fix(lsp): misleading logs in non-applicable filetypes #35749

Problem:
LSP logs show misleading "cannot start" messages when editing a filetype
NOT listed in the `config.filetypes` field.

    [ERROR][2025-09-13 18:55:56] …/runtime//lua/vim/lsp/log.lua:151
    "cannot start cssls due to config error: …/runtime//lua/vim/lsp.lua:423:
    cmd: expected expected function or table with executable command,
    got table: 0x0104701b18. Info: vscode-css-language-server is not executable"

Solution:
- `can_start`: check `config.filetypes` before checking the rest of the
  config.

runtime/lua/vim/lsp.lua

@@ -426,16 +426,21 @@ local function validate_config(config)
 end
 
 --- @param bufnr integer
---- @param name string
 --- @param config vim.lsp.Config
-local function can_start(bufnr, name, config)
+--- @param logging boolean
-  local config_ok, err = pcall(validate_config, config)
+local function can_start(bufnr, config, logging)
-  if not config_ok then
+  if
-    log.error(('cannot start %s due to config error: %s'):format(name, err))
+    type(config.filetypes) == 'table'
+    and not vim.tbl_contains(config.filetypes, vim.bo[bufnr].filetype)
+  then
     return false
   end
 
-  if config.filetypes and not vim.tbl_contains(config.filetypes, vim.bo[bufnr].filetype) then
+  local config_ok, err = pcall(validate_config, config)
+  if not config_ok then
+    if logging then
+      log.error(('invalid "%s" config: %s'):format(config.name, err))
+    end
     return false
   end
 
@@ -462,9 +467,7 @@ local function lsp_enable_callback(bufnr)
   -- Stop any clients that no longer apply to this buffer.
   local clients = lsp.get_clients({ bufnr = bufnr, _uninitialized = true })
   for _, client in ipairs(clients) do
-    if
+    if lsp.is_enabled(client.name) and not can_start(bufnr, lsp.config[client.name], false) then
-      lsp.is_enabled(client.name) and not can_start(bufnr, client.name, lsp.config[client.name])
-    then
       lsp.buf_detach_client(bufnr, client.id)
     end
   end
@@ -472,7 +475,7 @@ local function lsp_enable_callback(bufnr)
   -- Start any clients that apply to this buffer.
   for name in vim.spairs(lsp._enabled_configs) do
     local config = lsp.config[name]
-    if config and can_start(bufnr, name, config) then
+    if config and can_start(bufnr, config, true) then
       -- Deepcopy config so changes done in the client
       -- do not propagate back to the enabled configs.
       config = vim.deepcopy(config)

test/functional/plugin/lsp_spec.lua

@@ -3,7 +3,6 @@ local n = require('test.functional.testnvim')()
 
 local t_lsp = require('test.functional.plugin.lsp.testutil')
 
-local assert_log = t.assert_log
 local buf_lines = n.buf_lines
 local command = n.command
 local dedent = t.dedent
@@ -278,7 +277,7 @@ describe('LSP', function()
         on_exit = function(code, signal)
           eq(101, code, 'exit code') -- See fake-lsp-server.lua
           eq(0, signal, 'exit signal')
-          assert_log(
+          t.assert_log(
             pesc([[assert_eq failed: left == "\"shutdown\"", right == "\"test\""]]),
             fake_lsp_logfile
           )
@@ -6799,6 +6798,7 @@ describe('LSP', function()
     it('validates config on attach', function()
       local tmp1 = t.tmpname(true)
       exec_lua(function()
+        vim.fn.writefile({ '' }, fake_lsp_logfile)
         vim.lsp.log._set_filename(fake_lsp_logfile)
       end)
 
@@ -6808,22 +6808,32 @@ describe('LSP', function()
           vim.lsp.config('foo', cfg)
           vim.lsp.enable('foo')
           vim.cmd.edit(assert(tmp1))
+          vim.bo.filetype = 'non.applicable.filetype'
+        end)
+
+        -- Assert NO log for non-applicable 'filetype'. #35737
+        if type(cfg.filetypes) == 'table' then
+          t.assert_nolog(err, fake_lsp_logfile)
+        end
+
+        exec_lua(function()
           vim.bo.filetype = 'foo'
         end)
 
         retry(nil, 1000, function()
-          assert_log(err, fake_lsp_logfile)
+          t.assert_log(err, fake_lsp_logfile)
         end)
       end
 
       test_cfg({
+        filetypes = { 'foo' },
         cmd = { 'lolling' },
-      }, 'cannot start foo due to config error: .* lolling is not executable')
+      }, 'invalid "foo" config: .* lolling is not executable')
 
       test_cfg({
         cmd = { 'cat' },
         filetypes = true,
-      }, 'cannot start foo due to config error: .* filetypes: expected table, got boolean')
+      }, 'invalid "foo" config: .* filetypes: expected table, got boolean')
     end)
 
     it('does not start without workspace if workspace_required=true', function()