Warn if OTP built without doc chunks

Fixes #751

Author: lukaszsamson

apps/language_server/lib/language_server/cli.ex

@@ -38,6 +38,8 @@ defmodule ElixirLS.LanguageServer.CLI do
       "Running on elixir #{versions.current_elixir_version} on OTP #{versions.current_otp_version}"
     )
 
+    check_otp_doc_chunks()
+
     Launch.limit_num_schedulers()
 
     Mix.shell(ElixirLS.LanguageServer.MixShell)
@@ -82,4 +84,11 @@ defmodule ElixirLS.LanguageServer.CLI do
         raise message
     end
   end
+
+  def check_otp_doc_chunks() do
+    if match?({:error, _}, Code.fetch_docs(:erlang)) do
+      JsonRpc.show_message(:warning, "OTP compiled without EEP48 documentation chunks")
+      Logger.warn("OTP compiled without EEP48 documentation chunks. Language features for erlang modules will run in limited mode. Please reinstall or rebuild OTP with approperiate flags.")
+    end
+  end
 end