Add source extract to detailed exception page

actionpack/lib/action_dispatch/middleware/debug_exceptions.rb

@@ -42,7 +42,10 @@ module ActionDispatch
           :application_trace => wrapper.application_trace,
           :framework_trace => wrapper.framework_trace,
           :full_trace => wrapper.full_trace,
-          :routes => formatted_routes(exception)
+          :routes => formatted_routes(exception),
+          :source_extract => wrapper.source_extract,
+          :line_number => wrapper.line_number,
+          :file => wrapper.file
         )
 
         file = "rescues/#{wrapper.rescue_template}"

actionpack/lib/action_dispatch/middleware/exception_wrapper.rb

@@ -25,7 +25,7 @@ module ActionDispatch
       'ActionView::Template::Error'         => 'template_error'
     )
 
-    attr_reader :env, :exception
+    attr_reader :env, :exception, :line_number, :file
 
     def initialize(env, exception)
       @env = env
@@ -56,6 +56,15 @@ module ActionDispatch
       Rack::Utils.status_code(@@rescue_responses[class_name])
     end
 
+    def source_extract
+      if trace = application_trace.first
+        file, line, _ = trace.split(":")
+        @file = file
+        @line_number = line.to_i
+        source_fragment(@file, @line_number)
+      end
+    end
+
     private
 
     def original_exception(exception)
@@ -81,5 +90,16 @@ module ActionDispatch
     def backtrace_cleaner
       @backtrace_cleaner ||= @env['action_dispatch.backtrace_cleaner']
     end
+
+    def source_fragment(path, line)
+      full_path = Rails.root.join(path)
+      if File.exists?(full_path)
+        File.open(full_path, "r") do |file|
+          start = [line - 3, 0].max
+          lines = file.lines.drop(start).take(6)
+          Hash[*(start+1..(lines.count+start)).zip(lines).flatten]
+        end
+      end
+    end
   end
 end

actionpack/lib/action_dispatch/middleware/templates/rescues/_source.erb

@@ -0,0 +1,8 @@
+<% if @source_extract %>
+Extracted source (around line <strong>#<%= @line_number %></strong>):
+<pre>
+<% @source_extract.each do |line, source| %>
+<%= "#{(@line_number == line) ? "> " : "  "}#{line}: #{source}" -%>
+<% end %>
+</pre>
+<% end %>

actionpack/lib/action_dispatch/middleware/templates/rescues/diagnostics.erb

@@ -6,5 +6,6 @@
 </h1>
 <pre><%=h @exception.message %></pre>
 
+<%= render template: "rescues/_source" %>
 <%= render template: "rescues/_trace" %>
 <%= render template: "rescues/_request_and_response" %>