Disable Rails console IRB's autocompletion in production

Reasons behind this change:

1. Autocompletion increases data transmission significantly. This could
   cause noticeable slowdown when connecting to remote servers, which is
   usually the case in production.
2. The autocompletion feature still has many issues, as listed in
   ruby/irb#445. They may be just annoying
   when happened locally, but in production they could cause real
   issues (e.g. typos caused by slow backspacing).

Due to these reasons, I think it's safer to disable this feature in
production.

About the implementation:

Because `IRB.start` doesn't take configuration arguments and rebuilds
the `IRB.conf` object, the only way we can turn off autocompletion is
through the `IRB_USE_AUTOCOMPLETE` env var.

The env var was added in ruby/irb#469 and will
be available for IRB 1.6+ and Ruby 3.2+.

The name wasn't used before so it won't cause issues with older IRB
versions.

Note: Users can still turn it back on with `IRB_USE_AUTOCOMPLETE=true`

Author: st0012

railties/lib/rails/commands/console/console_command.rb

@@ -38,6 +38,10 @@ def initialize(app, options = {})
 
       if @console == IRB
         IRB::WorkSpace.prepend(BacktraceCleaner)
+
+        if Rails.env == "production"
+          ENV["IRB_USE_AUTOCOMPLETE"] ||= "false"
+        end
       end
     end
 

railties/test/commands/console_test.rb

@@ -58,6 +58,45 @@ def test_console_defaults_to_IRB
     assert_equal IRB, Rails::Console.new(app).console
   end
 
+  def test_console_disables_IRB_auto_completion_in_production
+    original_use_autocomplete = ENV["IRB_USE_AUTOCOMPLETE"]
+    ENV["IRB_USE_AUTOCOMPLETE"] = nil
+
+    with_rack_env "production" do
+      app = build_app(nil)
+      assert_equal IRB, Rails::Console.new(app).console
+      assert_equal "false", ENV["IRB_USE_AUTOCOMPLETE"]
+    end
+  ensure
+    ENV["IRB_USE_AUTOCOMPLETE"] = original_use_autocomplete
+  end
+
+  def test_console_accepts_override_on_IRB_auto_completion_flag
+    original_use_autocomplete = ENV["IRB_USE_AUTOCOMPLETE"]
+    ENV["IRB_USE_AUTOCOMPLETE"] = "true"
+
+    with_rack_env "production" do
+      app = build_app(nil)
+      assert_equal IRB, Rails::Console.new(app).console
+      assert_equal "true", ENV["IRB_USE_AUTOCOMPLETE"]
+    end
+  ensure
+    ENV["IRB_USE_AUTOCOMPLETE"] = original_use_autocomplete
+  end
+
+  def test_console_doesnt_disable_IRB_auto_completion_in_non_production
+    original_use_autocomplete = ENV["IRB_USE_AUTOCOMPLETE"]
+    ENV["IRB_USE_AUTOCOMPLETE"] = nil
+
+    with_rails_env nil do
+      app = build_app(nil)
+      assert_equal IRB, Rails::Console.new(app).console
+      assert_nil ENV["IRB_USE_AUTOCOMPLETE"]
+    end
+  ensure
+    ENV["IRB_USE_AUTOCOMPLETE"] = original_use_autocomplete
+  end
+
   def test_default_environment_with_no_rails_env
     with_rails_env nil do
       start