Emit warnings for unused match vars in assertions, closes #9787

Author: josevalim

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -352,7 +352,7 @@ defmodule ExUnit.Assertions do
           case right do
             unquote(left) ->
               unquote(check)
-              unquote(vars)
+              unquote(mark_as_generated(vars))
 
             _ ->
               left = unquote(Macro.escape(left))
@@ -477,7 +477,7 @@ defmodule ExUnit.Assertions do
       quote do
         case message do
           unquote(pattern) ->
-            _ = unquote(vars)
+            _ = unquote(mark_as_generated(vars))
             true
 
           _ ->
@@ -516,7 +516,7 @@ defmodule ExUnit.Assertions do
 
       {received, unquote(vars)} =
         receive do
-          unquote(pattern) -> {received, unquote(vars)}
+          unquote(pattern) -> {received, unquote(mark_as_generated(vars))}
         after
           timeout -> flunk(unquote(failure_message))
         end
@@ -651,14 +651,18 @@ defmodule ExUnit.Assertions do
         {[expanded], acc}
 
       {name, meta, context}, acc when is_atom(name) and is_atom(context) ->
-        {:ok, [{name, [generated: true] ++ meta, context} | acc]}
+        {:ok, [{name, meta, context} | acc]}
 
       node, acc ->
         {node, acc}
     end)
     |> elem(1)
   end
 
+  defp mark_as_generated(vars) do
+    for {name, meta, context} <- vars, do: {name, [generated: true] ++ meta, context }
+  end
+
   @doc false
   def __expand_pattern__({:when, meta, [left, right]}, caller) do
     left = prewalk_expand_pattern(left, Macro.Env.to_match(caller))

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -155,6 +155,23 @@ defmodule ExUnit.AssertionsTest do
     {2, 1} = assert {2, 1} = Value.tuple()
   end
 
+  test "assert match with unused var" do
+    assert ExUnit.CaptureIO.capture_io(:stderr, fn ->
+      Code.eval_string("""
+      defmodule ExSample do
+        import ExUnit.Assertions
+
+        def run do
+          {2, 1} = assert {2, var} = ExUnit.AssertionsTest.Value.tuple()
+        end
+      end
+      """)
+    end) =~ "variable \"var\" is unused"
+  after
+    :code.delete(ExSample)
+    :code.purge(ExSample)
+  end
+
   test "assert match expands argument in match context" do
     {x, y, z} = {1, 2, 3}
     assert vec(x: ^x, y: ^y) = vec(x: x, y: y, z: z)
@@ -308,7 +325,7 @@ defmodule ExUnit.AssertionsTest do
     timeout = ok(1)
 
     try do
-      assert_receive {~l(a)}, timeout
+      assert_receive {~l(_a)}, timeout
     rescue
       error in [ArgumentError] ->
         "timeout must be a non-negative integer, got: {:ok, 1}" = error.message