Provide more information on out of bound errors to aid debugging

Author: josevalim

lib/elixir/lib/access.ex

@@ -807,7 +807,7 @@ defmodule Access do
       iex> get_in([:a, :b, :c], [Access.at!(2)])
       :c
       iex> get_in([:a, :b, :c], [Access.at!(3)])
-      ** (Enum.OutOfBoundsError) out of bounds error
+      ** (Enum.OutOfBoundsError) out of bounds error at position 3 when traversing enumerable [:a, :b, :c]
 
   """
   @doc since: "1.11.0"
@@ -819,12 +819,14 @@ defmodule Access do
   defp at!(:get, data, index, next) when is_list(data) do
     case Enum.fetch(data, index) do
       {:ok, value} -> next.(value)
-      :error -> raise Enum.OutOfBoundsError
+      :error -> raise Enum.OutOfBoundsError, index: index, enumerable: data
     end
   end
 
   defp at!(:get_and_update, data, index, next) when is_list(data) do
-    get_and_update_at(data, index, next, [], fn -> raise Enum.OutOfBoundsError end)
+    get_and_update_at(data, index, next, [], fn ->
+      raise Enum.OutOfBoundsError, index: index, enumerable: data
+    end)
   end
 
   defp at!(_op, data, _index, _next) do

lib/elixir/lib/enum.ex

@@ -1074,14 +1074,14 @@ defmodule Enum do
       6
 
       iex> Enum.fetch!([2, 4, 6], 4)
-      ** (Enum.OutOfBoundsError) out of bounds error
+      ** (Enum.OutOfBoundsError) out of bounds error at position 4 when traversing enumerable [2, 4, 6]
 
   """
   @spec fetch!(t, index) :: element
   def fetch!(enumerable, index) when is_integer(index) do
     case slice_forward(enumerable, index, 1, 1) do
       [value] -> value
-      [] -> raise Enum.OutOfBoundsError
+      [] -> raise Enum.OutOfBoundsError, index: index, enumerable: enumerable
     end
   end
 

lib/elixir/lib/exception.ex

@@ -2216,7 +2216,25 @@ defmodule Enum.OutOfBoundsError do
   For example, this is raised by `Access.at!/1`.
   """
 
-  defexception message: "out of bounds error"
+  defexception [:enumerable, :index, :message]
+
+  @impl true
+  def message(exception = %{message: nil}), do: message(exception.index, exception.enumerable)
+  def message(%{message: message}), do: message
+
+  def message(index, enumerable) do
+    "out of bounds error" <>
+      if index do
+        " at position #{index}"
+      else
+        ""
+      end <>
+      if enumerable do
+        " when traversing enumerable #{inspect(enumerable)}"
+      else
+        ""
+      end
+  end
 end
 
 defmodule Enum.EmptyError do