Introduce full_cause_backtrace configuration

By default, RSpec only prints the first line of a 'caused by' backtrace. This commit introduces a new 'full_cause_backtrace' config option which will enable printing the entire cause backtrace.

lib/rspec/core/configuration.rb

@@ -458,6 +458,12 @@ def shared_context_metadata_behavior=(value)
       # return [Integer]
       add_setting :max_displayed_failure_line_count
 
+      # @macro full_cause_backtrace
+      # Display the full backtrace of causing exceptions
+      # (default false).
+      # return [Boolean]
+      add_setting :full_cause_backtrace
+
       # @macro add_setting
       # Format the output for pending examples. Can be set to:
       #  - :full (default) - pending examples appear similarly to failures
@@ -571,6 +577,7 @@ def initialize
         @derived_metadata_blocks = FilterableItemRepository::QueryOptimized.new(:any?)
         @threadsafe = true
         @max_displayed_failure_line_count = 10
+        @full_cause_backtrace = false
         @world = World::Null
         @shared_context_metadata_behavior = :trigger_inclusion
         @pending_failure_output = :full

lib/rspec/core/formatters/exception_presenter.rb

@@ -56,7 +56,12 @@ def formatted_cause(exception)
               end
 
               unless last_cause.backtrace.nil? || last_cause.backtrace.empty?
-                cause << ("  #{backtrace_formatter.format_backtrace(last_cause.backtrace, example.metadata).first}")
+                lines = backtrace_formatter.format_backtrace(last_cause.backtrace, example.metadata)
+                lines = [lines[0]] unless RSpec.configuration.full_cause_backtrace # rubocop:disable Metrics/BlockNesting
+
+                lines.each do |line|
+                  cause << ("  #{line}")
+                end
               end
             end
 

spec/rspec/core/formatters/exception_presenter_spec.rb

@@ -181,7 +181,7 @@ def initialize(message, backtrace = [], cause = nil)
       end
 
       caused_by_line_num = __LINE__ + 1
-      let(:first_exception) { FakeException.new("Real\nculprit", ["#{__FILE__}:#{__LINE__}"]) }
+      let(:first_exception) { FakeException.new("Real\nculprit", ["#{__FILE__}:#{__LINE__}", "#{__FILE__}:#{__LINE__}"]) }
 
       it 'includes the first exception that caused the failure', :if => RSpec::Support::RubyFeatures.supports_exception_cause? do
         the_presenter = Formatters::ExceptionPresenter.new(the_exception, example)
@@ -202,6 +202,32 @@ def initialize(message, backtrace = [], cause = nil)
         EOS
       end
 
+      context 'with RSpec.configuration.full_cause_backtrace enabled' do
+        before do
+          RSpec.configuration.full_cause_backtrace = true
+        end
+
+        it 'prints full cause backtrace', :if => RSpec::Support::RubyFeatures.supports_exception_cause? do
+          the_presenter = Formatters::ExceptionPresenter.new(the_exception, example)
+
+          expect(the_presenter.fully_formatted(1)).to eq(<<-EOS.gsub(/^ +\|/, ''))
+            |
+            |  1) Example
+            |     Failure/Error: # The failure happened here!#{ encoding_check }
+            |
+            |       Boom
+            |       Bam
+            |     # ./spec/rspec/core/formatters/exception_presenter_spec.rb:#{line_num}
+            |     # ------------------
+            |     # --- Caused by: ---
+            |     #   Real
+            |     #   culprit
+            |     #   ./spec/rspec/core/formatters/exception_presenter_spec.rb:#{caused_by_line_num}
+            |     #   ./spec/rspec/core/formatters/exception_presenter_spec.rb:#{caused_by_line_num}
+          EOS
+        end
+      end
+
       context "when the first exception doesn't have a backgrace" do
         let(:first_exception) { FakeException.new("Real\nculprit", backtrace) }