Inject required_rubygems_version >= 4.1.0.a into CA gems at build

[jenshenny]

Skinny gems built with --ruby-abi did not carry a required_rubygems_version constraint, so older RubyGems clients (4.0.x) could install them through the local path (gem install ./file.gem, vendor/cache, bundle install --local) with no warning. The content address was silently discarded and a gem scoped to one Ruby ABI would activate on an incompatible Ruby sharing that GEM_HOME.

Gem::Package.build now derives the required_rubygems_version for the built gem from the gemspec's own requirement:

• unset requirements become >= 4.1.0.a
• requirements that already satisfy that floor are left untouched
• weaker requirements are rewritten to the floor plus any declared upper bounds and exclusions, with a warning since the built gem's metadata will differ from the gemspec
• requirements that cap below the floor (e.g. < 4.0, ~> 3.5) raise at build time, since combined with the floor no RubyGems version could ever install the resulting gem

As with required_ruby_version, the derived value is propagated back to the original spec only after a successful build.

[jenshenny]

===== unset requirement (clean inject, no warning)
  Successfully built RubyGem
  Name: content_addressable_test
  Version: 0.1.1
  File: content_addressable_test-0.1.1-bfe80d24.gem

===== weak lower bound >= 3.0 (warning, raised to floor)
WARNING:  required_rubygems_version was changed from ">= 3.0" to ">= 4.1.0.beta1" for this build because content addressable gems can only be installed by RubyGems >= 4.1.0.beta1.
  Successfully built RubyGem
  Name: content_addressable_test
  Version: 0.1.1
  File: content_addressable_test-0.1.1-bfe80d24.gem

===== upper bound < 5.0 (warning, bound preserved)
WARNING:  required_rubygems_version was changed from "< 5.0" to ">= 4.1.0.beta1, < 5.0" for this build because content addressable gems can only be installed by RubyGems >= 4.1.0.beta1.
  Successfully built RubyGem
  Name: content_addressable_test
  Version: 0.1.1
  File: content_addressable_test-0.1.1-25c72c06.gem

===== already >= 4.2 (untouched, no warning)
  Successfully built RubyGem
  Name: content_addressable_test
  Version: 0.1.1
  File: content_addressable_test-0.1.1-96992460.gem

===== conflict: < 4.0
ERROR:  While executing gem ... (ArgumentError)
    Cannot build gem for Ruby ABI 4.0 because required_rubygems_version is set to < 4.0, which excludes RubyGems >= 4.1.0.beta1 required to install content addressable gems. Please remove or loosen the conflicting constraint.
	/Users/jennyshen/src/github.com/Shopify/rubygems/lib/rubygems/package.rb:756:in 'Gem::Package#normalized_required_rubygems_version'
	/Users/jennyshen/src/github.com/Shopify/rubygems/lib/rubygems/package.rb:402:in 'Gem::Package#build_content_addressable_file'

===== conflict: ~> 3.5
ERROR:  While executing gem ... (ArgumentError)
    Cannot build gem for Ruby ABI 4.0 because required_rubygems_version is set to ~> 3.5, which excludes RubyGems >= 4.1.0.beta1 required to install content addressable gems. Please remove or loosen the conflicting constraint.

===== conflict: = 3.5.9
ERROR:  While executing gem ... (ArgumentError)
    Cannot build gem for Ruby ABI 4.0 because required_rubygems_version is set to = 3.5.9, which excludes RubyGems >= 4.1.0.beta1 required to install content addressable gems. Please remove or loosen the conflicting constraint.

===== conflict: <= 4.1.0.beta0
ERROR:  While executing gem ... (ArgumentError)
    Cannot build gem for Ruby ABI 4.0 because required_rubygems_version is set to <= 4.1.0.beta0, which excludes RubyGems >= 4.1.0.beta1 required to install content addressable gems. Please remove or loosen the conflicting constraint.

===== ruby-abi mismatch with required_ruby_version
ERROR:  While executing gem ... (ArgumentError)
    Cannot build gem for Ruby ABI 4.0 because required_ruby_version is set to ~> 3.4.0. Please set required_ruby_version to "~> 4.0.0".

===== no platform set (default ruby platform)
ERROR:  While executing gem ... (ArgumentError)
    Cannot build a gem scoped to a single Ruby ABI as no platform or a Ruby platform has been set
      raise ArgumentError, "Cannot build a gem scoped to a single Ruby ABI as no platform or a Ruby platform has been set"

[Copilot]

🟡 Changes recommended

Pessimistic constraints are mishandled around prerelease upper bounds, potentially broadening or making requirements unsatisfiable.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds a RubyGems version floor to Ruby ABI-scoped content-addressable gems.

Changes:

• Normalizes and propagates required_rubygems_version.
• Rejects incompatible constraints and warns about rewrites.
• Adds package and command-level tests.

File summaries

File	Description
lib/rubygems/package.rb	Implements requirement normalization and validation.
test/rubygems/test_gem_package.rb	Tests metadata derivation and constraints.
test/rubygems/test_gem_commands_build_command.rb	Tests command-level conflict rejection.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Balanced

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[Copilot]

🟡 Changes recommended

Valid prerelease constraints are incorrectly rejected as conflicting with the declared minimum.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 2
• Review effort level: Balanced

[Copilot]

🟡 Changes recommended

Compound constraints capped at the version floor can produce an uninstallable gem instead of failing the build.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 1
• Review effort level: Balanced

[Copilot]

🟢 Approval recommended

The implementation matches the described behavior and includes comprehensive coverage of success and failure paths.

Review details

• Files reviewed: 3/3 changed files
• Comments generated: 0 new
• Review effort level: Balanced