Merge pull request #17077 from schneems/schneems/backtrace-string-allocations

Decrease backtrace cleaner object allocations

railties/lib/rails/backtrace_cleaner.rb

@@ -4,12 +4,16 @@ module Rails
   class BacktraceCleaner < ActiveSupport::BacktraceCleaner
     APP_DIRS_PATTERN = /^\/?(app|config|lib|test)/
     RENDER_TEMPLATE_PATTERN = /:in `_render_template_\w*'/
+    EMPTY_STRING = ''.freeze
+    SLASH        = '/'.freeze
+    DOT_SLASH    = './'.freeze
 
     def initialize
       super
-      add_filter   { |line| line.sub("#{Rails.root}/", '') }
-      add_filter   { |line| line.sub(RENDER_TEMPLATE_PATTERN, '') }
-      add_filter   { |line| line.sub('./', '/') } # for tests
+      @root = "#{Rails.root}/".freeze
+      add_filter { |line| line.sub(@root, EMPTY_STRING) }
+      add_filter { |line| line.sub(RENDER_TEMPLATE_PATTERN, EMPTY_STRING) }
+      add_filter { |line| line.sub(DOT_SLASH, SLASH) } # for tests
 
       add_gem_filters
       add_silencer { |line| line !~ APP_DIRS_PATTERN }
@@ -21,7 +25,8 @@ def add_gem_filters
         return if gems_paths.empty?
 
         gems_regexp = %r{(#{gems_paths.join('|')})/gems/([^/]+)-([\w.]+)/(.*)}
-        add_filter { |line| line.sub(gems_regexp, '\2 (\3) \4') }
+        gems_result = '\2 (\3) \4'.freeze
+        add_filter { |line| line.sub(gems_regexp, gems_result) }
       end
   end
 end