Address review comments

- Add a validator to config/check.lua
- Accept a string for `root_dir`
- Don't mention the default implementation as the private
  `rustaceanvim.cargo.get_root_dir` function.
- Pass an extra "default" argument with the default implementation.
- Update documentation/comments where relevant.

doc/rustaceanvim.txt

@@ -234,13 +234,13 @@ RustcOpts                                                            *RustcOpts*
 RustaceanLspClientOpts                                  *RustaceanLspClientOpts*
 
     Fields: ~
-        {auto_attach?}           (boolean|fun(bufnr:integer):boolean)                               Whether to automatically attach the LSP client. Defaults to `true` if the `rust-analyzer` executable is found.
-        {cmd?}                   (string[]|fun():string[])                                          Command and arguments for starting rust-analyzer
-        {root_dir?}              (fun(filename:string):string|nil)                                  Should return the directory to use for the attached LSP, or nil if no server should attach. Defaults to `rustaceanvim.cargo.get_root_dir`.
-        {settings?}              (table|fun(project_root:string|nil,default_settings:table):table)  Setting passed to rust-analyzer. Defaults to a function that looks for a `rust-analyzer.json` file or returns an empty table. See https://rust-analyzer.github.io/manual.html#configuration.
-        {standalone?}            (boolean)                                                          Standalone file support (enabled by default). Disabling it may improve rust-analyzer's startup time.
-        {logfile?}               (string)                                                           The path to the rust-analyzer log file.
-        {load_vscode_settings?}  (boolean)                                                          Whether to search (upward from the buffer) for rust-analyzer settings in .vscode/settings json. If found, loaded settings will override configured options. Default: false
+        {auto_attach?}           (boolean|fun(bufnr:integer):boolean)                                              Whether to automatically attach the LSP client. Defaults to `true` if the `rust-analyzer` executable is found.
+        {cmd?}                   (string[]|fun():string[])                                                         Command and arguments for starting rust-analyzer
+        {root_dir?}              (string|fun(filename:string,default:fun(filename:string):string|nil):string|nil)  The directory to use for the attached LSP. Can be a function, which may return nil if no server should attach. The second argument contains the default implementation, which can be used for fallback behavior.
+        {settings?}              (table|fun(project_root:string|nil,default_settings:table):table)                 Setting passed to rust-analyzer. Defaults to a function that looks for a `rust-analyzer.json` file or returns an empty table. See https://rust-analyzer.github.io/manual.html#configuration.
+        {standalone?}            (boolean)                                                                         Standalone file support (enabled by default). Disabling it may improve rust-analyzer's startup time.
+        {logfile?}               (string)                                                                          The path to the rust-analyzer log file.
+        {load_vscode_settings?}  (boolean)                                                                         Whether to search (upward from the buffer) for rust-analyzer settings in .vscode/settings json. If found, loaded settings will override configured options. Default: false
 
 
 RustaceanDapOpts                                              *RustaceanDapOpts*

lua/rustaceanvim/cargo.lua

@@ -28,7 +28,7 @@ local function get_mb_active_client_root(file_name)
 end
 
 ---Attempts to find the root for an existing active client. If no existing
----client root is found, returns the result of `config.root_dir(file_name)`.
+---client root is found, returns the result of evaluating `config.root_dir`.
 ---@param config RustaceanLspClientConfig
 ---@param file_name string
 ---@return string | nil root_dir
@@ -37,7 +37,13 @@ function cargo.get_config_root_dir(config, file_name)
   if reuse_active then
     return reuse_active
   end
-  return config.root_dir(file_name)
+
+  local config_root_dir = config.root_dir
+  if type(config_root_dir) == 'function' then
+    return config_root_dir(file_name, cargo.get_root_dir)
+  else
+    return config_root_dir
+  end
 end
 
 ---The default implementation used for `vim.g.rustaceanvim.server.root_dir`

lua/rustaceanvim/config/check.lua

@@ -89,6 +89,7 @@ function M.validate(cfg)
     cmd = { server.cmd, { 'function', 'table' } },
     standalone = { server.standalone, 'boolean' },
     settings = { server.settings, { 'function', 'table' }, true },
+    root_dir = { server.root_dir, { 'function', 'string' } },
   })
   if not ok then
     return false, err

lua/rustaceanvim/config/init.lua

@@ -110,7 +110,7 @@ vim.g.rustaceanvim = vim.g.rustaceanvim
 ---@class RustaceanLspClientOpts
 ---@field auto_attach? boolean | fun(bufnr: integer):boolean Whether to automatically attach the LSP client. Defaults to `true` if the `rust-analyzer` executable is found.
 ---@field cmd? string[] | fun():string[] Command and arguments for starting rust-analyzer
----@field root_dir? fun(filename: string):string|nil Should return the directory to use for the attached LSP, or nil if no server should attach. Defaults to `rustaceanvim.cargo.get_root_dir`.
+---@field root_dir? string | fun(filename: string, default: fun(filename: string):string|nil):string|nil The directory to use for the attached LSP. Can be a function, which may return nil if no server should attach. The second argument contains the default implementation, which can be used for fallback behavior.
 ---@field settings? table | fun(project_root:string|nil, default_settings: table):table Setting passed to rust-analyzer. Defaults to a function that looks for a `rust-analyzer.json` file or returns an empty table. See https://rust-analyzer.github.io/manual.html#configuration.
 ---@field standalone? boolean Standalone file support (enabled by default). Disabling it may improve rust-analyzer's startup time.
 ---@field logfile? string The path to the rust-analyzer log file.

lua/rustaceanvim/config/internal.lua

@@ -277,7 +277,7 @@ local RustaceanDefaultConfig = {
       return { 'rust-analyzer', '--log-file', RustaceanConfig.server.logfile }
     end,
 
-    ---@type fun(filename: string):string|nil
+    ---@type string | fun(filename: string, default: fun(filename: string):string|nil):string|nil
     root_dir = cargo.get_root_dir,
 
     --- standalone file support