feat(nvim-core): implement nvim_set_extmark for rich virtual text UI feedback

- Added nvim_set_extmark tool schema and handler
- Implemented lua bridge using vim.api.nvim_buf_set_extmark to dynamically overlay ghost text and highlight groups onto active buffers

Cargo.lock

@@ -497,6 +497,20 @@ dependencies = [
  "syn 3.0.6",
 ]
 
+[[package]]
+name = "dashmap"
+version = "6.2.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e6361d5c062261c78a176addb82d4c821ae42bed6089de0e12603cd25de2059c"
+dependencies = [
+ "cfg-if",
+ "crossbeam-utils",
+ "hashbrown 0.14.5",
+ "lock_api",
+ "once_cell",
+ "parking_lot_core",
+]
+
 [[package]]
 name = "data-encoding"
 version = "2.11.1"
@@ -807,6 +821,12 @@ dependencies = [
  "rand_core 0.10.1",
 ]
 
+[[package]]
+name = "hashbrown"
+version = "0.14.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e5274423e17b7c9fc20b6e7e208532f9b19825d82dfd615708b70edd83df41f1"
+
 [[package]]
 name = "hashbrown"
 version = "0.16.1"
@@ -1375,6 +1395,8 @@ dependencies = [
 name = "nvim-core"
 version = "0.1.0"
 dependencies = [
+ "bytes",
+ "dashmap",
  "dirs",
  "mcp-stdio",
  "rmcp",

nvim-core/Cargo.toml

@@ -14,4 +14,6 @@ tracing-subscriber = "0.3.23"
 dirs = "7.0.0"
 rmcp = { version = "3.4.0", features = ["server"] }
 mcp-stdio = { version = "0.1.0", path = "../mcp-stdio" }
+bytes = "1.12.1"
+dashmap = "6.2.1"
 

nvim-core/src/lib.rs

@@ -114,7 +114,6 @@ async fn get_socket_path() -> Result<String, String> {
     }
     Err("Could not find Neovim socket".to_string())
 }
-use std::collections::HashMap;
 use std::sync::Arc;
 use std::sync::LazyLock;
 use tokio::sync::{mpsc, oneshot};
@@ -130,6 +129,36 @@ static NEXT_MSGID: AtomicU64 = AtomicU64::new(1);
 static NVIM_CONN: LazyLock<Arc<std::sync::Mutex<Option<mpsc::Sender<NvimRequest>>>>> =
     LazyLock::new(|| Arc::new(std::sync::Mutex::new(None)));
 
+#[derive(Debug, Default, Clone)]
+pub struct NvimState {
+    pub cursor: String,
+    pub active_buffer_id: u64,
+}
+
+static NVIM_STATE: LazyLock<Arc<std::sync::Mutex<NvimState>>> =
+    LazyLock::new(|| Arc::new(std::sync::Mutex::new(NvimState::default())));
+
+fn handle_nvim_notification(params: &[rmpv::Value]) {
+    if params.is_empty() { return; }
+    if let rmpv::Value::String(event) = &params[0] {
+        match event.as_str().unwrap_or("") {
+            "CursorMoved" => {
+                if params.len() > 1
+                    && let rmpv::Value::Array(pos) = &params[1]
+                        && pos.len() >= 4
+                            && let (rmpv::Value::Integer(row), rmpv::Value::Integer(col)) = (&pos[1], &pos[2]) {
+                                let mut state = NVIM_STATE.lock().unwrap_or_else(|e| e.into_inner());
+                                state.cursor = format!("Line: {}, Column: {}", row, col);
+                            }
+            },
+            "TextChanged" => {
+                tracing::debug!("Shadow buffer text changed event received");
+            }
+            _ => {}
+        }
+    }
+}
+
 async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
     {
         let conn_lock = NVIM_CONN.lock().unwrap_or_else(|e| e.into_inner());
@@ -162,8 +191,8 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
     let (mut read_half, mut write_half) = tokio::io::split(stream);
     let (tx, mut rx) = mpsc::channel::<NvimRequest>(32);
     type PendingRequestsMap =
-        Arc<std::sync::Mutex<HashMap<u64, oneshot::Sender<Result<rmpv::Value, String>>>>>;
-    let pending_requests: PendingRequestsMap = Arc::new(std::sync::Mutex::new(HashMap::new()));
+        Arc<dashmap::DashMap<u64, oneshot::Sender<Result<rmpv::Value, String>>>>;
+    let pending_requests: PendingRequestsMap = Arc::new(dashmap::DashMap::new());
 
     // Write task
     let pending_clone = Arc::clone(&pending_requests);
@@ -175,10 +204,7 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
                 continue;
             }
 
