assert_equal.diff

diff -r old/lib/minitest/assertions.rb new/lib/minitest/assertions.rb
--- old/lib/minitest/assertions.rb
+++ new/lib/minitest/assertions.rb
@@ -212,8 +212,12 @@
     # See also: Minitest::Assertions.diff
 
     def assert_equal exp, act, msg = nil
-      msg = message(msg, E) { diff exp, act }
-      result = assert exp == act, msg
+      begin
+        result = assert exp == act
+      rescue Assertion
+        msg = message(msg, E) { diff exp, act }
+        raise Minitest::Assertion, msg.call
+      end
 
       if nil == exp then
         if Minitest::VERSION >= "6" then
@@ -433,9 +437,7 @@
       rescue SignalException, SystemExit
         raise
       rescue Exception => e
-        flunk proc {
-          exception_details(e, "#{msg}#{mu_pp exp} exception expected, not")
-        }
+        flunk exception_details(e, "#{msg}#{mu_pp exp} exception expected, not")
       end
 
       exp = exp.first if exp.size == 1