ruff server: Introduce the ruff.printDebugInformation command (#11831)

## Summary

Closes #11715.

Introduces a new command, `ruff.printDebugInformation`. This will print
useful information about the status of the server to `stderr`.

Right now, the information shown by this command includes:
* The path to the server executable
* The version of the executable
* The text encoding being used
* The number of open documents and workspaces
* A list of registered configuration files
* The capabilities of the client

## Test Plan

First, checkout and use [the corresponding `ruff-vscode`
PR](https://github.com/astral-sh/ruff-vscode/pull/495).

Running the `Print debug information` command in VS Code should show
something like the following in the Output channel:

<img width="991" alt="Screenshot 2024-06-11 at 11 41 46 AM"
src="ab93c009-bb7b-4291-b057-d44fdc6f9f86">

crates/ruff_server/src/session/capabilities.rs

@@ -1,4 +1,5 @@
 use lsp_types::ClientCapabilities;
+use ruff_linter::display_settings;
 
 #[derive(Debug, Clone, PartialEq, Eq, Default)]
 #[allow(clippy::struct_excessive_bools)]
@@ -65,3 +66,20 @@ impl ResolvedClientCapabilities {
         }
     }
 }
+
+impl std::fmt::Display for ResolvedClientCapabilities {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        display_settings! {
+            formatter = f,
+            namespace = "capabilities",
+            fields = [
+                self.code_action_deferred_edit_resolution,
+                self.apply_edit,
+                self.document_changes,
+                self.workspace_refresh,
+                self.pull_diagnostics,
+            ]
+        };
+        Ok(())
+    }
+}

crates/ruff_server/src/session/index.rs

@@ -187,6 +187,21 @@ impl Index {
         Self::register_workspace_settings(&mut self.settings, url, None, global_settings)
     }
 
+    pub(super) fn num_documents(&self) -> usize {
+        self.documents.len()
+    }
+
+    pub(super) fn num_workspaces(&self) -> usize {
+        self.settings.len()
+    }
+
+    pub(super) fn list_config_files(&self) -> Vec<&Path> {
+        self.settings
+            .values()
+            .flat_map(|WorkspaceSettings { ruff_settings, .. }| ruff_settings.list_files())
+            .collect()
+    }
+
     fn register_workspace_settings(
         settings_index: &mut SettingsIndex,
         workspace_url: &Url,

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

@@ -19,6 +19,9 @@ use walkdir::WalkDir;
 use crate::session::settings::{ConfigurationPreference, ResolvedEditorSettings};
 
 pub(crate) struct RuffSettings {
+    /// The path to this configuration file, used for debugging.
+    /// The default fallback configuration does not have a file path.
+    path: Option<PathBuf>,
     /// Settings used to manage file inclusion and exclusion.
     file_resolver: ruff_workspace::FileResolverSettings,
     /// Settings to pass into the Ruff linter.
@@ -48,14 +51,17 @@ impl std::fmt::Display for RuffSettings {
 
 impl RuffSettings {
     pub(crate) fn fallback(editor_settings: &ResolvedEditorSettings, root: &Path) -> RuffSettings {
+        let mut path = None;
         let fallback = find_user_settings_toml()
             .and_then(|user_settings| {
-                ruff_workspace::resolver::resolve_root_settings(
+                let settings = ruff_workspace::resolver::resolve_root_settings(
                     &user_settings,
                     Relativity::Cwd,
                     &EditorConfigurationTransformer(editor_settings, root),
                 )
-                .ok()
+                .ok();
+                path = Some(user_settings);
+                settings
             })
             .unwrap_or_else(|| {
                 let default_configuration = Configuration::default();
@@ -68,6 +74,7 @@ impl RuffSettings {
             });
 
         RuffSettings {
+            path,
             file_resolver: fallback.file_resolver,
             formatter: fallback.formatter,
             linter: fallback.linter,
@@ -108,6 +115,7 @@ impl RuffSettingsIndex {
                 index.insert(
                     directory.to_path_buf(),
                     Arc::new(RuffSettings {
+                        path: Some(pyproject),
                         file_resolver: settings.file_resolver,
                         linter: settings.linter,
                         formatter: settings.formatter,
@@ -176,6 +184,7 @@ impl RuffSettingsIndex {
                 index.insert(
                     directory,
                     Arc::new(RuffSettings {
+                        path: Some(pyproject),
                         file_resolver: settings.file_resolver,
                         linter: settings.linter,
                         formatter: settings.formatter,
@@ -198,6 +207,12 @@ impl RuffSettingsIndex {
             .clone()
     }
 
+    pub(crate) fn list_files(&self) -> impl Iterator<Item = &Path> {
+        self.index
+            .values()
+            .filter_map(|settings| settings.path.as_deref())
+    }
+
     pub(super) fn fallback(&self) -> Arc<RuffSettings> {
         self.fallback.clone()
     }