refactor: create function for deferred loading

The benefit of this is that users only pay for what they use. If e.g.
only `vim.lsp.buf_get_clients()` is called then they don't need to load
all modules under `vim.lsp` which could lead to significant startuptime
saving.

Also `vim.lsp.module` is a bit nicer to user compared to
`require("vim.lsp.module")`.

This isn't used for some nested modules such as `filetype` as it breaks
tests with error messages such as "attempt to index field 'detect'".
It's not entirely certain the reason for this, but it is likely it is
due to filetype being precompiled which would imply deferred loading
isn't needed for performance reasons.

runtime/lua/vim/lsp.lua

@@ -1,13 +1,4 @@
 ---@diagnostic disable: invisible
-local default_handlers = require('vim.lsp.handlers')
-local log = require('vim.lsp.log')
-local lsp_rpc = require('vim.lsp.rpc')
-local protocol = require('vim.lsp.protocol')
-local ms = protocol.Methods
-local util = require('vim.lsp.util')
-local changetracking = require('vim.lsp._changetracking')
-local semantic_tokens = require('vim.lsp.semantic_tokens')
-
 local api = vim.api
 local nvim_err_writeln, nvim_buf_get_lines, nvim_command, nvim_exec_autocmds =
   api.nvim_err_writeln, api.nvim_buf_get_lines, api.nvim_command, api.nvim_exec_autocmds
@@ -16,24 +7,34 @@ local tbl_isempty, tbl_extend = vim.tbl_isempty, vim.tbl_extend
 local validate = vim.validate
 local if_nil = vim.F.if_nil
 
-local lsp = {
-  protocol = protocol,
+local lsp = vim._defer_require('vim.lsp', {
+  _changetracking = ..., --- @module 'vim.lsp._changetracking'
+  _completion = ..., --- @module 'vim.lsp._completion'
+  _dynamic = ..., --- @module 'vim.lsp._dynamic'
+  _snippet_grammar = ..., --- @module 'vim.lsp._snippet_grammar'
+  _watchfiles = ..., --- @module 'vim.lsp._watchfiles'
+  buf = ..., --- @module 'vim.lsp.buf'
+  codelens = ..., --- @module 'vim.lsp.codelens'
+  diagnostic = ..., --- @module 'vim.lsp.diagnostic'
+  handlers = ..., --- @module 'vim.lsp.handlers'
+  inlay_hint = ..., --- @module 'vim.lsp.inlay_hint'
+  log = ..., --- @module 'vim.lsp.log'
+  protocol = ..., --- @module 'vim.lsp.protocol'
+  rpc = ..., --- @module 'vim.lsp.rpc'
+  semantic_tokens = ..., --- @module 'vim.lsp.semantic_tokens'
+  tagfunc = ..., --- @module 'vim.lsp.tagfunc'
+  util = ..., --- @module 'vim.lsp.util'
+})
 
-  handlers = default_handlers,
+local log = lsp.log
+local protocol = lsp.protocol
+local ms = protocol.Methods
+local util = lsp.util
+local changetracking = lsp._changetracking
 
-  buf = require('vim.lsp.buf'),
-  diagnostic = require('vim.lsp.diagnostic'),
-  codelens = require('vim.lsp.codelens'),
-  inlay_hint = require('vim.lsp.inlay_hint'),
-  semantic_tokens = semantic_tokens,
-  util = util,
-
-  -- Allow raw RPC access.
-  rpc = lsp_rpc,
-
-  -- Export these directly from rpc.
-  rpc_response_error = lsp_rpc.rpc_response_error,
-}
+-- Export these directly from rpc.
+---@nodoc
+lsp.rpc_response_error = lsp.rpc.rpc_response_error
 
 -- maps request name to the required server_capability in the client.
 lsp._request_name_to_capability = {
@@ -189,11 +190,11 @@ end
 --- @nodoc
 lsp.client_errors = tbl_extend(
   'error',
-  lsp_rpc.client_errors,
+  lsp.rpc.client_errors,
   vim.tbl_add_reverse_lookup({
-    BEFORE_INIT_CALLBACK_ERROR = table.maxn(lsp_rpc.client_errors) + 1,
-    ON_INIT_CALLBACK_ERROR = table.maxn(lsp_rpc.client_errors) + 2,
-    ON_ATTACH_ERROR = table.maxn(lsp_rpc.client_errors) + 3,
+    BEFORE_INIT_CALLBACK_ERROR = table.maxn(lsp.rpc.client_errors) + 1,
+    ON_INIT_CALLBACK_ERROR = table.maxn(lsp.rpc.client_errors) + 2,
+    ON_ATTACH_ERROR = table.maxn(lsp.rpc.client_errors) + 3,
   })
 )
 
@@ -800,7 +801,7 @@ function lsp.start_client(config)
   ---@param method (string) LSP method name
   ---@return lsp.Handler|nil handler for the given method, if defined, or the default from |vim.lsp.handlers|
   local function resolve_handler(method)
-    return handlers[method] or default_handlers[method]
+    return handlers[method] or lsp.handlers[method]
   end
 
   ---@private
@@ -958,7 +959,7 @@ function lsp.start_client(config)
   if type(cmd) == 'function' then
     rpc = cmd(dispatch)
   else
-    rpc = lsp_rpc.start(cmd, cmd_args, dispatch, {
+    rpc = lsp.rpc.start(cmd, cmd_args, dispatch, {
       cwd = config.cmd_cwd,
       env = config.cmd_env,
       detached = config.detached,
@@ -999,7 +1000,7 @@ function lsp.start_client(config)
 
     ---@deprecated use client.progress instead
     messages = { name = name, messages = {}, progress = {}, status = {} },
-    dynamic_capabilities = require('vim.lsp._dynamic').new(client_id),
+    dynamic_capabilities = vim.lsp._dynamic.new(client_id),
   }
 
   ---@type table<string|integer, string> title of unfinished progress sequences by token
@@ -1412,7 +1413,7 @@ function lsp.start_client(config)
     -- opt-out (deleting the semanticTokensProvider from capabilities)
     vim.schedule(function()
       if vim.tbl_get(client.server_capabilities, 'semanticTokensProvider', 'full') then
-        semantic_tokens.start(bufnr, client.id)
+        lsp.semantic_tokens.start(bufnr, client.id)
       end
     end)
 
@@ -1969,7 +1970,7 @@ function lsp.omnifunc(findstart, base)
   if log.debug() then
     log.debug('omnifunc.findstart', { findstart = findstart, base = base })
   end
-  return require('vim.lsp._completion').omnifunc(findstart, base)
+  return vim.lsp._completion.omnifunc(findstart, base)
 end
 
 --- Provides an interface between the built-in client and a `formatexpr` function.
@@ -2039,7 +2040,7 @@ end
 ---
 ---@return table[] tags A list of matching tags
 function lsp.tagfunc(pattern, flags)
-  return require('vim.lsp.tagfunc')(pattern, flags)
+  return vim.lsp.tagfunc(pattern, flags)
 end
 
 ---Checks whether a client is stopped.