feat(lsp): add fswatch watchfunc backend

Problem:
  vim._watch.watchdirs has terrible performance.

Solution:
  - On linux use fswatch as a watcher backend if available.

  - Add File watcher section to health:vim.lsp. Warn if watchfunc is
    libuv-poll.

1 files changed, 35 insertions, 5 deletions

diff --git a/runtime/lua/vim/lsp/health.lua b/runtime/lua/vim/lsp/health.lua
index 15e4555b55..797a1097f9 100644
--- a/runtime/lua/vim/lsp/health.lua
+++ b/runtime/lua/vim/lsp/health.lua
@@ -1,10 +1,9 @@
 local M = {}
 
---- Performs a healthcheck for LSP
-function M.check()
-  local report_info = vim.health.info
-  local report_warn = vim.health.warn
+local report_info = vim.health.info
+local report_warn = vim.health.warn
 
+local function check_log()
   local log = vim.lsp.log
   local current_log_level = log.get_level()
   local log_level_string = log.levels[current_log_level] ---@type string
@@ -27,9 +26,11 @@ function M.check()
 
   local report_fn = (log_size / 1000000 > 100 and report_warn or report_info)
   report_fn(string.format('Log size: %d KB', log_size / 1000))
+end
 
-  local clients = vim.lsp.get_clients()
+local function check_active_clients()
   vim.health.start('vim.lsp: Active Clients')
+  local clients = vim.lsp.get_clients()
   if next(clients) then
     for _, client in pairs(clients) do
       local attached_to = table.concat(vim.tbl_keys(client.attached_buffers or {}), ',')
@@ -48,4 +49,33 @@ function M.check()
   end
 end
 
+local function check_watcher()
+  vim.health.start('vim.lsp: File watcher')
+  local watchfunc = vim.lsp._watchfiles._watchfunc
+  assert(watchfunc)
+  local watchfunc_name --- @type string
+  if watchfunc == vim._watch.watch then
+    watchfunc_name = 'libuv-watch'
+  elseif watchfunc == vim._watch.watchdirs then
+    watchfunc_name = 'libuv-watchdirs'
+  elseif watchfunc == vim._watch.fswatch then
+    watchfunc_name = 'fswatch'
+  else
+    local nm = debug.getinfo(watchfunc, 'S').source
+    watchfunc_name = string.format('Custom (%s)', nm)
+  end
+
+  report_info('File watch backend: ' .. watchfunc_name)
+  if watchfunc_name == 'libuv-watchdirs' then
+    report_warn('libuv-watchdirs has known performance issues. Consider installing fswatch.')
+  end
+end
+
+--- Performs a healthcheck for LSP
+function M.check()
+  check_log()
+  check_active_clients()
+  check_watcher()
+end
+
 return M