linter::pyproject_toml::lint_pyproject_toml;
 use ruff_linter::registry::AsRule;
-use ruff_linter::settings::types::{ExtensionMapping, UnsafeFixes};
+use ruff_linter::settings::types::UnsafeFixes;
 use ruff_linter::settings::{flags, LinterSettings};
 use ruff_linter::source_kind::{SourceError, SourceKind};
 use ruff_linter::{fs, IOError, SyntaxError};
@@ -179,11 +179,6 @@ impl AddAssign for FixMap {
     }
 }
 
-fn override_source_type(path: Option<&Path>, extension: &ExtensionMapping) -> Option<PySourceType> {
-    let ext = path?.extension()?.to_str()?;
-    extension.get(ext).map(PySourceType::from)
-}
-
 /// Lint the source code at the given `Path`.
 pub(crate) fn lint_path(
     path: &Path,
@@ -228,7 +223,7 @@ pub(crate) fn lint_path(
 
     debug!("Checking: {}", path.display());
 
-    let source_type = match override_source_type(Some(path), &settings.extension) {
+    let source_type = match settings.extension.get(path).map(PySourceType::from) {
         Some(source_type) => source_type,
         None => match SourceType::from(path) {
             SourceType::Toml(TomlSourceType::Pyproject) => {
@@ -398,15 +393,14 @@ pub(crate) fn lint_stdin(
     fix_mode: flags::FixMode,
 ) -> Result<Diagnostics> {
     // TODO(charlie): Support `pyproject.toml`.
-    let source_type = if let Some(source_type) =
+    let source_type = match path.and_then(|path| settings.linter.extension.get(path)) {
-        override_source_type(path, &settings.linter.extension)
+        None => match path.map(SourceType::from).unwrap_or_default() {
-    {
+            SourceType::Python(source_type) => source_type,
-        source_type
+            SourceType::Toml(_) => {
-    } else {
-        let SourceType::Python(source_type) = path.map(SourceType::from).unwrap_or_default() else {
                 return Ok(Diagnostics::default());
-        };
+            }
-        source_type
+        },
+        Some(language) => PySourceType::from(language),
     };
 
     // Extract the sources from the file.

crates/ruff_cli/tests/format.rs

@@ -1468,3 +1468,73 @@ fn test_notebook_trailing_semicolon() {
     ----- stderr -----
     "###);
 }
+
+#[test]
+fn extension() -> Result<()> {
+    let tempdir = TempDir::new()?;
+
+    let ruff_toml = tempdir.path().join("ruff.toml");
+    fs::write(
+        &ruff_toml,
+        r#"
+include = ["*.ipy"]
+"#,
+    )?;
+
+    fs::write(
+        tempdir.path().join("main.ipy"),
+        r#"
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "ad6f36d9-4b7d-4562-8d00-f15a0f1fbb6d",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "x=1"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.12.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}
+"#,
+    )?;
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .current_dir(tempdir.path())
+        .arg("format")
+        .arg("--no-cache")
+        .args(["--config", &ruff_toml.file_name().unwrap().to_string_lossy()])
+        .args(["--extension", "ipy:ipynb"])
+        .arg("."), @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    1 file reformatted
+
+    ----- stderr -----
+    "###);
+    Ok(())
+}

crates/ruff_cli/tests/lint.rs

@@ -436,3 +436,75 @@ ignore = ["D203", "D212"]
     "###);
     Ok(())
 }
+
+#[test]
+fn extension() -> Result<()> {
+    let tempdir = TempDir::new()?;
+
+    let ruff_toml = tempdir.path().join("ruff.toml");
+    fs::write(
+        &ruff_toml,
+        r#"
+include = ["*.ipy"]
+"#,
+    )?;
+
+    fs::write(
+        tempdir.path().join("main.ipy"),
+        r#"
+{
+ "cells": [
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "ad6f36d9-4b7d-4562-8d00-f15a0f1fbb6d",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "import os"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "Python 3 (ipykernel)",
+   "language": "python",
+   "name": "python3"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.12.0"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}
+"#,
+    )?;
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .current_dir(tempdir.path())
+        .arg("check")
+        .args(STDIN_BASE_OPTIONS)
+        .args(["--config", &ruff_toml.file_name().unwrap().to_string_lossy()])
+        .args(["--extension", "ipy:ipynb"])
+        .arg("."), @r###"
+    success: false
+    exit_code: 1
+    ----- stdout -----
+    main.ipy:cell 1:1:8: F401 [*] `os` imported but unused
+    Found 1 error.
+    [*] 1 fixable with the `--fix` option.
+
+    ----- stderr -----
+    "###);
+    Ok(())
+}

