refactor(lsp): deprecate vim.lsp.util.lookup_section

This function is used only in the `workspace/configuration` handler,
and does not warrant a public API because of its confusing return types.

The only caller `vim.lsp.handlers["workspace.configuration"]` is also
refactored to use `vim.tbl_get()` instead.

runtime/doc/deprecated.txt

@@ -144,6 +144,8 @@ LSP FUNCTIONS
 						`progress` of |vim.lsp.client|
 - *vim.lsp.get_active_clients()*		Use |vim.lsp.get_clients()|
 - *vim.lsp.for_each_buffer_client()*		Use |vim.lsp.get_clients()|
+- *vim.lsp.util.lookup_section()*		Use |vim.tbl_get()| and
+						|vim.split()| with {plain=true} instead.
 - *vim.lsp.util.trim_empty_lines()*		Use |vim.split()| with `trimempty` instead.
 - *vim.lsp.util.try_trim_markdown_code_blocks()*
 - *vim.lsp.util.set_lines()*

runtime/doc/lsp.txt

@@ -1821,17 +1821,6 @@ locations_to_items({locations}, {offset_encoding})
     Return: ~
         (`vim.lsp.util.LocationItem[]`) list of items
 
-lookup_section({settings}, {section})          *vim.lsp.util.lookup_section()*
-    Helper function to return nested values in language server settings
-
-    Parameters: ~
-      • {settings}  (`table`) language server settings
-      • {section}   (`string`) indicating the field of the settings table
-
-    Return: ~
-        (`table|string|vim.NIL`) The value of settings accessed via section.
-        `vim.NIL` if not found.
-
                                   *vim.lsp.util.make_floating_popup_options()*
 make_floating_popup_options({width}, {height}, {opts})
     Creates a table with sensible default options for a floating window. The

runtime/lua/vim/lsp/handlers.lua

@@ -170,6 +170,14 @@ M[ms.workspace_applyEdit] = function(_, workspace_edit, ctx)
   }
 end
 
+---@param table   table e.g., { foo = { bar = "z" } }
+---@param section string indicating the field of the table, e.g., "foo.bar"
+---@return any|nil setting value read from the table, or `nil` not found
+local function lookup_section(table, section)
+  local keys = vim.split(section, '.', { plain = true }) --- @type string[]
+  return vim.tbl_get(table, unpack(keys))
+end
+
 --see: https://microsoft.github.io/language-server-protocol/specifications/specification-current/#workspace_configuration
 M[ms.workspace_configuration] = function(_, result, ctx)
   local client_id = ctx.client_id
@@ -189,10 +197,13 @@ M[ms.workspace_configuration] = function(_, result, ctx)
   local response = {}
   for _, item in ipairs(result.items) do
     if item.section then
-      local value = util.lookup_section(client.config.settings, item.section)
+      local value = lookup_section(client.config.settings, item.section)
       -- For empty sections with no explicit '' key, return settings as is
-      if value == vim.NIL and item.section == '' then
-        value = client.config.settings or vim.NIL
+      if value == nil and item.section == '' then
+        value = client.config.settings
+      end
+      if value == nil then
+        value = vim.NIL
       end
       table.insert(response, value)
     end

runtime/lua/vim/lsp/util.lua

@@ -2140,7 +2140,9 @@ end
 ---@param settings table language server settings
 ---@param section  string indicating the field of the settings table
 ---@return table|string|vim.NIL The value of settings accessed via section. `vim.NIL` if not found.
+---@deprecated
 function M.lookup_section(settings, section)
+  vim.deprecate('vim.lsp.util.lookup_section()', 'vim.tbl_get() with `vim.split`', '0.12')
   for part in vim.gsplit(section, '.', { plain = true }) do
     settings = settings[part]
     if settings == nil then