feat(lsp): defaults: tagfunc, omnifunc (#19003)

set `tagfunc` to `vim.lsp.tagfunc` and `omnifunc` to `vim.lsp.omnifunc` if empty when attaching a server

runtime/doc/lsp.txt

@@ -51,15 +51,14 @@ Starting a LSP client will automatically report diagnostics via
 |vim.diagnostic|. Read |vim.diagnostic.config| to learn how to customize the
 display.
 
-To get completion from the LSP server you can enable the |vim.lsp.omnifunc|:
+It also sets some buffer options if the options are otherwise empty and if the
->
+language server supports the functionality.
-    vim.bo.omnifunc = 'v:lua.vim.lsp.omnifunc'
-<
-To trigger completion, use |i_CTRL-X_CTRL-O|
 
-To get features like go-to-definition you can enable the |vim.lsp.tagfunc|
+- |omnifunc| is set to |vim.lsp.omnifunc|. This allows to trigger completion
-which changes commands like |:tjump| to utilize the language server and also
+  using |i_CTRL-X_CTRL-o|
-enables keymaps like |CTLR-]|, |CTRL-W_]|, |CTRL-W_}| and many more.
+- |tagfunc| is set to |vim.lsp.tagfunc|. This enables features like
+  go-to-definition, |:tjump|, and keymaps like |CTRL-]|, |CTRL-W_]|,
+  |CTRL-W_}| to utilize the language server.
 
 To use other LSP features like hover, rename, etc. you can setup some
 additional keymaps. It's recommended to setup them in a |LspAttach| autocmd to

runtime/lua/vim/lsp.lua

@@ -562,7 +562,7 @@ end
 ---@private
 --- Default handler for the 'textDocument/didOpen' LSP notification.
 ---
----@param bufnr (Number) Number of the buffer, or 0 for current
+---@param bufnr number Number of the buffer, or 0 for current
 ---@param client Client object
 local function text_document_did_open_handler(bufnr, client)
   changetracking.init(client, bufnr)
@@ -947,6 +947,28 @@ function lsp.start_client(config)
     end
   end
 
+  ---@private
+  local function set_defaults(client, bufnr)
+    if client.server_capabilities.definitionProvider and vim.bo[bufnr].tagfunc == '' then
+      vim.bo[bufnr].tagfunc = 'v:lua.vim.lsp.tagfunc'
+    end
+    if client.server_capabilities.completionProvider and vim.bo[bufnr].omnifunc == '' then
+      vim.bo[bufnr].omnifunc = 'v:lua.vim.lsp.omnifunc'
+    end
+  end
+
+  ---@private
+  --- Reset defaults set by `set_defaults`.
+  --- Must only be called if the last client attached to a buffer exits.
+  local function unset_defaults(bufnr)
+    if vim.bo[bufnr].tagfunc == 'v:lua.vim.lsp.tagfunc' then
+      vim.bo[bufnr].tagfunc = nil
+    end
+    if vim.bo[bufnr].omnifunc == 'v:lua.vim.lsp.omnifunc' then
+      vim.bo[bufnr].omnifunc = nil
+    end
+  end
+
   ---@private
   --- Invoked on client exit.
   ---
@@ -972,6 +994,11 @@ function lsp.start_client(config)
 
         client_ids[client_id] = nil
       end
+      if vim.tbl_isempty(client_ids) then
+        vim.schedule(function()
+          unset_defaults(bufnr)
+        end)
+      end
     end
 
     active_clients[client_id] = nil
@@ -1325,6 +1352,8 @@ function lsp.start_client(config)
   function client._on_attach(bufnr)
     text_document_did_open_handler(bufnr, client)
 
+    set_defaults(client, bufnr)
+
     nvim_exec_autocmds('LspAttach', {
       buffer = bufnr,
       modeline = false,