Include configured extensions in file discovery

crates/ruff/tests/cli/format.rs

@@ -2512,3 +2512,63 @@ fn markdown_formatting_stdin() -> Result<()> {
     "#);
     Ok(())
 }
+
+#[test]
+fn format_mapped_extension_files() -> Result<()> {
+    let test = CliTest::with_files([
+        (
+            "pyproject.toml",
+            r#"
+[tool.ruff]
+extension = {foo="python", bar="markdown"}
+"#,
+        ),
+        (
+            "test.foo",
+            r"
+print( 'hello' )
+",
+        ),
+        (
+            "test.bar",
+            r"
+Text string
+
+```py
+print( 'hello' )
+```
+",
+        ),
+    ])?;
+
+    assert_cmd_snapshot!(
+            test.format_command()
+                .args(["format", "--preview", "--check", "."]),
+            @r#"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+    io: [TMP]/format: No such file or directory (os error 2)
+    --> format:1:1
+
+    unformatted: File would be reformatted
+     --> test.bar:1:1
+    2 | Text string
+    3 | 
+    4 | ```py
+      - print( 'hello' )
+    5 + print("hello")
+    6 | ```
+
+    unformatted: File would be reformatted
+     --> test.foo:1:1
+      - 
+      - print( 'hello' )
+    1 + print("hello")
+
+    2 files would be reformatted
+
+    ----- stderr -----
+    "#);
+    Ok(())
+}

crates/ruff_linter/src/settings/types.rs

@@ -200,6 +200,7 @@ impl Deref for GlobPath {
 #[derive(Debug, Clone, CacheKey, PartialEq, PartialOrd, Eq, Ord)]
 pub enum FilePattern {
     Builtin(&'static str),
+    Config(String),
     User(String, GlobPath),
 }
 
@@ -209,6 +210,9 @@ impl FilePattern {
             FilePattern::Builtin(pattern) => {
                 builder.add(Glob::from_str(pattern)?);
             }
+            FilePattern::Config(pattern) => {
+                builder.add(Glob::new(&pattern)?);
+            }
             FilePattern::User(pattern, absolute) => {
                 // Add the absolute path.
                 builder.add(Glob::new(&absolute.to_string_lossy())?);
@@ -230,7 +234,7 @@ impl Display for FilePattern {
             "{:?}",
             match self {
                 Self::Builtin(pattern) => pattern,
-                Self::User(pattern, _) => pattern.as_str(),
+                Self::User(pattern, _) | Self::Config(pattern) => pattern.as_str(),
             }
         )
     }
@@ -498,6 +502,11 @@ impl From<ExtensionPair> for (String, Language) {
 pub struct ExtensionMapping(FxHashMap<String, Language>);
 
 impl ExtensionMapping {
+    /// Return the file extensions in the mapping.
+    pub fn extensions(&self) -> impl Iterator<Item = &String> {
+        self.0.keys()
+    }
+
     /// Return the [`Language`] for the given file.
     pub fn get(&self, path: &Path) -> Option<Language> {
         let ext = path.extension()?.to_str()?;

crates/ruff_workspace/src/configuration.rs

@@ -279,9 +279,19 @@ impl Configuration {
                     PreviewMode::Disabled => FilePatternSet::try_from_iter(
                         self.include.unwrap_or_else(|| INCLUDE.to_vec()),
                     )?,
-                    PreviewMode::Enabled => FilePatternSet::try_from_iter(
-                        self.include.unwrap_or_else(|| INCLUDE_PREVIEW.to_vec()),
-                    )?,
+                    PreviewMode::Enabled => {
+                        FilePatternSet::try_from_iter(self.include.unwrap_or_else(|| {
+                            let mut patterns = INCLUDE_PREVIEW.to_vec();
+                            if let Some(extension_map) = &self.extension {
+                                patterns.extend(
+                                    extension_map
+                                        .extensions()
+                                        .map(|ext| FilePattern::Config(format!("*.{ext}"))),
+                                );
+                            }
+                            patterns
+                        }))?
+                    }
                 },
                 respect_gitignore: self.respect_gitignore.unwrap_or(true),
                 project_root: project_root.to_path_buf(),

crates/ruff_workspace/src/options.rs

@@ -286,6 +286,10 @@ pub struct Options {
     /// by the `--extension` command-line flag).
     ///
     /// Supported file types include `python`, `pyi`, `ipynb`, and `markdown`.
+    ///
+    /// Any file extensions listed here will be automatically added to the
+    /// default `include` list as a `*.{ext}` glob, so that they are linted
+    /// and formatted without needing any additional configuration settings.
     #[option(
         default = "{}",
         value_type = "dict[str, Language]",