Freeze string literals when not mutated.

I wrote a utility that helps find areas where you could optimize your program using a frozen string instead of a string literal, it's called [let_it_go](https://github.com/schneems/let_it_go). After going through the output and adding `.freeze` I was able to eliminate the creation of 1,114 string objects on EVERY request to [codetriage](codetriage.com). How does this impact execution?

To look at memory:

```ruby
require 'get_process_mem'

mem = GetProcessMem.new
GC.start
GC.disable
1_114.times { " " }
before = mem.mb

after = mem.mb
GC.enable
puts "Diff: #{after - before} mb"

```

Creating 1,114 string objects results in `Diff: 0.03125 mb` of RAM allocated on every request. Or 1mb every 32 requests.

To look at raw speed:

```ruby
require 'benchmark/ips'

number_of_objects_reduced = 1_114

Benchmark.ips do |x|
  x.report("freeze")    { number_of_objects_reduced.times { " ".freeze } }
  x.report("no-freeze") { number_of_objects_reduced.times { " " } }
end
```

We get the results

```
Calculating -------------------------------------
              freeze     1.428k i/100ms
           no-freeze   609.000  i/100ms
-------------------------------------------------
              freeze     14.363k (± 8.5%) i/s -     71.400k
           no-freeze      6.084k (± 8.1%) i/s -     30.450k
```

Now we can do some maths:

```ruby
ips = 6_226k # iterations / 1 second
call_time_before = 1.0 / ips # seconds per iteration 

ips = 15_254 # iterations / 1 second
call_time_after = 1.0 / ips # seconds per iteration 

diff = call_time_before - call_time_after

number_of_objects_reduced * diff * 100

# => 0.4530373333993266 miliseconds saved per request
```

So we're shaving off 1 second of execution time for every 220 requests. 

Is this going to be an insane speed boost to any Rails app: nope. Should we merge it: yep. 

p.s. If you know of a method call that doesn't modify a string input such as [String#gsub](https://github.com/schneems/let_it_go/blob/b0e2da69f0cca87ab581022baa43291cdf48638c/lib/let_it_go/core_ext/string.rb#L37) please [give me a pull request to the appropriate file](https://github.com/schneems/let_it_go/blob/b0e2da69f0cca87ab581022baa43291cdf48638c/lib/let_it_go/core_ext/string.rb#L37), or open an issue in LetItGo so we can track and freeze more strings. 

Keep those strings Frozen

![](https://www.dropbox.com/s/z4dj9fdsv213r4v/let-it-go.gif?dl=1)

6 files changed, 13 insertions, 13 deletions

diff --git a/actionpack/lib/action_dispatch/http/parameter_filter.rb b/actionpack/lib/action_dispatch/http/parameter_filter.rb
index 6e058b829e..e826551f4b 100644
--- a/actionpack/lib/action_dispatch/http/parameter_filter.rb
+++ b/actionpack/lib/action_dispatch/http/parameter_filter.rb
@@ -34,11 +34,11 @@ module ActionDispatch
             end
           end
 
-          deep_regexps, regexps = regexps.partition { |r| r.to_s.include?("\\.") }
-          deep_strings, strings = strings.partition { |s| s.include?("\\.") }
+          deep_regexps, regexps = regexps.partition { |r| r.to_s.include?("\\.".freeze) }
+          deep_strings, strings = strings.partition { |s| s.include?("\\.".freeze) }
 
-          regexps << Regexp.new(strings.join('|'), true) unless strings.empty?
-          deep_regexps << Regexp.new(deep_strings.join('|'), true) unless deep_strings.empty?
+          regexps << Regexp.new(strings.join('|'.freeze), true) unless strings.empty?
+          deep_regexps << Regexp.new(deep_strings.join('|'.freeze), true) unless deep_strings.empty?
 
