Wierd problem when an exception is raised inside a template that has a UTF-8 encoded identifier. Using module_eval with an UTF-8 filename as in (https://github.com/rails/rails/blob/master/actionview/lib/action_view/template.rb#L295), will result in ASCII-8Bit entries in the `e.backtrace` array.

backtrace_encoding.rb

module EncodingBug
end

filename = ARGV[0]
puts "using encoding: #{filename.encoding}"

EncodingBug.module_eval "def self.say; raise end", filename, 1

begin
  EncodingBug.say
rescue => e
  frame = e.backtrace.first
  puts "frame: #{frame}"
  puts "backtrace encoding: #{frame.encoding}"
end

output.txt

activesupport :: (master*) » ruby reproduction.rb ä
using encoding: UTF-8
frame: ä:1:in `say'
backtrace encoding: ASCII-8BIT