Correctly read the `cache_format_version` setting on boot

Fixes https://github.com/rails/rails/issues/45289

activesupport/lib/active_support/cache.rb

@@ -873,7 +873,7 @@ module ActiveSupport
           when 7.0
             Rails70Coder
           else
-            raise ArgumentError, "Unknown ActiveSupport::Cache.format_version #{Cache.format_version.inspect}"
+            raise ArgumentError, "Unknown ActiveSupport::Cache.format_version: #{Cache.format_version.inspect}"
           end
         end
       end

railties/lib/rails/application/bootstrap.rb

@@ -63,6 +63,9 @@ module Rails
 
       # Initialize cache early in the stack so railties can make use of it.
       initializer :initialize_cache, group: :all do
+        cache_format_version = config.active_support.delete(:cache_format_version)
+        ActiveSupport.cache_format_version = cache_format_version if cache_format_version
+
         unless Rails.cache
           Rails.cache = ActiveSupport::Cache.lookup_store(*config.cache_store)
 

railties/test/application/configuration_test.rb

@@ -1758,7 +1758,7 @@ module ApplicationTests
     test "config.log_file_size returns no limit in production" do
       app "production"
 
-      assert_equal nil, app.config.log_file_size
+      assert_nil app.config.log_file_size
     end
 
     test "rake_tasks block works at instance level" do
@@ -3787,6 +3787,52 @@ module ApplicationTests
       assert_equal :fiber, ActiveSupport::IsolatedExecutionState.isolation_level
     end
 
+    test "cache_format_version in a new app" do
+      add_to_config <<-RUBY
+        config.cache_store = :null_store
+      RUBY
+      app "development"
+
+      assert_equal ActiveSupport::Cache::Coders::Rails70Coder, Rails.cache.instance_variable_get(:@coder)
+    end
+
+    test "cache_format_version with explicit 7.0 defaults" do
+      add_to_config <<-RUBY
+        config.cache_store = :null_store
+      RUBY
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "7.0"'
+      app "development"
+
+      assert_equal ActiveSupport::Cache::Coders::Rails70Coder, Rails.cache.instance_variable_get(:@coder)
+    end
+
+    test "cache_format_version with 6.1 defaults" do
+      add_to_config <<-RUBY
+        config.cache_store = :null_store
+      RUBY
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "6.1"'
+      app "development"
+
+      assert_equal ActiveSupport::Cache::Coders::Rails61Coder, Rails.cache.instance_variable_get(:@coder)
+    end
+
+    test "cache_format_version **cannot** be set via new framework defaults" do
+      add_to_config <<-RUBY
+        config.cache_store = :null_store
+      RUBY
+      remove_from_config '.*config\.load_defaults.*\n'
+      add_to_config 'config.load_defaults "6.1"'
+      app_file "config/initializers/new_framework_defaults_7_0.rb", <<-RUBY
+        Rails.application.config.active_support.cache_format_version = 7.0
+      RUBY
+
+      app "development"
+
+      assert_equal ActiveSupport::Cache::Coders::Rails61Coder, Rails.cache.instance_variable_get(:@coder)
+    end
+
     private
       def set_custom_config(contents, config_source = "custom".inspect)
         app_file "config/custom.yml", contents