Do not leak external variables used in guards in assert_receive

Author: José Valim

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -427,6 +427,12 @@ defmodule ExUnit.Assertions do
     Enum.uniq_by(pins, &elem(&1, 0))
   end
 
+  defp collect_vars_from_pattern({:when, _, [left, right]}) do
+    pattern = collect_vars_from_pattern(left)
+    guards = for var <- collect_vars_from_pattern(right), var in pattern, do: var
+    pattern ++ guards
+  end
+
   defp collect_vars_from_pattern(expr) do
     Macro.prewalk(expr, [], fn
       {:::, _, [left, _]}, acc ->

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -266,6 +266,13 @@ defmodule ExUnit.AssertionsTest do
     :world = world
   end
 
+  test "assert received does not leak external variables used in guards" do
+    send self, {:hello, :world}
+    guard_world = :world
+    assert_received {:hello, world} when world == guard_world
+    :world = world
+  end
+
   test "refute received does not wait" do
     false = refute_received :hello
   end