For recent Ruby, show nested errors properly in backtraces

lib/minitest.rb

@@ -836,8 +836,24 @@ def error # :nodoc:
     end
 
     def message # :nodoc:
-      bt = Minitest.filter_backtrace(self.backtrace).join "\n    "
-      "#{self.exception.class}: #{self.exception.message}\n    #{bt}"
+      if self.exception.respond_to?(:cause) && self.exception.cause
+        # Build a chain of exception causes starting from the outermost
+        exc = self.exception
+        cause_chain = []
+        while exc
+          cause_chain.push(exc)
+          exc = exc.cause
+        end
+
+        bt_lines = cause_chain.map { |c|
+          [c.message] + Minitest.filter_backtrace(c.backtrace)
+        }.inject() { |acc, bt| acc + ["...Caused By..."] + bt }
+        bt_out = bt_lines.join "\n    "
+        "#{self.exception.class}: #{self.exception.message}\n    #{bt_out}"
+      else
+        bt = Minitest.filter_backtrace(self.backtrace).join "\n    "
+        "#{self.exception.class}: #{self.exception.message}\n    #{bt}"
+      end
     end
 
     def result_label # :nodoc:

test/minitest/test_minitest_test.rb

@@ -263,6 +263,40 @@ def test_error
     assert_report expected
   end
 
+  def test_run_nested_error
+    @tu =
+    Class.new FakeNamedTest do
+      def test_something
+        assert true
+      end
+
+      def test_error
+        begin
+          raise "internal exception"
+        rescue
+          raise "external exception"
+        end
+      end
+    end
+
+    expected = clean <<-EOM
+      E.
+
+      Finished in 0.00
+
+        1) Error:
+      FakeNamedTestXX#test_error:
+      RuntimeError: external exception
+          FILE:LINE:in \`rescue in test_error\'
+          FILE:LINE:in \`test_error\'
+          FILE:LINE:in \`test_error\'
+
+      2 runs, 1 assertions, 0 failures, 1 errors, 0 skips
+    EOM
+
+    assert_report expected
+  end
+
   def test_run_error_teardown
     @tu =
     Class.new FakeNamedTest do