Refactor ActionView::Template::Types to avoid delegation

The `Type` class was introduced in https://github.com/rails/rails/pull/23085
for the sole purpose of breaking the dependency of Action View on Action Dispatch.

Unless you are somehow running Action View standalone, this is actually
never used.

So instead of delegating, we can use constant swapping, this saves us
a useless layer.

Ultimately we could consider moving `Mime::Types` into Active Support
but it requires some more thoughts.

actionmailer/lib/action_mailer.rb

@@ -73,6 +73,6 @@ def self.eager_load!
 
 ActiveSupport.on_load(:action_view) do
   ActionView::Base.default_formats ||= Mime::SET.symbols
-  ActionView::Template::Types.delegate_to Mime
+  ActionView::Template.mime_types_implementation = Mime
   ActionView::LookupContext::DetailsKey.clear
 end

actionpack/lib/action_dispatch.rb

@@ -147,6 +147,6 @@ def eager_load!
 
 ActiveSupport.on_load(:action_view) do
   ActionView::Base.default_formats ||= Mime::SET.symbols
-  ActionView::Template::Types.delegate_to Mime
+  ActionView::Template.mime_types_implementation = Mime
   ActionView::LookupContext::DetailsKey.clear
 end

actionpack/lib/action_dispatch/http/mime_type.rb

@@ -42,6 +42,10 @@ def [](type)
       Type.lookup_by_extension(type)
     end
 
+    def symbols
+      SET.symbols
+    end
+
     def fetch(type, &block)
       return type if type.is_a?(Type)
       EXTENSION_LOOKUP.fetch(type.to_s, &block)

actionview/lib/action_view/template.rb

@@ -109,6 +109,7 @@ class Template
       autoload :Handlers
       autoload :HTML
       autoload :Inline
+      autoload :Types
       autoload :Sources
       autoload :Text
       autoload :Types
@@ -119,6 +120,17 @@ class Template
     singleton_class.attr_accessor :frozen_string_literal
     @frozen_string_literal = false
 
+    class << self # :nodoc:
+      def mime_types_implementation=(implementation)
+        # This method isn't thread-safe, but it's not supposed
+        # to be called after initialization
+        if self::Types != implementation
+          remove_const(:Types)
+          const_set(:Types, implementation)
+        end
+      end
+    end
+
     attr_reader :identifier, :handler
     attr_reader :variable, :format, :variant, :virtual_path
 

actionview/lib/action_view/template/types.rb

@@ -4,53 +4,38 @@
 
 module ActionView
   class Template # :nodoc:
-    module Types
-      class Type
-        SET = Struct.new(:symbols).new([ :html, :text, :js, :css, :xml, :json ])
+    class SimpleType # :nodoc:
+      SET = Struct.new(:symbols).new([ :html, :text, :js, :css, :xml, :json ])
 
-        def self.[](type)
-          if type.is_a?(self)
-            type
-          else
-            new(type)
-          end
-        end
-
-        attr_reader :symbol
-
-        def initialize(symbol)
-          @symbol = symbol.to_sym
-        end
-
-        def to_s
-          @symbol.to_s
-        end
-        alias to_str to_s
-
-        def ref
-          @symbol
-        end
-        alias to_sym ref
-
-        def ==(type)
-          @symbol == type.to_sym unless type.blank?
+      def self.[](type)
+        if type.is_a?(self)
+          type
+        else
+          new(type)
         end
       end
 
-      class << self
-        attr_reader :symbols
+      attr_reader :symbol
 
-        def delegate_to(klass)
-          @symbols = klass::SET.symbols
-          @type_klass = klass
-        end
-
-        def [](type)
-          @type_klass[type]
-        end
+      def initialize(symbol)
+        @symbol = symbol.to_sym
       end
 
-      delegate_to Type
+      def to_s
+        @symbol.to_s
+      end
+      alias to_str to_s
+
+      def ref
+        @symbol
+      end
+      alias to_sym ref
+
+      def ==(type)
+        @symbol == type.to_sym unless type.blank?
+      end
     end
+
+    Types = SimpleType # :nodoc:
   end
 end

actionview/test/template/asset_tag_helper_test.rb

@@ -4,7 +4,7 @@
 require "active_support/ordered_options"
 
 require "action_dispatch"
-ActionView::Template::Types.delegate_to Mime
+ActionView::Template.mime_types_implementation = Mime
 
 module AssetTagHelperTestHelpers
   def with_preload_links_header(new_preload_links_header = true)