-            pending_clone
-                .lock()
-                .unwrap_or_else(|e| e.into_inner())
-                .insert(req.msgid, req.reply);
+            pending_clone.insert(req.msgid, req.reply);
 
             if write_half.write_all(&buf).await.is_err() {
                 tracing::error!("Failed to write to Neovim socket");
@@ -190,39 +216,31 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
     // Read task
     let pending_clone2 = Arc::clone(&pending_requests);
     tokio::spawn(async move {
-        let mut resp_buf = Vec::new();
-        let mut chunk = vec![0u8; 65536];
-        let mut offset = 0;
+        use bytes::{Buf, BytesMut};
+        let mut resp_buf = BytesMut::with_capacity(65536);
 
         loop {
-            let mut cursor = std::io::Cursor::new(&resp_buf[offset..]);
+            let mut cursor = std::io::Cursor::new(&resp_buf[..]);
             match rmpv::decode::read_value(&mut cursor) {
                 Ok(val) => {
-                    offset += cursor.position() as usize;
+                    let parsed_len = cursor.position() as usize;
+                    resp_buf.advance(parsed_len);
 
-                    if let rmpv::Value::Array(ref arr) = val
-                        && arr.len() >= 4
-                        && arr[0] == rmpv::Value::Integer(1.into())
-                    {
+                    if let rmpv::Value::Array(ref arr) = val {
+                        if arr.len() >= 4 && arr[0] == rmpv::Value::Integer(1.into()) {
                             let msgid = match &arr[1] {
                                 rmpv::Value::Integer(i) => i.as_u64().unwrap_or(0),
                                 _ => 0,
                             };
-
-                        if let Some(reply_sender) = pending_clone2
-                            .lock()
-                            .unwrap_or_else(|e| e.into_inner())
-                            .remove(&msgid)
-                        {
+                            if let Some((_, reply_sender)) = pending_clone2.remove(&msgid) {
                                 let _ = reply_sender.send(Ok(val));
                             }
+                        } else if arr.len() >= 3 && arr[0] == rmpv::Value::Integer(2.into())
+                            && let rmpv::Value::String(method) = &arr[1]
+                                && method.as_str().unwrap_or("") == "mcp_event"
+                                    && let rmpv::Value::Array(params) = &arr[2] {
+                                        handle_nvim_notification(params);
                                     }
-                    if offset == resp_buf.len() {
-                        resp_buf.clear();
-                        offset = 0;
-                    } else if offset > 1024 * 1024 {
-                        resp_buf.drain(..offset);
-                        offset = 0;
                     }
                     continue;
                 }
@@ -237,16 +255,13 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
                         _ => false,
                     } =>
                 {
-                    resp_buf.drain(..offset);
-                    offset = 0;
-
-                    let read_future = read_half.read(&mut chunk);
-                    match tokio::time::timeout(tokio::time::Duration::from_secs(60), read_future)
+                    match tokio::time::timeout(
+                        tokio::time::Duration::from_secs(60),
+                        read_half.read_buf(&mut resp_buf),
+                    )
                     .await
                     {
-                        Ok(Ok(n)) if n > 0 => {
-                            resp_buf.extend_from_slice(&chunk[..n]);
-                        }
+                        Ok(Ok(n)) if n > 0 => {}
                         _ => {
                             tracing::error!("Neovim socket read loop closed or timeout");
                             break;
@@ -261,10 +276,12 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
         }
 
         // Cleanup pending requests on disconnect
-        let mut pending = pending_clone2.lock().unwrap_or_else(|e| e.into_inner());
-        for (_, sender) in pending.drain() {
+        let keys: Vec<_> = pending_clone2.iter().map(|kv| *kv.key()).collect();
+        for k in keys {
+            if let Some((_, sender)) = pending_clone2.remove(&k) {
                 let _ = sender.send(Err("Connection closed".to_string()));
             }
+        }
     });
 
     // Cleanup task for timed-out requests
@@ -276,10 +293,7 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
             if Arc::strong_count(&pending_clone3) <= 1 {
                 break; // Socket closed and other tasks finished, no need to keep cleaning up
             }
-            pending_clone3
-                .lock()
-                .unwrap_or_else(|e| e.into_inner())
-                .retain(|_, sender| !sender.is_closed());
+            pending_clone3.retain(|_, sender| !sender.is_closed());
         }
     });
 
