Raise if message in AssertionError is not a binary

Closes #14695.

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -38,6 +38,16 @@ defmodule ExUnit.AssertionError do
     @no_value
   end
 
+  @impl true
+  def exception(opts) do
+    with {:ok, message} when not is_binary(message) <- Keyword.fetch(opts, :message) do
+      raise ArgumentError,
+            ":message in ExUnit.Assertion must be a binary, got: #{inspect(message)}"
+    end
+
+    struct(__MODULE__, opts)
+  end
+
   @impl true
   def message(exception) do
     "\n\n" <> ExUnit.Formatter.format_assertion_error(exception)

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -857,6 +857,22 @@ defmodule ExUnit.AssertionsTest do
       "assertion" = error.message
   end
 
+  test "assert operator with custom options" do
+    assert 1 > 2, message: "assertion"
+    flunk("This should never be tested")
+  rescue
+    error in [ExUnit.AssertionError] ->
+      "assertion" = error.message
+  end
+
+  test "assert operator with invalid options" do
+    assert 1 > 2, message: 123
+    flunk("This should never be tested")
+  rescue
+    error in [ArgumentError] ->
+      ":message in ExUnit.Assertion must be a binary, got: 123" = error.message
+  end
+
   test "assert lack of equality" do
     try do
       assert "one" != "one"