improves the reloading disabled error message

The original message from Zeitwerk is "can't reload, please call
loader.enable_reloading before setup (Zeitwerk::Error)", which is not
very informative for Rails programmers.

Rails should err with a message worded in terms of its interface.

Gemfile.lock

@@ -71,7 +71,7 @@ PATH
       i18n (>= 0.7, < 2)
       minitest (~> 5.1)
       tzinfo (~> 1.1)
-      zeitwerk (~> 2.1)
+      zeitwerk (~> 2.1, >= 2.1.2)
     rails (6.0.0.beta3)
       actioncable (= 6.0.0.beta3)
       actionmailbox (= 6.0.0.beta3)
@@ -526,7 +526,7 @@ GEM
     websocket-extensions (0.1.3)
     xpath (3.2.0)
       nokogiri (~> 1.8)
-    zeitwerk (2.1.0)
+    zeitwerk (2.1.2)
 
 PLATFORMS
   java

activesupport/activesupport.gemspec

@@ -34,5 +34,5 @@ Gem::Specification.new do |s|
   s.add_dependency "tzinfo",          "~> 1.1"
   s.add_dependency "minitest",        "~> 5.1"
   s.add_dependency "concurrent-ruby", "~> 1.0", ">= 1.0.2"
-  s.add_dependency "zeitwerk",        "~> 2.1"
+  s.add_dependency "zeitwerk",        "~> 2.1", ">= 2.1.2"
 end

activesupport/lib/active_support/dependencies/zeitwerk_integration.rb

@@ -9,7 +9,11 @@ module ActiveSupport
       module Decorations
         def clear
           Dependencies.unload_interlock do
-            Rails.autoloaders.main.reload
+            begin
+              Rails.autoloaders.main.reload
+            rescue Zeitwerk::ReloadingDisabledError
+              raise "reloading is disabled because config.cache_classes is true"
+            end
           end
         end
 

railties/test/application/zeitwerk_integration_test.rb

@@ -156,6 +156,15 @@ class ZeitwerkIntegrationTest < ActiveSupport::TestCase
     assert_not Rails.autoloaders.once.reloading_enabled?
   end
 
+  test "reloading raises if config.cache_classes is true" do
+    boot("production")
+
+    e = assert_raises(StandardError) do
+      deps.clear
+    end
+    assert_equal "reloading is disabled because config.cache_classes is true", e.message
+  end
+
   test "eager loading loads code in engines" do
     $test_blog_engine_eager_loaded = false