crates/ruff_linter/src/settings/mod.rs

@@ -41,6 +41,7 @@ pub mod types;
 #[derive(Debug, CacheKey)]
 pub struct LinterSettings {
     pub exclude: FilePatternSet,
+    pub extension: ExtensionMapping,
     pub project_root: PathBuf,
 
     pub rules: RuleTable,
@@ -50,7 +51,6 @@ pub struct LinterSettings {
     pub target_version: PythonVersion,
     pub preview: PreviewMode,
     pub explicit_preview_rules: bool,
-    pub extension: ExtensionMapping,
 
     // Rule-specific settings
     pub allowed_confusables: FxHashSet<char>,

crates/ruff_linter/src/settings/types.rs

@@ -388,9 +388,10 @@ pub struct ExtensionMapping {
 }
 
 impl ExtensionMapping {
-    /// Return the [`Language`] for the given extension.
+    /// Return the [`Language`] for the given file.
-    pub fn get(&self, extension: &str) -> Option<Language> {
+    pub fn get(&self, path: &Path) -> Option<Language> {
-        self.mapping.get(extension).copied()
+        let ext = path.extension()?.to_str()?;
+        self.mapping.get(ext).copied()
     }
 }
 

crates/ruff_workspace/src/configuration.rs

@@ -117,6 +117,7 @@ pub struct Configuration {
     pub output_format: Option<SerializationFormat>,
     pub preview: Option<PreviewMode>,
     pub required_version: Option<Version>,
+    pub extension: Option<ExtensionMapping>,
     pub show_fixes: Option<bool>,
     pub show_source: Option<bool>,
 
@@ -174,6 +175,7 @@ impl Configuration {
 
         let formatter = FormatterSettings {
             exclude: FilePatternSet::try_from_iter(format.exclude.unwrap_or_default())?,
+            extension: self.extension.clone().unwrap_or_default(),
             preview: format_preview,
             target_version: match target_version {
                 PythonVersion::Py37 => ruff_python_formatter::PythonVersion::Py37,
@@ -241,7 +243,7 @@ impl Configuration {
             linter: LinterSettings {
                 rules: lint.as_rule_table(lint_preview),
                 exclude: FilePatternSet::try_from_iter(lint.exclude.unwrap_or_default())?,
-                extension: lint.extension.unwrap_or_default(),
+                extension: self.extension.unwrap_or_default(),
                 preview: lint_preview,
                 target_version,
                 project_root: project_root.to_path_buf(),
@@ -496,6 +498,9 @@ impl Configuration {
                 .map(|src| resolve_src(&src, project_root))
                 .transpose()?,
             target_version: options.target_version,
+            // `--extension` is a hidden command-line argument that isn't supported in configuration
+            // files at present.
+            extension: None,
 
             lint: LintConfiguration::from_options(lint, project_root)?,
             format: FormatConfiguration::from_options(
@@ -538,6 +543,7 @@ impl Configuration {
             src: self.src.or(config.src),
             target_version: self.target_version.or(config.target_version),
             preview: self.preview.or(config.preview),
+            extension: self.extension.or(config.extension),
 
             lint: self.lint.combine(config.lint),
             format: self.format.combine(config.format),
@@ -549,7 +555,6 @@ impl Configuration {
 pub struct LintConfiguration {
     pub exclude: Option<Vec<FilePattern>>,
     pub preview: Option<PreviewMode>,
-    pub extension: Option<ExtensionMapping>,
 
     // Rule selection
     pub extend_per_file_ignores: Vec<PerFileIgnore>,
@@ -616,9 +621,6 @@ impl LintConfiguration {
             .chain(options.common.extend_unfixable.into_iter().flatten())
             .collect();
         Ok(LintConfiguration {
-            // `--extension` is a hidden command-line argument that isn't supported in configuration
-            // files at present.
-            extension: None,
             exclude: options.exclude.map(|paths| {
                 paths
                     .into_iter()
@@ -954,7 +956,6 @@ impl LintConfiguration {
         Self {
             exclude: self.exclude.or(config.exclude),
             preview: self.preview.or(config.preview),
-            extension: self.extension.or(config.extension),
             rule_selections: config
                 .rule_selections
                 .into_iter()
@@ -1031,6 +1032,7 @@ impl LintConfiguration {
 pub struct FormatConfiguration {
     pub exclude: Option<Vec<FilePattern>>,
     pub preview: Option<PreviewMode>,
+    pub extension: Option<ExtensionMapping>,
 