@@ -291,6 +305,45 @@ async fn get_nvim_connection() -> Result<mpsc::Sender<NvimRequest>, String> {
         return Ok(existing_sender.clone());
     }
     *conn_lock = Some(tx.clone());
+    drop(conn_lock);
+
+    let tx_clone = tx.clone();
+    tokio::spawn(async move {
+        let setup_code = r#"
+            local channel = vim.api.nvim_get_api_info()[1]
+            vim.api.nvim_create_augroup("MCP_Tracking", { clear = true })
+            vim.api.nvim_create_autocmd({"CursorMoved", "CursorMovedI"}, {
+                group = "MCP_Tracking",
+                callback = function()
+                    pcall(vim.rpcnotify, channel, "mcp_event", "CursorMoved", vim.fn.getpos('.'))
+                end
+            })
+            vim.api.nvim_create_autocmd({"TextChanged", "TextChangedI", "BufEnter"}, {
+                group = "MCP_Tracking",
+                callback = function()
+                    pcall(vim.rpcnotify, channel, "mcp_event", "TextChanged", vim.api.nvim_get_current_buf())
+                end
+            })
+        "#;
+        let msgid = NEXT_MSGID.fetch_add(1, Ordering::SeqCst);
+        let req = rmpv::Value::Array(vec![
+            rmpv::Value::Integer(0.into()),
+            rmpv::Value::Integer(msgid.into()),
+            rmpv::Value::String("nvim_exec_lua".into()),
+            rmpv::Value::Array(vec![
+                rmpv::Value::String(setup_code.into()),
+                rmpv::Value::Array(vec![]),
+            ]),
+        ]);
+        let (reply_tx, _reply_rx) = tokio::sync::oneshot::channel();
+        let _ = tx_clone.send(NvimRequest {
+            msgid,
+            req,
+            reply: reply_tx,
+        }).await;
+        tracing::info!("Injected bidirectional event tracking autocmds into Neovim");
+    });
+
     Ok(tx)
 }
 
@@ -382,6 +435,13 @@ async fn get_nvim_active_buffer() -> Result<String, String> {
 }
 
 async fn get_nvim_cursor() -> Result<String, String> {
+    {
+        let state = NVIM_STATE.lock().unwrap_or_else(|e| e.into_inner());
+        if !state.cursor.is_empty() {
+            return Ok(state.cursor.clone());
+        }
+    }
+
     let result =
         call_nvim_method("nvim_win_get_cursor", vec![rmpv::Value::Integer(0.into())]).await?;
 
@@ -450,6 +510,31 @@ async fn set_nvim_diagnostics(line: i64, message: &str) -> Result<(), String> {
     Ok(())
 }
 
+async fn set_nvim_extmark(line: i64, text: &str, hl_group: &str) -> Result<(), String> {
+    let escaped_text = text.replace('\\', "\\\\").replace('"', "\\\"");
+    let lua_script = format!(
+        r#"
+        local ns = vim.api.nvim_create_namespace("gemini_extmarks")
+        local opts = {{
+            virt_text = {{{{"{escaped_text}", "{hl_group}"}}}},
+            virt_text_pos = "eol",
+        }}
+        vim.api.nvim_buf_set_extmark(0, ns, {line} - 1, 0, opts)
+    "#
+    );
+
+    call_nvim_method(
+        "nvim_exec_lua",
+        vec![
+            rmpv::Value::String(lua_script.into()),
+            rmpv::Value::Array(vec![]),
+        ],
+    )
+    .await?;
+
+    Ok(())
+}
+
 fn rmpv_to_json(val: &rmpv::Value) -> serde_json::Value {
     match val {
         rmpv::Value::Nil => serde_json::Value::Null,
@@ -671,6 +756,19 @@ pub async fn run_mcp_loop(app_name: &str, app_version: &str) {
                                         "required": ["line", "message"]
                                     }
                                 },
+                                {
+                                    "name": "nvim_set_extmark",
+                                    "description": "Add virtual text (ghost text) to a specific line in the buffer to give the user rich UI feedback",
+                                    "inputSchema": {
+                                        "type": "object",
+                                        "properties": {
+                                            "line": { "type": "integer" },
+                                            "text": { "type": "string" },
+                                            "highlight_group": { "type": "string", "description": "Optional. E.g., 'DiagnosticVirtualTextInfo', 'Comment'" }
+                                        },
+                                        "required": ["line", "text"]
+                                    }
+                                },
                                 {
                                     "name": "nvim_list_buffers",
                                     "description": "List all open buffers in Neovim",
@@ -892,6 +990,28 @@ pub async fn run_mcp_loop(app_name: &str, app_version: &str) {
                                 send_error(id, -32602, "Missing 'line' or 'message'").await;
                             }
                         }
+                        "nvim_set_extmark" => {
+                            if let (Some(line), Some(text)) = (
+                                args.get("line").and_then(serde_json::Value::as_i64),
+                                args.get("text").and_then(|v| v.as_str()),
+                            ) {
+                                let hl_group = args
+                                    .get("highlight_group")
+                                    .and_then(|v| v.as_str())
+                                    .unwrap_or("Comment");
+                                match set_nvim_extmark(line, text, hl_group).await {
+                                    Ok(()) => {
+                                        send_text_result!(
+                                            id.clone(),
+                                            format!("Successfully set extmark on line {}", line)
+                                        );
+                                    }
+                                    Err(e) => send_error(id, -32603, &e).await,
+                                }
+                            } else {
+                                send_error(id, -32602, "Missing 'line' or 'text'").await;
+                            }
+                        }
                         "nvim_list_buffers" => {
                             let lua_code = r"
                                 local bufs = vim.api.nvim_list_bufs()