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.

runtime/lua/vim/_watch.lua

@@ -222,5 +222,81 @@ function M.watchdirs(path, opts, callback)
   return cancel
 end
 
-return M
+--- @param data string
+--- @param opts vim._watch.Opts?
+--- @param callback vim._watch.Callback
+local function fswatch_output_handler(data, opts, callback)
+  local d = vim.split(data, '%s+')
 
+  -- only consider the last reported event
+  local fullpath, event = d[1], d[#d]
+
+  if skip(fullpath, opts) then
+    return
+  end
+
+  --- @type integer
+  local change_type
+
+  if event == 'Created' then
+    change_type = M.FileChangeType.Created
+  elseif event == 'Removed' then
+    change_type = M.FileChangeType.Deleted
+  elseif event == 'Updated' then
+    change_type = M.FileChangeType.Changed
+  elseif event == 'Renamed' then
+    local _, staterr, staterrname = uv.fs_stat(fullpath)
+    if staterrname == 'ENOENT' then
+      change_type = M.FileChangeType.Deleted
+    else
+      assert(not staterr, staterr)
+      change_type = M.FileChangeType.Created
+    end
+  end
+
+  if change_type then
+    callback(fullpath, change_type)
+  end
+end
+
+--- @param path string The path to watch. Must refer to a directory.
+--- @param opts vim._watch.Opts?
+--- @param callback vim._watch.Callback Callback for new events
+--- @return fun() cancel Stops the watcher
+function M.fswatch(path, opts, callback)
+  -- debounce isn't the same as latency but close enough
+  local latency = 0.5 -- seconds
+  if opts and opts.debounce then
+    latency = opts.debounce / 1000
+  end
+
+  local obj = vim.system({
+    'fswatch',
+    '--event=Created',
+    '--event=Removed',
+    '--event=Updated',
+    '--event=Renamed',
+    '--event-flags',
+    '--recursive',
+    '--latency=' .. tostring(latency),
+    '--exclude',
+    '/.git/',
+    path,
+  }, {
+    stdout = function(err, data)
+      if err then
+        error(err)
+      end
+
+      for line in vim.gsplit(data or '', '\n', { plain = true, trimempty = true }) do
+        fswatch_output_handler(line, opts, callback)
+      end
+    end,
+  })
+
+  return function()
+    obj:kill(2)
+  end
+end
+
+return M

runtime/lua/vim/lsp/_watchfiles.lua

@@ -9,6 +9,8 @@ local M = {}
 
 if vim.fn.has('win32') == 1 or vim.fn.has('mac') == 1 then
   M._watchfunc = watch.watch
+elseif vim.fn.executable('fswatch') == 1 then
+  M._watchfunc = watch.fswatch
 else
   M._watchfunc = watch.watchdirs
 end
@@ -177,4 +179,3 @@ function M.cancel(client_id)
 end
 
 return M
-

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