Fix completion crash on higher unicode characters (#466)

* Fix completion crash on higher unicode characters

* Safely rescue from intellisense errors

lib/livebook/evaluator.ex

@@ -13,6 +13,8 @@ defmodule Livebook.Evaluator do
 
   use GenServer, restart: :temporary
 
+  require Logger
+
   alias Livebook.Evaluator
 
   @type t :: GenServer.server()
@@ -214,7 +216,15 @@ defmodule Livebook.Evaluator do
 
   def handle_cast({:handle_intellisense, send_to, ref, request, evaluation_ref}, state) do
     context = get_context(state, evaluation_ref)
-    response = Livebook.Intellisense.handle_request(request, context.binding, context.env)
+
+    # Safely rescue from intellisense errors
+    response =
+      try do
+        Livebook.Intellisense.handle_request(request, context.binding, context.env)
+      rescue
+        error -> Logger.error(Exception.format(:error, error, __STACKTRACE__))
+      end
+
     send(send_to, {:intellisense_response, ref, response})
 
     {:noreply, state}

lib/livebook/intellisense.ex

@@ -172,6 +172,7 @@ defmodule Livebook.Intellisense do
 
   defp subject_range(line, index) do
     {left, right} = String.split_at(line, index)
+    bytes_until_index = byte_size(left)
 
     left =
       left
@@ -186,7 +187,7 @@ defmodule Livebook.Intellisense do
       |> consume_until(@space ++ @operators ++ @punctuation, @closing_identifier)
       |> List.to_string()
 
-    {index - byte_size(left), index + byte_size(right)}
+    {bytes_until_index - byte_size(left), bytes_until_index + byte_size(right)}
   end
 
   defp consume_until(acc \\ [], chars, stop, stop_include)

test/livebook/intellisense_test.exs

@@ -1064,5 +1064,11 @@ defmodule Livebook.IntellisenseTest do
       assert %{contents: [file_read]} = Intellisense.get_details(":file.read()", 8, binding, env)
       assert file_read =~ "Typical error reasons:"
     end
+
+    test "properly parses unicode" do
+      {binding, env} = eval(do: nil)
+
+      assert nil == Intellisense.get_details("msg = '🍵'", 8, binding, env)
+    end
   end
 end