Add richer LSP server example (#20017)

crates/rust-analyzer/Cargo.toml

@@ -106,3 +106,7 @@ in-rust-tree = [
 
 [lints]
 workspace = true
+
+[[example]]
+name = "rust_analyzer_lsp_example"
+path = "examples/rust_analyzer_lsp_example/main.rs"

crates/rust-analyzer/examples/rust_analyzer_lsp_example/main.rs

@@ -0,0 +1,69 @@
+//! rust-analyzer LSP example
+//!
+//! A minimal Language Server Protocol server demonstrating how to embed
+//! `rust-analyzer` as a library. To run it:
+//! ```bash
+//! cargo run --example rust_analyzer_lsp_example
+//
+#![allow(clippy::print_stderr)]
+
+use anyhow::Result;
+use lsp_server::{Connection, Message};
+use lsp_types::{
+    CompletionParams, DidChangeTextDocumentParams, DocumentFormattingParams, OneOf,
+    ServerCapabilities, TextDocumentSyncKind, TextDocumentSyncOptions,
+    request::{Completion, Formatting, Request as LspRequest},
+};
+use serde_json::json;
+
+fn main() -> Result<()> {
+    // Set up a JSON-RPC connection over stdio
+    let (connection, io_threads) = Connection::stdio();
+
+    // Perform the initialize handshake and advertise capabilities
+    let (init_id, _init_params) = connection.initialize_start()?;
+    let capabilities = ServerCapabilities {
+        text_document_sync: Some(
+            TextDocumentSyncOptions {
+                change: Some(TextDocumentSyncKind::FULL),
+                ..Default::default()
+            }
+            .into(),
+        ),
+        completion_provider: Some(Default::default()),
+        document_formatting_provider: Some(OneOf::Left(true)),
+        ..Default::default()
+    };
+    connection.initialize_finish(init_id, json!({ "capabilities": capabilities }))?;
+
+    // Enter the main message loop
+    for message in connection.receiver.iter() {
+        match message {
+            Message::Notification(notification)
+                if notification.method == "textDocument/didChange" =>
+            {
+                let params: DidChangeTextDocumentParams =
+                    serde_json::from_value(notification.params).unwrap();
+                eprintln!("Document changed: {}", params.text_document.uri);
+            }
+            Message::Request(request) if request.method == Completion::METHOD => {
+                let _: CompletionParams = serde_json::from_value(request.params).unwrap();
+                let response = lsp_server::Response::new_ok(
+                    request.id,
+                    json!({ "isIncomplete": false, "items": [] }),
+                );
+                connection.sender.send(Message::Response(response)).unwrap();
+            }
+            Message::Request(request) if request.method == Formatting::METHOD => {
+                let _: DocumentFormattingParams = serde_json::from_value(request.params).unwrap();
+                let response = lsp_server::Response::new_ok(request.id, json!([]));
+                connection.sender.send(Message::Response(response)).unwrap();
+            }
+            Message::Request(request) if connection.handle_shutdown(&request)? => break,
+            _ => {} // ignore all other messages
+        }
+    }
+
+    io_threads.join().unwrap();
+    Ok(())
+}