Build settings index in parallel for the native server (#12299)

## Summary

This PR updates the server to build the settings index in parallel using
similar logic as `python_files_in_path`.

This should help with https://github.com/astral-sh/ruff/issues/11366 but
ideally we would want to build it lazily.

## Test Plan

`cargo insta test`

crates/ruff_server/src/session/index/ruff_settings.rs

@@ -1,3 +1,9 @@
+use std::collections::BTreeMap;
+use std::path::{Path, PathBuf};
+use std::sync::Arc;
+
+use ignore::{WalkBuilder, WalkState};
+
 use ruff_linter::{
     display_settings, fs::normalize_path_to, settings::types::FilePattern,
     settings::types::PreviewMode,
@@ -8,12 +14,6 @@ use ruff_workspace::{
     pyproject::{find_user_settings_toml, settings_toml},
     resolver::{ConfigurationTransformer, Relativity},
 };
-use std::{
-    collections::BTreeMap,
-    path::{Path, PathBuf},
-    sync::Arc,
-};
-use walkdir::WalkDir;
 
 use crate::session::settings::{ConfigurationPreference, ResolvedEditorSettings};
 
@@ -30,7 +30,7 @@ pub struct RuffSettings {
 }
 
 pub(super) struct RuffSettingsIndex {
-    /// Index from folder to the resoled ruff settings.
+    /// Index from folder to the resolved ruff settings.
     index: BTreeMap<PathBuf, Arc<RuffSettings>>,
     fallback: Arc<RuffSettings>,
 }
@@ -100,6 +100,7 @@ impl RuffSettings {
 impl RuffSettingsIndex {
     pub(super) fn new(root: &Path, editor_settings: &ResolvedEditorSettings) -> Self {
         let mut index = BTreeMap::default();
+        let mut respect_gitignore = true;
 
         // Add any settings from above the workspace root.
         for directory in root.ancestors() {
@@ -112,6 +113,7 @@ impl RuffSettingsIndex {
                     continue;
                 };
 
+                respect_gitignore = settings.file_resolver.respect_gitignore;
                 index.insert(
                     directory.to_path_buf(),
                     Arc::new(RuffSettings {
@@ -126,70 +128,88 @@ impl RuffSettingsIndex {
         }
 
         // Add any settings within the workspace itself
-        let mut walker = WalkDir::new(root).into_iter();
+        let mut builder = WalkBuilder::new(root);
+        builder.standard_filters(respect_gitignore);
+        builder.hidden(false);
+        builder.threads(
+            std::thread::available_parallelism()
+                .map_or(1, std::num::NonZeroUsize::get)
+                .min(12),
+        );
+        let walker = builder.build_parallel();
 
-        while let Some(entry) = walker.next() {
-            let Ok(entry) = entry else {
-                continue;
-            };
+        let index = std::sync::RwLock::new(index);
+        walker.run(|| {
+            Box::new(|result| {
+                let Ok(entry) = result else {
+                    return WalkState::Continue;
+                };
 
-            // Skip non-directories.
-            if !entry.file_type().is_dir() {
-                continue;
-            }
-
-            let directory = entry.into_path();
-
-            // If the directory is excluded from the workspace, skip it.
-            if let Some(file_name) = directory.file_name() {
-                if let Some((_, settings)) = index
-                    .range(..directory.clone())
-                    .rfind(|(path, _)| directory.starts_with(path))
+                // Skip non-directories.
+                if !entry
+                    .file_type()
+                    .is_some_and(|file_type| file_type.is_dir())
                 {
-                    if match_exclusion(&directory, file_name, &settings.file_resolver.exclude) {
-                        tracing::debug!("Ignored path via `exclude`: {}", directory.display());
+                    return WalkState::Continue;
+                }
 
-                        walker.skip_current_dir();
-                        continue;
-                    } else if match_exclusion(
-                        &directory,
-                        file_name,
-                        &settings.file_resolver.extend_exclude,
-                    ) {
-                        tracing::debug!(
-                            "Ignored path via `extend-exclude`: {}",
-                            directory.display()
-                        );
+                let directory = entry.into_path();
+                tracing::debug!("Visiting: {}", directory.display());
 
-                        walker.skip_current_dir();
-                        continue;
+                // If the directory is excluded from the workspace, skip it.
+                if let Some(file_name) = directory.file_name() {
+                    if let Some((_, settings)) = index
+                        .read()
+                        .unwrap()
+                        .range(..directory.clone())
+                        .rfind(|(path, _)| directory.starts_with(path))
+                    {
+                        if match_exclusion(&directory, file_name, &settings.file_resolver.exclude) {
+                            tracing::debug!("Ignored path via `exclude`: {}", directory.display());
+                            return WalkState::Continue;
+                        } else if match_exclusion(
+                            &directory,
+                            file_name,
+                            &settings.file_resolver.extend_exclude,
+                        ) {
+                            tracing::debug!(
+                                "Ignored path via `extend-exclude`: {}",
+                                directory.display()
+                            );
+                            return WalkState::Continue;
+                        }
                     }
                 }
-            }
 
-            if let Some(pyproject) = settings_toml(&directory).ok().flatten() {
-                let Ok(settings) = ruff_workspace::resolver::resolve_root_settings(
-                    &pyproject,
-                    Relativity::Parent,
-                    &EditorConfigurationTransformer(editor_settings, root),
-                ) else {
-                    continue;
-                };
-                index.insert(
-                    directory,
-                    Arc::new(RuffSettings {
-                        path: Some(pyproject),
-                        file_resolver: settings.file_resolver,
-                        linter: settings.linter,
-                        formatter: settings.formatter,
-                    }),
-                );
-            }
-        }
+                if let Some(pyproject) = settings_toml(&directory).ok().flatten() {
+                    let Ok(settings) = ruff_workspace::resolver::resolve_root_settings(
+                        &pyproject,
+                        Relativity::Parent,
+                        &EditorConfigurationTransformer(editor_settings, root),
+                    ) else {
+                        return WalkState::Continue;
+                    };
+                    index.write().unwrap().insert(
+                        directory,
+                        Arc::new(RuffSettings {
+                            path: Some(pyproject),
+                            file_resolver: settings.file_resolver,
+                            linter: settings.linter,
+                            formatter: settings.formatter,
+                        }),
+                    );
+                }
+
+                WalkState::Continue
+            })
+        });
 
         let fallback = Arc::new(RuffSettings::fallback(editor_settings, root));
 
-        Self { index, fallback }
+        Self {
+            index: index.into_inner().unwrap(),
+            fallback,
+        }
     }
 
     pub(super) fn get(&self, document_path: &Path) -> Arc<RuffSettings> {