     pub indent_style: Option<IndentStyle>,
     pub quote_style: Option<QuoteStyle>,
@@ -1044,6 +1046,9 @@ impl FormatConfiguration {
     #[allow(clippy::needless_pass_by_value)]
     pub fn from_options(options: FormatOptions, project_root: &Path) -> Result<Self> {
         Ok(Self {
+            // `--extension` is a hidden command-line argument that isn't supported in configuration
+            // files at present.
+            extension: None,
             exclude: options.exclude.map(|paths| {
                 paths
                     .into_iter()
@@ -1077,18 +1082,19 @@ impl FormatConfiguration {
 
     #[must_use]
     #[allow(clippy::needless_pass_by_value)]
-    pub fn combine(self, other: Self) -> Self {
+    pub fn combine(self, config: Self) -> Self {
         Self {
-            exclude: self.exclude.or(other.exclude),
+            exclude: self.exclude.or(config.exclude),
-            preview: self.preview.or(other.preview),
+            preview: self.preview.or(config.preview),
-            indent_style: self.indent_style.or(other.indent_style),
+            extension: self.extension.or(config.extension),
-            quote_style: self.quote_style.or(other.quote_style),
+            indent_style: self.indent_style.or(config.indent_style),
-            magic_trailing_comma: self.magic_trailing_comma.or(other.magic_trailing_comma),
+            quote_style: self.quote_style.or(config.quote_style),
-            line_ending: self.line_ending.or(other.line_ending),
+            magic_trailing_comma: self.magic_trailing_comma.or(config.magic_trailing_comma),
-            docstring_code_format: self.docstring_code_format.or(other.docstring_code_format),
+            line_ending: self.line_ending.or(config.line_ending),
+            docstring_code_format: self.docstring_code_format.or(config.docstring_code_format),
             docstring_code_line_width: self
                 .docstring_code_line_width
-                .or(other.docstring_code_line_width),
+                .or(config.docstring_code_line_width),
         }
     }
 }

crates/ruff_workspace/src/settings.rs

@@ -1,7 +1,9 @@
 use path_absolutize::path_dedot;
 use ruff_cache::cache_dir;
 use ruff_formatter::{FormatOptions, IndentStyle, IndentWidth, LineWidth};
-use ruff_linter::settings::types::{FilePattern, FilePatternSet, SerializationFormat, UnsafeFixes};
+use ruff_linter::settings::types::{
+    ExtensionMapping, FilePattern, FilePatternSet, SerializationFormat, UnsafeFixes,
+};
 use ruff_linter::settings::LinterSettings;
 use ruff_macros::CacheKey;
 use ruff_python_ast::PySourceType;
@@ -116,6 +118,7 @@ impl FileResolverSettings {
 #[derive(CacheKey, Clone, Debug)]
 pub struct FormatterSettings {
     pub exclude: FilePatternSet,
+    pub extension: ExtensionMapping,
     pub preview: PreviewMode,
     pub target_version: ruff_python_formatter::PythonVersion,
 
@@ -177,6 +180,7 @@ impl Default for FormatterSettings {
 
         Self {
             exclude: FilePatternSet::default(),
+            extension: ExtensionMapping::default(),
             target_version: default_options.target_version(),
             preview: PreviewMode::Disabled,
             line_width: default_options.line_width(),

docs/configuration.md

@@ -528,6 +528,8 @@ Options:
           Enable preview mode; checks will include unstable rules and fixes. Use `--no-preview` to disable
       --config <CONFIG>
           Path to the `pyproject.toml` or `ruff.toml` file to use for configuration
+      --extension <EXTENSION>
+          List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]). For example, to treat `.ipy` files as IPython notebooks, use `--extension ipy:ipynb`
       --statistics
           Show counts for every rule with at least one violation
       --add-noqa
@@ -604,6 +606,8 @@ Options:
           Avoid writing any formatted files back; instead, exit with a non-zero status code and the difference between the current file and how the formatted file would look like
       --config <CONFIG>
           Path to the `pyproject.toml` or `ruff.toml` file to use for configuration
+      --extension <EXTENSION>
+          List of mappings from file extension to language (one of ["python", "ipynb", "pyi"]). For example, to treat `.ipy` files as IPython notebooks, use `--extension ipy:ipynb`
       --target-version <TARGET_VERSION>
           The minimum Python version that should be supported [possible values: py37, py38, py39, py310, py311, py312]
       --preview