Support ellipsis in doctest exceptions

lib/ex_unit/lib/ex_unit/doc_test.ex

@@ -1,5 +1,5 @@
 defmodule ExUnit.DocTest do
-  @moduledoc """
+  @moduledoc ~S"""
   Extract test cases from the documentation.
 
   Doctests allow us to generate tests from code examples found
@@ -131,7 +131,7 @@ defmodule ExUnit.DocTest do
 
   You can also showcase expressions raising an exception, for example:
 
-      iex(1)> raise "some error"
+      iex> raise "some error"
       ** (RuntimeError) some error
 
   Doctest will look for a line starting with `** (` and it will parse it
@@ -141,6 +141,19 @@ defmodule ExUnit.DocTest do
   Therefore, it is possible to match on multiline messages as long as there
   are no empty lines on the message itself.
 
+  Asserting on the full exception message might not be possible because it is
+  non-deterministic, or it might result in brittle tests if the exact message
+  changes and gets more detailed.
+  Since Elixir 1.19.0, doctests allow the use of an ellipsis (`...`) at the
+  end of messages:
+
+      iex> raise "some error in pid: #{inspect(self())}"
+      ** (RuntimeError) some error in pid: ...
+
+      iex> raise "some error in pid:\n#{inspect(self())}"
+      ** (RuntimeError) some error in pid:
+      ...
+
   ## When not to use doctest
 
   In general, doctests are not recommended when your code examples contain
@@ -565,7 +578,7 @@ defmodule ExUnit.DocTest do
               "Doctest failed: expected exception #{inspect(exception)} but got " <>
                 "#{inspect(actual_exception)} with message #{inspect(actual_message)}"
 
-            actual_message != message ->
+            not error_message_matches?(actual_message, message) ->
               "Doctest failed: wrong message for #{inspect(actual_exception)}\n" <>
                 "expected:\n" <>
                 "  #{inspect(message)}\n" <>
@@ -588,6 +601,17 @@ defmodule ExUnit.DocTest do
     end
   end
 
+  defp error_message_matches?(actual, expected) when actual == expected, do: true
+
+  defp error_message_matches?(actual, expected) do
+    if String.ends_with?(expected, "...") do
+      ellipsis_removed = binary_slice(expected, 0..-4//1)
+      String.starts_with?(actual, ellipsis_removed)
+    else
+      false
+    end
+  end
+
   defp test_import(_mod, false), do: []
   defp test_import(mod, _), do: [quote(do: import(unquote(mod)))]
 

lib/ex_unit/test/ex_unit/doc_test_test.exs

@@ -548,6 +548,34 @@ defmodule ExUnit.DocTestTest.PatternMatching do
 end
 |> ExUnit.BeamHelpers.write_beam()
 
+defmodule ExUnit.DocTestTest.Ellipsis do
+  @doc """
+      iex> ExUnit.DocTestTest.Ellipsis.same_line_err(self())
+      ** (ArgumentError) Unexpected: ...
+  """
+  def same_line_err(arg) do
+    raise ArgumentError, message: "Unexpected: #{inspect(arg)}"
+  end
+
+  @doc """
+      iex> ExUnit.DocTestTest.Ellipsis.multi_line_err(self())
+      ** (ArgumentError) Unexpected:
+      ...
+  """
+  def multi_line_err(arg) do
+    raise ArgumentError, message: "Unexpected:\n#{inspect(arg)}"
+  end
+
+  @doc """
+      iex> ExUnit.DocTestTest.Ellipsis.triple_dot_err(:foo)
+      ** (ArgumentError) :foo ...
+  """
+  def triple_dot_err(arg) do
+    raise ArgumentError, message: "#{inspect(arg)} ..."
+  end
+end
+|> ExUnit.BeamHelpers.write_beam()
+
 defmodule ExUnit.DocTestTest do
   use ExUnit.Case
 
@@ -574,6 +602,8 @@ defmodule ExUnit.DocTestTest do
   doctest ExUnit.DocTestTest.HaikuIndent4UsingInspectOpts,
     inspect_opts: [custom_options: [indent: 4]]
 
+  doctest ExUnit.DocTestTest.Ellipsis
+
   import ExUnit.CaptureIO
 
   test "multiple functions filtered with :only" do