Fix MixEnv for module attributes

Refs #907

lib/credo/check/warning/mix_env.ex

@@ -49,6 +49,18 @@ defmodule Credo.Check.Warning.MixEnv do
   defp matches?(directory, %Regex{} = regex), do: Regex.match?(regex, directory)
   defp matches?(directory, path) when is_binary(path), do: String.starts_with?(directory, path)
 
+  defp traverse(
+         {:@, _,
+          [
+            {name, _, args}
+          ]},
+         issues,
+         _issue_meta
+       )
+       when is_atom(name) and is_list(args) do
+    {nil, issues}
+  end
+
   defp traverse(
          {{:., _, [{:__aliases__, _, [:Mix]}, :env]}, meta, _arguments} = ast,
          issues,

test/credo/check/warning/mix_env_test.exs

@@ -59,6 +59,21 @@ defmodule Credo.Check.Warning.MixEnvTest do
     |> refute_issues()
   end
 
+  test "it should NOT report on instance in module attributes" do
+    """
+    defmodule CredoSampleModule do
+      @myvar Mix.env() == :test
+
+      def test do
+        @myvar
+      end
+    end
+    """
+    |> to_source_file()
+    |> run_check(@described_check)
+    |> refute_issues()
+  end
+
   #
   # cases raising issues
   #