1 files changed, 99 insertions, 0 deletions
diff --git a/actionpack/lib/action_view/template/error.rb b/actionpack/lib/action_view/template/error.rb
new file mode 100644
index 0000000000..37cb1c7c6c
--- /dev/null
+++ b/actionpack/lib/action_view/template/error.rb
@@ -0,0 +1,99 @@
+module ActionView
+  # The TemplateError exception is raised when the compilation of the template fails. This exception then gathers a
+  # bunch of intimate details and uses it to report a very precise exception message.
+  class TemplateError < ActionViewError #:nodoc:
+    SOURCE_CODE_RADIUS = 3
+
+    attr_reader :original_exception
+
+    def initialize(template, assigns, original_exception)
+      @template, @assigns, @original_exception = template, assigns.dup, original_exception
+      @backtrace = compute_backtrace
+    end
+
+    def file_name
+      @template.relative_path
+    end
+
+    def message
+      ActiveSupport::Deprecation.silence { original_exception.message }
+    end
+
+    def clean_backtrace
+      if defined?(Rails) && Rails.respond_to?(:backtrace_cleaner)
+        Rails.backtrace_cleaner.clean(original_exception.backtrace)
+      else
+        original_exception.backtrace
+      end
+    end
+
+    def sub_template_message
+      if @sub_templates
+        "Trace of template inclusion: " +
+        @sub_templates.collect { |template| template.relative_path }.join(", ")
+      else
+        ""
+      end
+    end
+
+    def source_extract(indentation = 0)
+      return unless num = line_number
+      num = num.to_i
+
+      source_code = @template.source.split("\n")
+
+      start_on_line = [ num - SOURCE_CODE_RADIUS - 1, 0 ].max
+      end_on_line   = [ num + SOURCE_CODE_RADIUS - 1, source_code.length].min
+
+      indent = ' ' * indentation
+      line_counter = start_on_line
+      return unless source_code = source_code[start_on_line..end_on_line]
+
+      source_code.sum do |line|
+        line_counter += 1
+        "#{indent}#{line_counter}: #{line}\n"
+      end
+    end
+
+    def sub_template_of(template_path)
+      @sub_templates ||= []
+      @sub_templates << template_path
+    end
+
+    def line_number
+      @line_number ||=
+        if file_name
+          regexp = /#{Regexp.escape File.basename(file_name)}:(\d+)/
+
+          $1 if message =~ regexp or clean_backtrace.find { |line| line =~ regexp }
+        end
+    end
+
+    def to_s
+      "\n#{self.class} (#{message}) #{source_location}:\n" + 
+      "#{source_extract}\n    #{clean_backtrace.join("\n    ")}\n\n"
+    end
+
+    # don't do anything nontrivial here. Any raised exception from here becomes fatal 
+    # (and can't be rescued).
+    def backtrace
+      @backtrace
+    end
+
+    private
+      def compute_backtrace
+        [
+          "#{source_location.capitalize}\n\n#{source_extract(4)}\n    " +
+          clean_backtrace.join("\n    ")
+        ]
+      end
+
+      def source_location
+        if line_number
+          "on line ##{line_number} of "
+        else
+          'in '
+        end + file_name
+      end
+  end
+end
+\ No newline at end of file