Ensure language servers from extension properly start on workspace restoration (#51308)

Closes #49877

Before you mark this PR as ready for review, make sure that you have:
- [x] Added a solid test coverage and/or screenshots from doing manual
testing
- [x] Done a self-review taking into account security and performance
aspects
- [ ] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Fixed extension language servers not starting when Zed launches with
files already open from a restored session.

crates/language/src/language_registry.rs

@@ -319,6 +319,8 @@ impl LanguageRegistry {
         state
             .all_lsp_adapters
             .insert(cached.name.clone(), cached.clone());
+        state.version += 1;
+        *state.subscription.0.borrow_mut() = ();
     }
 
     /// Register a fake language server and adapter
@@ -354,6 +356,8 @@ impl LanguageRegistry {
         state
             .all_lsp_adapters
             .insert(cached_adapter.name(), cached_adapter);
+        state.version += 1;
+        *state.subscription.0.borrow_mut() = ();
     }
 
     /// Register a fake language server (without the adapter)

crates/project/src/lsp_store.rs

@@ -4747,6 +4747,7 @@ impl LspStore {
 
                     this.update(cx, |this, cx| {
                         let mut plain_text_buffers = Vec::new();
+                        let mut buffers_with_language = Vec::new();
                         let mut buffers_with_unknown_injections = Vec::new();
                         for handle in this.buffer_store.read(cx).buffers() {
                             let buffer = handle.read(cx);
@@ -4754,8 +4755,11 @@ impl LspStore {
                                 || buffer.language() == Some(&*language::PLAIN_TEXT)
                             {
                                 plain_text_buffers.push(handle);
-                            } else if buffer.contains_unknown_injections() {
-                                buffers_with_unknown_injections.push(handle);
+                            } else {
+                                if buffer.contains_unknown_injections() {
+                                    buffers_with_unknown_injections.push(handle.clone());
+                                }
+                                buffers_with_language.push(handle);
                             }
                         }
 
@@ -4785,6 +4789,24 @@ impl LspStore {
                             }
                         }
 
+                        // Also register buffers that already have a language with
+                        // any newly-available language servers (e.g., from extensions
+                        // that finished loading after buffers were restored).
+                        if let Some(local) = this.as_local_mut() {
+                            for buffer in buffers_with_language {
+                                if local
+                                    .registered_buffers
+                                    .contains_key(&buffer.read(cx).remote_id())
+                                {
+                                    local.register_buffer_with_language_servers(
+                                        &buffer,
+                                        HashSet::default(),
+                                        cx,
+                                    );
+                                }
+                            }
+                        }
+
                         for buffer in buffers_with_unknown_injections {
                             buffer.update(cx, |buffer, cx| buffer.reparse(cx, false));
                         }

crates/project/tests/integration/project_tests.rs

@@ -1915,6 +1915,84 @@ async fn test_managing_language_servers(cx: &mut gpui::TestAppContext) {
     );
 }
 
+#[gpui::test]
+async fn test_late_lsp_adapter_registration(cx: &mut gpui::TestAppContext) {
+    init_test(cx);
+
+    let fs = FakeFs::new(cx.executor());
+    fs.insert_tree(
+        path!("/dir"),
+        json!({
+            "test.rs": "const A: i32 = 1;",
+        }),
+    )
+    .await;
+
+    let project = Project::test(fs.clone(), [path!("/dir").as_ref()], cx).await;
+    let language_registry = project.read_with(cx, |project, _| project.languages().clone());
+
+    // Add the language first so the buffer gets assigned a language.
+    language_registry.add(rust_lang());
+    cx.executor().run_until_parked();
+
+    // Open a buffer — it gets assigned the Rust language but there is no LSP adapter yet.
+    let (rust_buffer, _handle) = project
+        .update(cx, |project, cx| {
+            project.open_local_buffer_with_lsp(path!("/dir/test.rs"), cx)
+        })
+        .await
+        .unwrap();
+
+    rust_buffer.update(cx, |buffer, _| {
+        assert_eq!(buffer.language().map(|l| l.name()), Some("Rust".into()));
+    });
+
+    // Now register the LSP adapter late (simulating an extension loading after startup).
+    let mut fake_rust_servers = language_registry.register_fake_lsp(
+        "Rust",
+        FakeLspAdapter {
+            name: "the-rust-language-server",
+            capabilities: lsp::ServerCapabilities {
+                completion_provider: Some(lsp::CompletionOptions {
+                    trigger_characters: Some(vec![".".to_string(), "::".to_string()]),
+                    ..Default::default()
+                }),
+                ..Default::default()
+            },
+            ..Default::default()
+        },
+    );
+    cx.executor().run_until_parked();
+
+    // The language server should start and receive a DidOpenTextDocument notification
+    // for the already-open buffer.
+    let mut fake_rust_server = fake_rust_servers.next().await.unwrap();
+    assert_eq!(
+        fake_rust_server
+            .receive_notification::<lsp::notification::DidOpenTextDocument>()
+            .await
+            .text_document,
+        lsp::TextDocumentItem {
+            uri: lsp::Uri::from_file_path(path!("/dir/test.rs")).unwrap(),
+            version: 0,
+            text: "const A: i32 = 1;".to_string(),
+            language_id: "rust".to_string(),
+        }
+    );
+
+    // The buffer should be configured with the language server's capabilities.
+    rust_buffer.update(cx, |buffer, _| {
+        assert_eq!(
+            buffer
+                .completion_triggers()
+                .iter()
+                .cloned()
+                .collect::<Vec<_>>(),
+            &[".".to_string(), "::".to_string()]
+        );
+    });
+}
+
 #[gpui::test]
 async fn test_language_server_relative_path(cx: &mut gpui::TestAppContext) {
     init_test(cx);