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)

5 files changed, 11 insertions, 11 deletions

diff --git a/actionview/lib/action_view/helpers/asset_tag_helper.rb b/actionview/lib/action_view/helpers/asset_tag_helper.rb
index e32f8e219e..3041ad3ef7 100644
--- a/actionview/lib/action_view/helpers/asset_tag_helper.rb
+++ b/actionview/lib/action_view/helpers/asset_tag_helper.rb
@@ -237,7 +237,7 @@ module ActionView
       #   image_alt('underscored_file_name.png')
       #   # => Underscored file name
       def image_alt(src)
-        File.basename(src, '.*').sub(/-[[:xdigit:]]{32}\z/, '').tr('-_', ' ').capitalize
+        File.basename(src, '.*'.freeze).sub(/-[[:xdigit:]]{32}\z/, ''.freeze).tr('-_'.freeze, ' '.freeze).capitalize
       end
 
       # Returns an HTML video tag for the +sources+. If +sources+ is a string,
diff --git a/actionview/lib/action_view/helpers/asset_url_helper.rb b/actionview/lib/action_view/helpers/asset_url_helper.rb
index b19dc25025..3beb7fefb1 100644
--- a/actionview/lib/action_view/helpers/asset_url_helper.rb
+++ b/actionview/lib/action_view/helpers/asset_url_helper.rb
@@ -127,7 +127,7 @@ module ActionView
         return "" unless source.present?
         return source if source =~ URI_REGEXP
 
-        tail, source = source[/([\?#].+)$/], source.sub(/([\?#].+)$/, '')
+        tail, source = source[/([\?#].+)$/], source.sub(/([\?#].+)$/, ''.freeze)
 
         if extname = compute_asset_extname(source, options)
           source = "#{source}#{extname}"
diff --git a/actionview/lib/action_view/lookup_context.rb b/actionview/lib/action_view/lookup_context.rb
index 1c8359005b..f4a10aa393 100644
--- a/actionview/lib/action_view/lookup_context.rb
+++ b/actionview/lib/action_view/lookup_context.rb
@@ -173,13 +173,13 @@ module ActionView
       # name instead of the prefix.
       def normalize_name(name, prefixes) #:nodoc:
         prefixes = prefixes.presence
-        parts    = name.to_s.split('/')
+        parts    = name.to_s.split('/'.freeze)
         parts.shift if parts.first.empty?
         name     = parts.pop
 
         return name, prefixes || [""] if parts.empty?
 
-        parts    = parts.join('/')
+        parts    = parts.join('/'.freeze)
         prefixes = prefixes ? prefixes.map { |p| "#{p}/#{parts}" } : [parts]
 
         return name, prefixes
@@ -204,7 +204,7 @@ module ActionView
     # add :html as fallback to :js.
     def formats=(values)
       if values
-        values.concat(default_formats) if values.delete "*/*"
+        values.concat(default_formats) if values.delete "*/*".freeze
         if values == [:js]
           values << :html
           @html_fallback_for_js = true
diff --git a/actionview/lib/action_view/template.rb b/actionview/lib/action_view/template.rb
index d8585514d5..1ce9f94b13 100644
--- a/actionview/lib/action_view/template.rb
+++ b/actionview/lib/action_view/template.rb
@@ -190,7 +190,7 @@ module ActionView
     end
 
     def inspect
-      @inspect ||= defined?(Rails.root) ? identifier.sub("#{Rails.root}/", '') : identifier
+      @inspect ||= defined?(Rails.root) ? identifier.sub("#{Rails.root}/", ''.freeze) : identifier
     end
 
     # This method is responsible for properly setting the encoding of the
@@ -337,13 +337,13 @@ module ActionView
       def method_name #:nodoc:
         @method_name ||= begin
           m = "_#{identifier_method_name}__#{@identifier.hash}_#{__id__}"
-          m.tr!('-', '_')
+          m.tr!('-'.freeze, '_'.freeze)
           m
         end
       end
 
       def identifier_method_name #:nodoc:
-        inspect.tr('^a-z_', '_')
+        inspect.tr('^a-z_'.freeze, '_'.freeze)
       end
 
       def instrument(action, &block)
@@ -366,7 +366,7 @@ module ActionView
       end
 
       def inferred_cache_name
-        @inferred_cache_name ||= @virtual_path.split('/').last.sub('_', '')
+        @inferred_cache_name ||= @virtual_path.split('/'.freeze).last.sub('_'.freeze, ''.freeze)
       end
   end
 end
diff --git a/actionview/lib/action_view/template/resolver.rb b/actionview/lib/action_view/template/resolver.rb
index 2b00b0303d..e7208cce49 100644
--- a/actionview/lib/action_view/template/resolver.rb
+++ b/actionview/lib/action_view/template/resolver.rb
@@ -222,7 +222,7 @@ module ActionView
     end
 
     def escape_entry(entry)
-      entry.gsub(/[*?{}\[\]]/, '\\\\\\&')
+      entry.gsub(/[*?{}\[\]]/, '\\\\\\&'.freeze)
     end
 
     # Returns the file mtime from the filesystem.
@@ -234,7 +234,7 @@ module ActionView
     # from the path, or the handler, we should return the array of formats given
     # to the resolver.
     def extract_handler_and_format_and_variant(path, default_formats)
-      pieces = File.basename(path).split('.')
+      pieces = File.basename(path).split('.'.freeze)
       pieces.shift
 
       extension = pieces.pop