Raise a helpful error if mneme was not started

.github/workflows/ci.yml

@@ -93,3 +93,5 @@ jobs:
       - run: mix deps.compile
 
       - run: mix test
+
+      - run: mix test.mneme_not_started

lib/mneme/options.ex

@@ -215,7 +215,7 @@ defmodule Mneme.Options do
     |> collect_attributes(Map.get(attrs, @test_attr, []))
     |> collect_attributes(Map.get(attrs, @describe_attr, []))
     |> collect_attributes(Map.get(attrs, @module_attr, []))
-    |> collect_attributes([:persistent_term.get(@config_cache)])
+    |> collect_attributes([persistent_term_get!(@config_cache)])
   end
 
   defp collect_attributes(_), do: %{}
@@ -240,4 +240,11 @@ defmodule Mneme.Options do
 
     Map.merge(acc, new, fn _, vs1, vs2 -> vs1 ++ vs2 end)
   end
+
+  defp persistent_term_get!(key) do
+    :persistent_term.get(key)
+  rescue
+    ArgumentError ->
+      raise "Config key not found (#{inspect(@config_cache)}). Did you start Mneme in test_helper.exs?"
+  end
 end

mix.exs

@@ -76,14 +76,19 @@ defmodule Mneme.MixProject do
       "coveralls.html": [
         &export_integration_coverage/1,
         "coveralls.html --import-cover cover"
+      ],
+      "test.mneme_not_started": [
+        fn _ -> System.put_env("START_MNEME", "false") end,
+        "test --only mneme_not_started test/mneme_not_started_test.exs"
       ]
     ]
   end
 
   defp preferred_cli_env do
     [
       coveralls: :test,
-      "coveralls.html": :test
+      "coveralls.html": :test,
+      "test.mneme_not_started": :test
     ]
   end
 

test/mneme_not_started_test.exs

@@ -0,0 +1,22 @@
+defmodule Mneme.MnemeNotStartedTest do
+  use ExUnit.Case
+  use Mneme
+
+  @tag :mneme_not_started
+  test "error" do
+    assert_raise RuntimeError, ~r/Did you start Mneme/, fn ->
+      code = """
+      defmodule Mneme.StartedTest.ExampleTest do
+        use ExUnit.Case
+        use Mneme
+
+        test "a test" do
+          auto_assert 2 + 2
+        end
+      end
+      """
+
+      assert [{_, _}] = Code.compile_string(code)
+    end
+  end
+end

test/test_helper.exs

@@ -1,2 +1,5 @@
-ExUnit.start()
-Mneme.start()
+ExUnit.start(exclude: [mneme_not_started: true])
+
+if System.get_env("START_MNEME") != "false" do
+  Mneme.start()
+end