Prevent malformed exception messages from exiting RSpec

Changelog.md

@@ -19,6 +19,8 @@ Bug Fixes:
 * Predicates for pending examples, (in `RSpec::Core::Example`, `#pending?`, `#skipped?` and
   `#pending_fixed?`) now return boolean values rather than truthy values.
   (Marc-André Lafortune, #2756, #2758)
+* Exceptions which have a message which cannot be cast to a string will no longer
+  cause a crash. (Jon Rowe, #2761)
 
 ### 3.9.2 / 2020-05-02
 [Full Changelog](http://github.com/rspec/rspec-core/compare/v3.9.1...v3.9.2)

lib/rspec/core/formatters/exception_presenter.rb

@@ -51,7 +51,7 @@ def formatted_cause(exception)
               cause << '--- Caused by: ---'
               cause << "#{exception_class_name(last_cause)}:" unless exception_class_name(last_cause) =~ /RSpec/
 
-              encoded_string(last_cause.message.to_s).split("\n").each do |line|
+              encoded_string(exception_message_string(last_cause)).split("\n").each do |line|
                 cause << "  #{line}"
               end
 
@@ -174,11 +174,19 @@ def failure_slash_error_lines
           lines
         end
 
+        # rubocop:disable Lint/RescueException
+        def exception_message_string(exception)
+          exception.message.to_s
+        rescue Exception => other
+          "A #{exception.class} for which `exception.message.to_s` raises #{other.class}."
+        end
+        # rubocop:enable Lint/RescueException
+
         def exception_lines
           @exception_lines ||= begin
             lines = []
             lines << "#{exception_class_name}:" unless exception_class_name =~ /RSpec/
-            encoded_string(exception.message.to_s).split("\n").each do |line|
+            encoded_string(exception_message_string(exception)).split("\n").each do |line|
               lines << (line.empty? ? line : "  #{line}")
             end
             lines

spec/rspec/core/formatters/exception_presenter_spec.rb

@@ -260,6 +260,32 @@ def initialize(message, backtrace = [], cause = nil)
         EOS
       end
 
+      it 'will work then the message to_s raises a looped exception' do
+        raising_to_s_klass =
+          Class.new do
+            def to_s
+              raise StandardError, self
+            end
+          end
+
+        if RSpec::Support::Ruby.jruby?
+          expected_error = Java::JavaLang::StackOverflowError
+        else
+          expected_error = StandardError
+        end
+
+        incorrect_message_exception = FakeException.new(raising_to_s_klass.new, [])
+
+        the_presenter = Formatters::ExceptionPresenter.new(incorrect_message_exception, example)
+
+        expect(the_presenter.fully_formatted(1)).to eq(<<-EOS.gsub(/^ +\|/, ''))
+          |
+          |  1) Example
+          |     Failure/Error: Unable to find matching line from backtrace
+          |       A #{FakeException} for which `exception.message.to_s` raises #{expected_error}.
+        EOS
+      end
+
       it "adds extra failure lines from the example metadata" do
         extra_example = example.clone
         failure_line = 'http://www.example.com/job_details/123'