[ty] Add documentation for server traits (#19137)

This PR adds some basic documentation for the traits in the server
implementation.

crates/ruff_server/src/server/api.rs

@@ -26,8 +26,11 @@ use self::traits::{NotificationHandler, RequestHandler};
 
 use super::{Result, schedule::BackgroundSchedule};
 
-/// Defines the `document_url` method for implementers of [`traits::Notification`] and [`traits::Request`],
-/// given the parameter type used by the implementer.
+/// Defines the `document_url` method for implementers of [`Notification`] and [`Request`], given
+/// the request or notification parameter type.
+///
+/// This would only work if the parameter type has a `text_document` field with a `uri` field
+/// that is of type [`lsp_types::Url`].
 macro_rules! define_document_url {
     ($params:ident: &$p:ty) => {
         fn document_url($params: &$p) -> std::borrow::Cow<lsp_types::Url> {

crates/ruff_server/src/server/api/traits.rs

@@ -1,4 +1,31 @@
-//! A stateful LSP implementation that calls into the Ruff API.
+//! Traits for handling requests and notifications from the LSP client.
+//!
+//! This module defines the trait abstractions used by the language server to handle incoming
+//! requests and notifications from clients. It provides a type-safe way to implement LSP handlers
+//! with different execution models (synchronous or asynchronous) and automatic retry capabilities.
+//!
+//! All request and notification handlers must implement the base traits [`RequestHandler`] and
+//! [`NotificationHandler`], respectively, which associate them with specific LSP request or
+//! notification types. These base traits are then extended by more specific traits that define
+//! the execution model of the handler.
+//!
+//! The [`SyncRequestHandler`] and [`SyncNotificationHandler`] traits are for handlers that
+//! executes synchronously on the main loop, providing mutable access to the [`Session`] that
+//! contains the current state of the server. This is useful for handlers that need to modify
+//! the server state such as when the content of a file changes.
+//!
+//! The [`BackgroundDocumentRequestHandler`] and [`BackgroundDocumentNotificationHandler`] traits
+//! are for handlers that operate on a single document and can be executed on a background thread.
+//! These handlers will have access to a snapshot of the document at the time of the request or
+//! notification, allowing them to perform operations without blocking the main loop.
+//!
+//! The [`SyncNotificationHandler`] is the most common trait that would be used because most
+//! notifications are specific to a single document and require updating the server state.
+//! Similarly, the [`BackgroundDocumentRequestHandler`] is the most common request handler that
+//! would be used as most requests are document-specific and can be executed in the background.
+//!
+//! See the `./requests` and `./notifications` directories for concrete implementations of these
+//! traits in action.
 
 use crate::session::{Client, DocumentSnapshot, Session};
 
@@ -12,9 +39,10 @@ pub(super) trait RequestHandler {
 }
 
 /// A request handler that needs mutable access to the session.
-/// This will block the main message receiver loop, meaning that no
-/// incoming requests or notifications will be handled while `run` is
-/// executing. Try to avoid doing any I/O or long-running computations.
+///
+/// This will block the main message receiver loop, meaning that no incoming requests or
+/// notifications will be handled while `run` is executing. Try to avoid doing any I/O or
+/// long-running computations.
 pub(super) trait SyncRequestHandler: RequestHandler {
     fn run(
         session: &mut Session,
@@ -24,10 +52,14 @@ pub(super) trait SyncRequestHandler: RequestHandler {
 }
 
 /// A request handler that can be run on a background thread.
+///
+/// This handler is specific to requests that operate on a single document.
 pub(super) trait BackgroundDocumentRequestHandler: RequestHandler {
-    /// `document_url` can be implemented automatically with
-    /// `define_document_url!(params: &<YourParameterType>)` in the trait
-    /// implementation.
+    /// Returns the URL of the document that this request handler operates on.
+    ///
+    /// This method can be implemented automatically using the [`define_document_url`] macro.
+    ///
+    /// [`define_document_url`]: super::define_document_url
     fn document_url(
         params: &<<Self as RequestHandler>::RequestType as Request>::Params,
     ) -> std::borrow::Cow<lsp_types::Url>;
@@ -47,9 +79,10 @@ pub(super) trait NotificationHandler {
 }
 
 /// A notification handler that needs mutable access to the session.
-/// This will block the main message receiver loop, meaning that no
-/// incoming requests or notifications will be handled while `run` is
-/// executing. Try to avoid doing any I/O or long-running computations.
+///
+/// This will block the main message receiver loop, meaning that no incoming requests or
+/// notifications will be handled while `run` is executing. Try to avoid doing any I/O or
+/// long-running computations.
 pub(super) trait SyncNotificationHandler: NotificationHandler {
     fn run(
         session: &mut Session,
@@ -60,9 +93,11 @@ pub(super) trait SyncNotificationHandler: NotificationHandler {
 
 /// A notification handler that can be run on a background thread.
 pub(super) trait BackgroundDocumentNotificationHandler: NotificationHandler {
-    /// `document_url` can be implemented automatically with
-    /// `define_document_url!(params: &<YourParameterType>)` in the trait
-    /// implementation.
+    /// Returns the URL of the document that this notification handler operates on.
+    ///
+    /// This method can be implemented automatically using the [`define_document_url`] macro.
+    ///
+    /// [`define_document_url`]: super::define_document_url
     fn document_url(
         params: &<<Self as NotificationHandler>::NotificationType as LSPNotification>::Params,
     ) -> std::borrow::Cow<lsp_types::Url>;