Display full backtrace when every line filtered.

Restores previous behavior and adds a message to the backtrace
explaining that we're showing the full backtrace because every line was
filtered.

lib/rspec/core/backtrace_formatter.rb

@@ -37,7 +37,16 @@ def format_backtrace(backtrace, options = {})
         backtrace.
           take_while {|l| l != RSpec::Core::Runner::AT_EXIT_HOOK_BACKTRACE_LINE}.
           map        {|l| backtrace_line(l)}.
-          compact
+          compact.
+          tap do |filtered|
+            if filtered.empty?
+              filtered.concat backtrace
+              filtered << ""
+              filtered << "  Showing full backtrace because every line was filtered out."
+              filtered << "  See docs for RSpec::Configuration#backtrace_exclusion_patterns and"
+              filtered << "  RSpec::Configuration#backtrace_inclusion_patterns for more information."
+            end
+          end
       end
 
       # @api private

spec/rspec/core/backtrace_formatter_spec.rb

@@ -61,6 +61,28 @@ def make_backtrace_formatter(exclusion_patterns=nil, inclusion_patterns=nil)
         ]
         expect(BacktraceFormatter.new.format_backtrace(backtrace)).to eq(["./spec/my_spec.rb:7", "./spec/my_spec.rb:5"])
       end
+
+      context "when every line is filtered out" do
+        it "includes full backtrace" do
+          backtrace = [
+            "/path/to/rspec-expectations/lib/rspec/expectations/foo.rb:37",
+            "/path/to/rspec-expectations/lib/rspec/matchers/foo.rb:37",
+            "/path/to/rspec-mocks/lib/rspec/mocks/foo.rb:37",
+            "/path/to/rspec-core/lib/rspec/core/foo.rb:37"
+          ]
+          expect(BacktraceFormatter.new.format_backtrace(backtrace).take(4)).to eq backtrace
+        end
+
+        it "adds a message explaining everything was filtered" do
+          backtrace = [
+            "/path/to/rspec-expectations/lib/rspec/expectations/foo.rb:37",
+            "/path/to/rspec-expectations/lib/rspec/matchers/foo.rb:37",
+            "/path/to/rspec-mocks/lib/rspec/mocks/foo.rb:37",
+            "/path/to/rspec-core/lib/rspec/core/foo.rb:37"
+          ]
+          expect(BacktraceFormatter.new.format_backtrace(backtrace).drop(4).join).to match(/Showing full backtrace/)
+        end
+      end
     end
 
     describe "#full_backtrace=true" do