           new regexps, deep_regexps, blocks
         end
diff --git a/actionpack/lib/action_dispatch/http/url.rb b/actionpack/lib/action_dispatch/http/url.rb
index f5b709ccd6..6fcf49030b 100644
--- a/actionpack/lib/action_dispatch/http/url.rb
+++ b/actionpack/lib/action_dispatch/http/url.rb
@@ -245,7 +245,7 @@ module ActionDispatch
       #   req = Request.new 'HTTP_HOST' => 'example.com:8080'
       #   req.host # => "example.com"
       def host
-        raw_host_with_port.sub(/:\d+$/, '')
+        raw_host_with_port.sub(/:\d+$/, ''.freeze)
       end
 
       # Returns a \host:\port string for this request, such as "example.com" or
diff --git a/actionpack/lib/action_dispatch/journey/nodes/node.rb b/actionpack/lib/action_dispatch/journey/nodes/node.rb
index bb01c087bc..cf6542b370 100644
--- a/actionpack/lib/action_dispatch/journey/nodes/node.rb
+++ b/actionpack/lib/action_dispatch/journey/nodes/node.rb
@@ -30,7 +30,7 @@ module ActionDispatch
         end
 
         def name
-          left.tr '*:', ''
+          left.tr '*:'.freeze, ''.freeze
         end
 
         def type
diff --git a/actionpack/lib/action_dispatch/journey/router/utils.rb b/actionpack/lib/action_dispatch/journey/router/utils.rb
index d02ed96d0d..9793ca1c7a 100644
--- a/actionpack/lib/action_dispatch/journey/router/utils.rb
+++ b/actionpack/lib/action_dispatch/journey/router/utils.rb
@@ -14,10 +14,10 @@ module ActionDispatch
         #   normalize_path("/%ab")  # => "/%AB"
         def self.normalize_path(path)
           path = "/#{path}"
-          path.squeeze!('/')
-          path.sub!(%r{/+\Z}, '')
+          path.squeeze!('/'.freeze)
+          path.sub!(%r{/+\Z}, ''.freeze)
           path.gsub!(/(%[a-f0-9]{2})/) { $1.upcase }
-          path = '/' if path == ''
+          path = '/' if path == ''.freeze
           path
         end
 
diff --git a/actionpack/lib/action_dispatch/middleware/static.rb b/actionpack/lib/action_dispatch/middleware/static.rb
index b098ea389f..f38da4fdf6 100644
--- a/actionpack/lib/action_dispatch/middleware/static.rb
+++ b/actionpack/lib/action_dispatch/middleware/static.rb
@@ -35,7 +35,7 @@ module ActionDispatch
       paths = [path, "#{path}#{ext}", "#{path}/#{@index}#{ext}"]
 
       if match = paths.detect { |p|
-        path = File.join(@root, p.force_encoding('UTF-8'))
+        path = File.join(@root, p.force_encoding('UTF-8'.freeze))
         begin
           File.file?(path) && File.readable?(path)
         rescue SystemCallError
@@ -76,7 +76,7 @@ module ActionDispatch
       end
 
       def content_type(path)
-        ::Rack::Mime.mime_type(::File.extname(path), 'text/plain')
+        ::Rack::Mime.mime_type(::File.extname(path), 'text/plain'.freeze)
       end
 
       def gzip_encoding_accepted?(env)
@@ -112,7 +112,7 @@ module ActionDispatch
     def call(env)
       case env['REQUEST_METHOD']
       when 'GET', 'HEAD'
-        path = env['PATH_INFO'].chomp('/')
+        path = env['PATH_INFO'].chomp('/'.freeze)
         if match = @file_handler.match?(path)
           env['PATH_INFO'] = match
           return @file_handler.call(env)
diff --git a/actionpack/lib/action_dispatch/routing/route_set.rb b/actionpack/lib/action_dispatch/routing/route_set.rb
index 454593b59f..42512cad91 100644
--- a/actionpack/lib/action_dispatch/routing/route_set.rb
+++ b/actionpack/lib/action_dispatch/routing/route_set.rb
@@ -671,7 +671,7 @@ module ActionDispatch
 
         # Remove leading slashes from controllers
         def normalize_controller!
-          @options[:controller] = controller.sub(%r{^/}, '') if controller
+          @options[:controller] = controller.sub(%r{^/}, ''.freeze) if controller
         end
 
         # Move 'index' action from options to recall