"can't modify frozen Array" error in Rails 5

[Will Jordan]

Error is raised whenever require 'sass/plugin/rack' is included anywhere in a Rails 5 application.
Minimal repro bash script (just adds require 'sass/plugin/rack' in an initializer and starts a new rails server):

#!/bin/bash

# Run in a temp dir and clean up after
TEST_DIR=$(mktemp -d)
pushd ${TEST_DIR} > /dev/null
trap "rm -rf ${TEST_DIR}; popd > /dev/null" EXIT

# Generate new Rails project (override version with RAILS_VERSION)
: ${RAILS_VERSION:=5.0.0.1}
rails _${RAILS_VERSION}_ new ${TEST_DIR} -q

# Add initializer
cat <<RB > config/initializers/sass_middleware.rb
require 'sass/plugin/rack'
RB

# Start Rails server
bin/rails server

Raises "can't modify frozen Array" error in Rails 5 (RAILS_VERSION=5.0.0.1 ./test.sh), doesn't throw any error in Rails 4 (RAILS_VERSION=4.2.7.1 ./test.sh).

Full stacktrace:

[GEM_PATH]/actionpack-5.0.0.1/lib/action_dispatch/middleware/stack.rb:92:in `delete_if': can't modify frozen Array (RuntimeError)
    from [GEM_PATH]/actionpack-5.0.0.1/lib/action_dispatch/middleware/stack.rb:92:in `delete'
    from [GEM_PATH]/sass-rails-5.0.6/lib/sass/rails/railtie.rb:35:in `block in <class:Railtie>'
    from [GEM_PATH]/activesupport-5.0.0.1/lib/active_support/lazy_load_hooks.rb:36:in `execute_hook'
    from [GEM_PATH]/activesupport-5.0.0.1/lib/active_support/lazy_load_hooks.rb:45:in `block in run_load_hooks'
    from [GEM_PATH]/activesupport-5.0.0.1/lib/active_support/lazy_load_hooks.rb:44:in `each'
    from [GEM_PATH]/activesupport-5.0.0.1/lib/active_support/lazy_load_hooks.rb:44:in `run_load_hooks'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/application/finisher.rb:65:in `block in <module:Finisher>'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/initializable.rb:30:in `instance_exec'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/initializable.rb:30:in `run'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/initializable.rb:55:in `block in run_initializers'
    from [RUBY]/tsort.rb:228:in `block in tsort_each'
    from [RUBY]/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
    from [RUBY]/tsort.rb:431:in `each_strongly_connected_component_from'
    from [RUBY]/tsort.rb:349:in `block in each_strongly_connected_component'
    from [RUBY]/tsort.rb:347:in `each'
    from [RUBY]/tsort.rb:347:in `call'
    from [RUBY]/tsort.rb:347:in `each_strongly_connected_component'
    from [RUBY]/tsort.rb:226:in `tsort_each'
    from [RUBY]/tsort.rb:205:in `tsort_each'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/initializable.rb:54:in `run_initializers'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/application.rb:352:in `initialize!'
    from [RAILS]/config/environment.rb:5:in `<top (required)>'
    from [RAILS]/config.ru:3:in `require_relative'
    from [RAILS]/config.ru:3:in `block in <main>'
    from [GEM_PATH]/rack-2.0.1/lib/rack/builder.rb:55:in `instance_eval'
    from [GEM_PATH]/rack-2.0.1/lib/rack/builder.rb:55:in `initialize'
    from [RAILS]/config.ru:in `new'
    from [RAILS]/config.ru:in `<main>'
    from [GEM_PATH]/rack-2.0.1/lib/rack/builder.rb:49:in `eval'
    from [GEM_PATH]/rack-2.0.1/lib/rack/builder.rb:49:in `new_from_string'
    from [GEM_PATH]/rack-2.0.1/lib/rack/builder.rb:40:in `parse_file'
    from [GEM_PATH]/rack-2.0.1/lib/rack/server.rb:318:in `build_app_and_options_from_config'
    from [GEM_PATH]/rack-2.0.1/lib/rack/server.rb:218:in `app'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/server.rb:59:in `app'
    from [GEM_PATH]/rack-2.0.1/lib/rack/server.rb:353:in `wrapped_app'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/server.rb:124:in `log_to_stdout'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/server.rb:77:in `start'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/commands_tasks.rb:90:in `block in server'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/commands_tasks.rb:85:in `tap'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/commands_tasks.rb:85:in `server'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands/commands_tasks.rb:49:in `run_command!'
    from [GEM_PATH]/railties-5.0.0.1/lib/rails/commands.rb:18:in `<top (required)>'
    from [RAILS]/bin/rails:9:in `require'
    from [RAILS]/bin/rails:9:in `<top (required)>'
    from [GEM_PATH]/spring-1.7.2/lib/spring/client/rails.rb:28:in `load'
    from [GEM_PATH]/spring-1.7.2/lib/spring/client/rails.rb:28:in `call'
    from [GEM_PATH]/spring-1.7.2/lib/spring/client/command.rb:7:in `call'
    from [GEM_PATH]/spring-1.7.2/lib/spring/client.rb:30:in `run'
    from [GEM_PATH]/spring-1.7.2/bin/spring:49:in `<top (required)>'
    from [GEM_PATH]/spring-1.7.2/lib/spring/binstub.rb:11:in `load'
    from [GEM_PATH]/spring-1.7.2/lib/spring/binstub.rb:11:in `<top (required)>'
    from [RUBY]/rubygems/core_ext/kernel_require.rb:55:in `require'
    from [RUBY]/rubygems/core_ext/kernel_require.rb:55:in `require'
    from [RAILS]/bin/spring:13:in `<top (required)>'
    from bin/rails:3:in `load'
    from bin/rails:3:in `<main>'

[Rafael Mendonça França]

require 'sass/plugin/rack'

Why would a Rails 5 application do this? Just asking to understand why we need that code at all.

[Will Jordan]

My particular use case is a lightweight Sinatra application designed to be inserted into a Rack middleware stack that may or may not include a Rails application.