commands/wtf,pry: add support for Exception#cause

Fixes #1449 (support exceptions with cause)
Replaces #1525

lib/pry.rb

@@ -46,6 +46,15 @@ class Pry
     else
       output.puts "#{exception.class}: #{exception.message}"
       output.puts "from #{exception.backtrace.first}"
+
+      if exception.respond_to? :cause
+        cause = exception.cause
+        while cause
+          output.puts "Caused by #{cause.class}: #{cause}\n"
+          output.puts "from #{cause.backtrace.first}"
+          cause = cause.cause
+        end
+      end
     end
   end
 

lib/pry/commands/wtf.rb

@@ -32,6 +32,19 @@ class Pry
       else
         output.puts with_line_numbers(backtrace.first(size_of_backtrace))
       end
+
+      if exception.respond_to? :cause
+        cause = exception.cause
+        while cause
+          output.puts "#{text.bold('Caused by:')} #{cause.class}: #{cause}\n--"
+          if opts.verbose?
+            output.puts with_line_numbers(cause.backtrace)
+          else
+            output.puts with_line_numbers(cause.backtrace.first(size_of_backtrace))
+          end
+          cause = cause.cause
+        end
+      end
     end
 
     private

spec/commands/wtf_spec.rb

@@ -0,0 +1,39 @@
+require_relative '../helper'
+
+describe "wtf?!" do
+  let(:tester) do
+    pry_tester do
+      def last_exception=(ex)
+        @pry.last_exception = ex
+      end
+
+      def last_exception
+        @pry.last_exception
+      end
+    end
+  end
+
+  it "unwinds nested exceptions" do
+    if Gem::Version.new(RUBY_VERSION) <= Gem::Version.new('2.0.0')
+      skip('Exception#cause is not supported')
+    end
+
+    begin
+      begin
+        begin
+          raise 'inner'
+        rescue RuntimeError
+          raise 'outer'
+        end
+      end
+    rescue RuntimeError => ex
+      tester.last_exception = ex
+    end
+
+    expect(tester.eval('wtf -v')).to match(/
+      Exception:\sRuntimeError:\souter
+      .+
+      Caused\sby:\sRuntimeError:\sinner
+    /xm)
+  end
+end