2 files changed, 62 insertions, 31 deletions

diff --git a/runtime/lua/vim/lsp.lua b/runtime/lua/vim/lsp.lua
index 91f2895f04..6c23c24220 100644
--- a/runtime/lua/vim/lsp.lua
+++ b/runtime/lua/vim/lsp.lua
@@ -374,26 +374,31 @@ lsp.config = setmetatable({ _configs = {} }, {
     validate('name', name, 'string')
 
     local rconfig = lsp._enabled_configs[name] or {}
-    self._configs[name] = self._configs[name] or {}
 
     if not rconfig.resolved_config then
       -- Resolve configs from lsp/*.lua
       -- Calls to vim.lsp.config in lsp/* have a lower precedence than calls from other sites.
-      local rtp_config = {} ---@type vim.lsp.Config
+      local rtp_config --- @type vim.lsp.Config?
       for _, v in ipairs(api.nvim_get_runtime_file(('lsp/%s.lua'):format(name), true)) do
         local config = assert(loadfile(v))() ---@type any?
         if type(config) == 'table' then
-          rtp_config = vim.tbl_deep_extend('force', rtp_config, config)
+          --- @type vim.lsp.Config?
+          rtp_config = vim.tbl_deep_extend('force', rtp_config or {}, config)
         else
           log.warn(string.format('%s does not return a table, ignoring', v))
         end
       end
 
+      if not rtp_config and not self._configs[name] then
+        log.warn(string.format('%s does not have a configuration', name))
+        return
+      end
+
       rconfig.resolved_config = vim.tbl_deep_extend(
         'force',
         lsp.config._configs['*'] or {},
-        rtp_config,
-        lsp.config._configs[name] or {}
+        rtp_config or {},
+        self._configs[name] or {}
       )
       rconfig.resolved_config.name = name
     end
@@ -424,39 +429,60 @@ lsp.config = setmetatable({ _configs = {} }, {
 
 local lsp_enable_autocmd_id --- @type integer?
 
---- @param bufnr integer
-local function lsp_enable_callback(bufnr)
-  -- Only ever attach to buffers that represent an actual file.
-  if vim.bo[bufnr].buftype ~= '' then
-    return
+local function validate_cmd(v)
+  if type(v) == 'table' then
+    if vim.fn.executable(v[1]) == 0 then
+      return false, v[1] .. ' is not executable'
+    end
+    return true
   end
+  return type(v) == 'function'
+end
 
-  --- @param config vim.lsp.Config
-  local function can_start(config)
-    if config.filetypes and not vim.tbl_contains(config.filetypes, vim.bo[bufnr].filetype) then
-      return false
-    elseif type(config.cmd) == 'table' and vim.fn.executable(config.cmd[1]) == 0 then
-      return false
-    end
+--- @param config vim.lsp.Config
+local function validate_config(config)
+  validate('cmd', config.cmd, validate_cmd, 'expected function or table with executable command')
+  validate('reuse_client', config.reuse_client, 'function', true)
+  validate('filetypes', config.filetypes, 'table', true)
+end
 
-    return true
+--- @param bufnr integer
+--- @param name string
+--- @param config vim.lsp.Config
+local function can_start(bufnr, name, config)
+  local config_ok, err = pcall(validate_config, config)
+  if not config_ok then
+    log.error(('cannot start %s due to config error: %s'):format(name, err))
+    return false
   end
 
-  --- @param config vim.lsp.Config
-  local function start(config)
-    return vim.lsp.start(config, {
-      bufnr = bufnr,
-      reuse_client = config.reuse_client,
-      _root_markers = config.root_markers,
-    })
+  if config.filetypes and not vim.tbl_contains(config.filetypes, vim.bo[bufnr].filetype) then
+    return false
+  end
+
+  return true
+end
+
+--- @param bufnr integer
+--- @param config vim.lsp.Config
+local function start_config(bufnr, config)
+  return vim.lsp.start(config, {
+    bufnr = bufnr,
+    reuse_client = config.reuse_client,
+    _root_markers = config.root_markers,
+  })
+end
+
+--- @param bufnr integer
+local function lsp_enable_callback(bufnr)
+  -- Only ever attach to buffers that represent an actual file.
+  if vim.bo[bufnr].buftype ~= '' then
+    return
   end
 
   for name in vim.spairs(lsp._enabled_configs) do
     local config = lsp.config[name]
-    validate('cmd', config.cmd, { 'function', 'table' })
-    validate('cmd', config.reuse_client, 'function', true)
-
-    if can_start(config) then
+    if config and can_start(bufnr, name, config) then
       -- Deepcopy config so changes done in the client
       -- do not propagate back to the enabled configs.
       config = vim.deepcopy(config)
@@ -466,11 +492,11 @@ local function lsp_enable_callback(bufnr)
         config.root_dir(bufnr, function(root_dir)
           config.root_dir = root_dir
           vim.schedule(function()
-            start(config)
+            start_config(bufnr, config)
           end)
         end)
       else
-        start(config)
+        start_config(bufnr, config)
       end
     end
   end
diff --git a/runtime/lua/vim/lsp/log.lua b/runtime/lua/vim/lsp/log.lua
index ec78dd3dc5..0f93369a0c 100644
--- a/runtime/lua/vim/lsp/log.lua
+++ b/runtime/lua/vim/lsp/log.lua
@@ -45,6 +45,11 @@ function log.get_filename()
   return logfilename
 end
 
+--- @param s string
+function log._set_filename(s)
+  logfilename = s
+end
+
 --- @type file*?, string?
 local logfile, openerr