Make assert_raise fail with broken message/1 implementation

Until now, all errors would be swallowed in `Exception.message/1` and would
never reach the user.

Author: michalmuskala

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -628,6 +628,7 @@ defmodule ExUnit.Assertions do
 
         cond do
           name == exception ->
+            check_error_message(name, error)
             error
           name == ExUnit.AssertionError ->
             reraise(error, stacktrace)
@@ -639,6 +640,15 @@ defmodule ExUnit.Assertions do
     end
   end
 
+  defp check_error_message(module, error) do
+    module.message(error)
+  catch
+    kind, reason ->
+      stacktrace = System.stacktrace()
+
+      flunk "Got exception #{inspect module} but it failed to produce a message with:\n\n" <> Exception.format(kind, reason, stacktrace)
+  end
+
   @doc """
   Asserts that `value1` and `value2` differ by no more than `delta`.
 

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -6,7 +6,15 @@ defmodule ExUnit.AssertionsTest.Value do
   def truthy, do: true
 end
 
-alias ExUnit.AssertionsTest.Value
+defmodule ExUnit.AssertionsTest.BrokenError do
+  defexception [:message]
+
+  def message(_) do
+    raise "error"
+  end
+end
+
+alias ExUnit.AssertionsTest.{BrokenError, Value}
 
 defmodule ExUnit.AssertionsTest do
   use ExUnit.Case, async: true
@@ -548,6 +556,16 @@ defmodule ExUnit.AssertionsTest do
       "\n  \"bar\"" = error.message
   end
 
+  test "assert raise with an exception with bad message/1 implementation" do
+    assert_raise BrokenError, fn ->
+      raise BrokenError
+    end
+  rescue
+    error in [ExUnit.AssertionError] ->
+      "Got exception ExUnit.AssertionsTest.BrokenError but it failed to produce a message with:" <>
+      "\n\n** (RuntimeError) error\n" <> _ = error.message
+  end
+
   test "assert greater than operator" do
     true = assert 2 > 1
   end