Bundler cannot evaluate local gemspecs with RUBY_BOX=1

[katsyoshi]

Description

Bundler fails to evaluate a local gemspec when Ruby Box is enabled with
RUBY_BOX=1.

The failure occurs before the requested command runs:

Bundler::GemspecError:
There was an error while loading `foo.gemspec`:
uninitialized constant Gem::Specification

This reproduces with an unmodified gem project generated by bundle gem. I
also reproduced the same problem in two unrelated existing gem projects,
Caotral and Oj.

Minimal reproduction

Generate a new gem using Bundler:

bundle gem foo
cd foo
bundle install

Running a command normally succeeds:

$ bundle exec ruby -e 'puts :ok'
ok

Enable Ruby Box and run the same command:

$ RUBY_BOX=1 bundle exec ruby -e 'puts :ok'

Bundler fails while evaluating the generated, unmodified foo.gemspec:

ruby: warning: Ruby::Box is experimental, and the behavior may change in the future!
See https://docs.ruby-lang.org/en/4.0/Ruby/Box.html for known issues, etc.

[!] There was an error while loading `foo.gemspec`:
uninitialized constant Gem::Specification. Bundler cannot continue.

 #  from /path/to/foo/foo.gemspec:5
 #  -------------------------------------------
 #
 >  Gem::Specification.new do |spec|
 #    spec.name = "foo"
 #  -------------------------------------------

Expected behavior

Bundler evaluates the gemspec and runs the requested command:

ok

Actual behavior

Bundler raises Bundler::GemspecError because Gem::Specification is not
available while evaluating the gemspec under Ruby Box.

Subprocess reproduction

The same issue occurs in Oj when its build invokes a Ruby subprocess:

RUBY_BOX=1 bundle exec rake

The outer Rake process starts, but the Ruby subprocess spawned for
extconf.rb fails:

/path/to/ruby -I. ../../../../ext/oj/extconf.rb

[!] There was an error while loading `oj.gemspec`:
uninitialized constant Gem::Specification. Bundler cannot continue.

 #  from /path/to/oj/oj.gemspec:6
 #  -------------------------------------------
 #
 >  Gem::Specification.new do |s|
 #    s.name = "oj"
 #  -------------------------------------------

This suggests that Ruby subprocesses inheriting both the Bundler environment
and RUBY_BOX=1 are affected as well.

Environment

The minimal bundle gem foo reproduction was confirmed with:

Ruby: 4.0.6
RubyGems: 4.0.17
Bundler: 4.0.17
OS: Linux

The same failure was also reproduced in Caotral with Bundler 4.0.12.

Attempted workarounds

Adding this to the gemspec does not fix the problem:

require "rubygems/specification"

It proceeds further but then fails because Gem::Deprecate is unavailable.

Adding this instead also does not provide a usable workaround:

require "rubygems"

It causes a circular require, after which Bundler fails to materialize the
local gemspec.

I confirmed that the application test itself works if Bundler is started
without Ruby Box and a child Ruby process is started separately with:

• RUBY_BOX=1
• Bundler's resolved $LOAD_PATH passed through RUBYLIB
• inherited RUBYOPT cleared
• --disable-gems

For example:

env = {
  "RUBY_BOX" => "1",
  "RUBYOPT" => nil,
  "RUBYLIB" => $LOAD_PATH.join(File::PATH_SEPARATOR),
}

exec(
  env,
  RbConfig.ruby,
  "--disable-gems",
  "-Ilib",
  "path/to/test.rb",
)

This avoids evaluating the gemspec inside Ruby Box, but it is not a practical
general workaround for bundle exec.

Additional context

Ruby Box must be enabled when the Ruby process starts. It cannot be enabled
after Bundler has initialized:

https://docs.ruby-lang.org/en/4.0/Ruby/Box.html

Is Bundler expected to support running under RUBY_BOX=1? If so, it appears
that the RubyGems constants required by Bundler.eval_gemspec are not
initialized in the relevant box.