feat(lsp): support function for client root_dir

If root_dir is a function it is evaluated when the client is created to
determine the root directory.

This enables dynamically determining the root directory based on e.g.
project or directory structure (example: finding a parent Cargo.toml
file that contains "[workspace]" in a Rust project).

runtime/doc/lsp.txt

@@ -1317,9 +1317,10 @@ Lua module: vim.lsp.client                                        *lsp-client*
                                 request before sending kill -15. If set to
                                 false, nvim exits immediately after sending
                                 the "shutdown" request to the server.
-      • {root_dir}?           (`string`) Directory where the LSP server will
-                              base its workspaceFolders, rootUri, and rootPath
-                              on initialization.
+      • {root_dir}?           (`string|fun():string`) Directory (or a function
+                              which returns a directory) where the LSP server
+                              will base its workspaceFolders, rootUri, and
+                              rootPath on initialization.
 
 
 Client:cancel_request({id})                          *Client:cancel_request()*

runtime/lua/vim/lsp.lua

@@ -200,12 +200,17 @@ local function reuse_client_default(client, config)
     return false
   end
 
-  local config_folders = lsp._get_workspace_folders(config.workspace_folders or config.root_dir)
+  local root_dir = config.root_dir
+  if type(root_dir) == 'function' then
+    root_dir = root_dir()
+  end
+
+  local config_folders = lsp._get_workspace_folders(config.workspace_folders or root_dir)
 
   if not config_folders or not next(config_folders) then
     -- Reuse if the client was configured with no workspace folders
     local client_config_folders =
-      lsp._get_workspace_folders(client.config.workspace_folders or client.config.root_dir)
+      lsp._get_workspace_folders(client.config.workspace_folders or client.root_dir)
     return not client_config_folders or not next(client_config_folders)
   end
 

runtime/lua/vim/lsp/client.lua

@@ -130,8 +130,9 @@ local validate = vim.validate
 --- A table with flags for the client. The current (experimental) flags are:
 --- @field flags? vim.lsp.Client.Flags
 ---
---- Directory where the LSP server will base its workspaceFolders, rootUri, and rootPath on initialization.
---- @field root_dir? string
+--- Directory (or a function which returns a directory) where the LSP server will base its
+--- workspaceFolders, rootUri, and rootPath on initialization.
+--- @field root_dir? string|fun():string
 
 --- @class vim.lsp.Client.Progress: vim.Ringbuf<{token: integer|string, value: any}>
 --- @field pending table<lsp.ProgressToken,lsp.LSPAny>
@@ -373,6 +374,11 @@ function Client.create(config)
   local id = client_index
   local name = get_name(id, config)
 
+  local root_dir = config.root_dir
+  if type(root_dir) == 'function' then
+    root_dir = root_dir()
+  end
+
   --- @class vim.lsp.Client
   local self = {
     id = id,
@@ -390,8 +396,8 @@ function Client.create(config)
     flags = config.flags or {},
     get_language_id = config.get_language_id or default_get_language_id,
     capabilities = config.capabilities,
-    workspace_folders = lsp._get_workspace_folders(config.workspace_folders or config.root_dir),
-    root_dir = config.root_dir,
+    workspace_folders = lsp._get_workspace_folders(config.workspace_folders or root_dir),
+    root_dir = root_dir,
     _before_init_cb = config.before_init,
     _on_init_cbs = vim._ensure_list(config.on_init),
     _on_exit_cbs = vim._ensure_list(config.on_exit),

test/functional/plugin/lsp_spec.lua

@@ -271,6 +271,33 @@ describe('LSP', function()
       }
     end)
 
+    it('supports a function for root_dir', function()
+      clear()
+      exec_lua(create_server_definition)
+      local result = exec_lua(function()
+        local server = _G._create_server({})
+
+        local client_id = vim.lsp.start({
+          name = 'dummy',
+          cmd = server.cmd,
+          root_dir = function()
+            return 'some_directory'
+          end
+        })
+
+        if not client_id then
+          return 'vim.lsp.start did not return client_id'
+        end
+
+        local client = vim.lsp.get_client_by_id(client_id)
+        if not client then
+          return 'No client found with id ' .. client_id
+        end
+        return client.root_dir
+      end)
+      eq('some_directory', result)
+    end)
+
     it('should send didChangeConfiguration after initialize if there are settings', function()
       test_rpc_server({
         test_name = 'basic_init_did_change_configuration',