Auto-pin variables when replacing blanks in assert_receive

lib/blanks.ex

@@ -7,10 +7,25 @@ defmodule Blanks do
     |> elem(0)
   end
 
+  defp pre({:assert_receive, _, args} = node, replacements) do
+    {args, replacements} = Macro.prewalk(args, replacements, &pre_pin/2)
+    {put_elem(node, 2, args), replacements}
+  end
   defp pre(:___, [first | remainder]), do: {first, remainder}
   defp pre({:___, _, _}, [first | remainder]), do: {first, remainder}
   defp pre(node, acc), do: {node, acc}
 
+  defp pre_pin(:___, [first | remainder]), do: {pin(first), remainder}
+  defp pre_pin({:___, _, _}, [first | remainder]), do: {pin(first), remainder}
+  defp pre_pin(node, acc), do: {node, acc}
+
+  defp pin(var) when is_tuple(var) do
+    quote do
+      ^unquote(var)
+    end
+  end
+  defp pin(var), do: var
+
   def count(ast) do
     ast
     |> Macro.prewalk(0, &count/2)

lib/koans/10_processes.ex

@@ -13,7 +13,7 @@ defmodule Processes do
 
   koan "You can send messages to any process you want" do
     send self(), "hola!"
-    assert_receive ^___
+    assert_receive ___
   end
 
   koan "A common pattern is to include the sender in the message" do
@@ -24,7 +24,7 @@ defmodule Processes do
                  end)
 
     send pid, {:hello, self()}
-    assert_receive ^___
+    assert_receive ___
   end
 
   koan "Waiting for a message can get boring" do
@@ -35,7 +35,7 @@ defmodule Processes do
                 end
            end)
 
-    assert_receive ^___
+    assert_receive ___
   end
 
   koan "Killing a process will terminate it" do
@@ -64,7 +64,7 @@ defmodule Processes do
     wait()
     Process.exit(pid, :random_reason)
 
-    assert_receive ^___
+    assert_receive ___
   end
 
   koan "Trying to quit normally has no effect" do
@@ -96,7 +96,7 @@ defmodule Processes do
             end
      end)
 
-    assert_receive ^___
+    assert_receive ___
   end
 
   koan "If you monitor your children, you'll be automatically informed for their depature" do
@@ -108,7 +108,7 @@ defmodule Processes do
             end
      end)
 
-    assert_receive ^___
+    assert_receive ___
   end
 
   def wait do

test/blanks_test.exs

@@ -24,6 +24,16 @@ defmodule BlanksTest do
     assert Blanks.replace(ast, [true, false]) == quote(do: assert true == false)
   end
 
+  test "pins variables in assert_receive replacement" do
+    ast = quote do: assert_receive ___
+    assert Blanks.replace(ast, Macro.var(:answer, __MODULE__)) == quote(do: assert_receive ^answer)
+  end
+
+  test "does not pin values in assert_receive replacement" do
+    ast = quote do: assert_receive ___
+    assert Blanks.replace(ast, :lolwat) == quote(do: assert_receive :lolwat)
+  end
+
   test "counts simple blanks" do
     ast = quote do: 1 + ___