Make completions an opt-in LSP feature (#17921)

## Summary

We now expect the client to send initialization options to opt-in to
experimental (but LSP-standardized) features, like completion support.
Specifically, the client should set `"experimental.completions.enable":
true`.

Closes https://github.com/astral-sh/ty/issues/74.

crates/ty_ide/src/completion.rs

@@ -6,6 +6,7 @@ use ruff_text_size::TextSize;
 
 use crate::Db;
 
+#[derive(Debug, Clone)]
 pub struct Completion {
     pub label: String,
 }

crates/ty_server/src/server.rs

@@ -15,7 +15,7 @@ use lsp_types::{
 
 use self::connection::{Connection, ConnectionInitializer};
 use self::schedule::event_loop_thread;
-use crate::session::{AllSettings, ClientSettings, Session};
+use crate::session::{AllSettings, ClientSettings, Experimental, Session};
 use crate::PositionEncoding;
 
 mod api;
@@ -41,19 +41,6 @@ impl Server {
 
         let (id, init_params) = connection.initialize_start()?;
 
-        let client_capabilities = init_params.capabilities;
-        let position_encoding = Self::find_best_position_encoding(&client_capabilities);
-        let server_capabilities = Self::server_capabilities(position_encoding);
-
-        let connection = connection.initialize_finish(
-            id,
-            &server_capabilities,
-            crate::SERVER_NAME,
-            crate::version(),
-        )?;
-
-        crate::message::init_messenger(connection.make_sender());
-
         let AllSettings {
             global_settings,
             mut workspace_settings,
@@ -63,6 +50,19 @@ impl Server {
                 .unwrap_or_else(|| serde_json::Value::Object(serde_json::Map::default())),
         );
 
+        let client_capabilities = init_params.capabilities;
+        let position_encoding = Self::find_best_position_encoding(&client_capabilities);
+        let server_capabilities =
+            Self::server_capabilities(position_encoding, global_settings.experimental.as_ref());
+
+        let connection = connection.initialize_finish(
+            id,
+            &server_capabilities,
+            crate::SERVER_NAME,
+            crate::version(),
+        )?;
+
+        crate::message::init_messenger(connection.make_sender());
         crate::logging::init_logging(
             global_settings.tracing.log_level.unwrap_or_default(),
             global_settings.tracing.log_file.as_deref(),
@@ -206,7 +206,10 @@ impl Server {
             .unwrap_or_default()
     }
 
-    fn server_capabilities(position_encoding: PositionEncoding) -> ServerCapabilities {
+    fn server_capabilities(
+        position_encoding: PositionEncoding,
+        experimental: Option<&Experimental>,
+    ) -> ServerCapabilities {
         ServerCapabilities {
             position_encoding: Some(position_encoding.into()),
             diagnostic_provider: Some(DiagnosticServerCapabilities::Options(DiagnosticOptions {
@@ -226,7 +229,9 @@ impl Server {
             inlay_hint_provider: Some(lsp_types::OneOf::Right(
                 InlayHintServerCapabilities::Options(InlayHintOptions::default()),
             )),
-            completion_provider: Some(lsp_types::CompletionOptions {
+            completion_provider: experimental
+                .is_some_and(Experimental::is_completions_enabled)
+                .then_some(lsp_types::CompletionOptions {
                     ..Default::default()
                 }),
             ..Default::default()

crates/ty_server/src/session.rs

@@ -21,6 +21,7 @@ pub(crate) use self::capabilities::ResolvedClientCapabilities;
 pub use self::index::DocumentQuery;
 pub(crate) use self::settings::AllSettings;
 pub use self::settings::ClientSettings;
+pub(crate) use self::settings::Experimental;
 
 mod capabilities;
 pub(crate) mod index;

crates/ty_server/src/session/settings.rs

@@ -7,11 +7,35 @@ use serde::Deserialize;
 /// Maps a workspace URI to its associated client settings. Used during server initialization.
 pub(crate) type WorkspaceSettingsMap = FxHashMap<Url, ClientSettings>;
 
+#[derive(Debug, Deserialize, Default)]
+#[cfg_attr(test, derive(PartialEq, Eq))]
+#[serde(rename_all = "camelCase")]
+struct Completions {
+    enable: Option<bool>,
+}
+
+#[derive(Debug, Deserialize, Default)]
+#[cfg_attr(test, derive(PartialEq, Eq))]
+#[serde(rename_all = "camelCase")]
+pub(crate) struct Experimental {
+    completions: Option<Completions>,
+}
+
+impl Experimental {
+    /// Returns `true` if completions are enabled in the settings.
+    pub(crate) fn is_completions_enabled(&self) -> bool {
+        self.completions
+            .as_ref()
+            .is_some_and(|completions| completions.enable.unwrap_or_default())
+    }
+}
+
 /// This is a direct representation of the settings schema sent by the client.
 #[derive(Debug, Deserialize, Default)]
 #[cfg_attr(test, derive(PartialEq, Eq))]
 #[serde(rename_all = "camelCase")]
 pub struct ClientSettings {
+    pub(crate) experimental: Option<Experimental>,
     // These settings are only needed for tracing, and are only read from the global configuration.
     // These will not be in the resolved settings.
     #[serde(flatten)]