Support for gems.rb is causing a NameError for the `Bundler` constant in generators

[elia]

The use of Bundler in #588 is causing:

foo-app:master $ cd /tmp/foo-app/; spring stop; trash tmp/cache/boot*; bin/rails g -h                                                                                                 /t/foo-app
Spring stopped.
fish: No matches for wildcard 'tmp/cache/boot*'. See `help expand`.
cd /tmp/foo-app/; spring stop; trash tmp/cache/boot*; bin/rails g -h
                                       ^
/Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/configuration.rb:8:in `gemfile': uninitialized constant #<Class:Spring>::Bundler (NameError)
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/configuration.rb:47:in `find_project_root'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/configuration.rb:41:in `project_root_path'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/configuration.rb:32:in `application_root_path'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:288:in `loaded_application_features'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:119:in `ensure in preload'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:126:in `preload'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:157:in `serve'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:145:in `block in run'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:139:in `loop'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application.rb:139:in `run'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/gems/2.6.0/gems/spring-2.1.1/lib/spring/application/boot.rb:19:in `<top (required)>'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:72:in `require'
	from /Users/elia/.asdf/installs/ruby/2.6.3/lib/ruby/site_ruby/2.6.0/rubygems/core_ext/kernel_require.rb:72:in `require'
	from -e:1:in `<main>'

The app comes from a vanilla rails new plus solidus added to the gemfile (I was testing a generator).
I think it just needs to require Bundler where it's used. Locally I solved by downgrading to v2.1.0.

[Sixeight]

Forked process doesn't know Bundler. Then ProcessTitleUpdater on boot.rb failed to update process title because app.app_name uses Bundler.default_gemfile through Spring.gemfile .

Spring is running:

57589 spring server | sample | started 29 secs ago
57590 ruby -I /Users/tomohiro/.rbenv/versions/2.7.2/lib/ruby/gems/2.7.0/gems/spring-2.1.1/lib -e require 'spring/application/boot'

I except

Spring is running:

55038 spring server | sample | started 8 secs ago
55040 spring app    | sample | started 8 secs ago | development mode

[alexbel]

Looks like the PR is not going to be merged any time soon.
Workaround: add require 'bundler' in ~/.spring.rb