Do not expand variables to argless macros in assert

lib/ex_unit/lib/ex_unit/assertions.ex

@@ -98,7 +98,7 @@ defmodule ExUnit.Assertions do
   defmacro assert({:=, _, [left, right]} = assertion) do
     code = escape_quoted(:assert, assertion)
 
-    left = Macro.prewalk(left, &Macro.expand(&1, __CALLER__))
+    left = expand_pattern(left, __CALLER__)
     vars = collect_vars_from_pattern(left)
     pins = collect_pins_from_pattern(left, __CALLER__.vars)
 
@@ -398,7 +398,7 @@ defmodule ExUnit.Assertions do
     binary = Macro.to_string(pattern)
 
     # Expand before extracting metadata
-    pattern = Macro.prewalk(pattern, &Macro.expand(&1, caller))
+    pattern = expand_pattern(pattern, caller)
     vars = collect_vars_from_pattern(pattern)
     pins = collect_pins_from_pattern(pattern, caller.vars)
 
@@ -560,6 +560,16 @@ defmodule ExUnit.Assertions do
     |> elem(1)
   end
 
+  defp expand_pattern(expr, caller) do
+    Macro.prewalk(expr, fn
+      {var, _, context} = node when is_atom(var) and is_atom(context) ->
+        node
+
+      other ->
+        Macro.expand(other, caller)
+    end)
+  end
+
   defp suppress_warning({name, meta, [expr, [do: clauses]]}) do
     clauses =
       Enum.map(clauses, fn {:->, meta, args} ->

lib/ex_unit/test/ex_unit/assertions_test.exs

@@ -20,6 +20,7 @@ defmodule ExUnit.AssertionsTest do
   use ExUnit.Case, async: true
 
   defmacro sigil_l({:<<>>, _, [string]}, _), do: Code.string_to_quoted!(string, [])
+  defmacro argless_macro(), do: raise "should not be invoked"
 
   defmacrop assert_ok(arg) do
     quote do
@@ -85,6 +86,11 @@ defmodule ExUnit.AssertionsTest do
     assert c == 3
   end
 
+  test "assert does not expand variables" do
+    assert argless_macro = 1
+    assert argless_macro == 1
+  end
+
   test "refute when value is false" do
     false = refute false
   end