Add Ruby 4.0

[lloeki]

What does this PR do?

Add Ruby 4.0 by moving from 3.5 (preview1) to 4.0 (preview2)

Motivation:

Ruby 3.5 isn't going to be a thing! Instead it's going to be 4.0.

Change log entry

None

Additional Notes:

How to test the change?

CI

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-11-27 17:34:30

Comparing candidate commit d3cc2ad in PR branch lloeki/add-ruby-4-0 with baseline commit dcb0b4f in branch master.

Found 1 performance improvements and 0 performance regressions! Performance is the same for 43 metrics, 2 unstable metrics.

scenario:profiling - gvl benchmark samples

• 🟩 throughput [+832.598op/s; +843.373op/s] or [+6.696%; +6.782%]

[ivoanjo]

Nice! I'm working on the profiling bits, feel free to throw in a disable for those and I'll take care to re-enable them once I have my fixes!

[lloeki]

Two fun ones yelling:

  2) Datadog::Core::Environment::Execution.development? when not in an RSpec test for Cucumber returns true
     Failure/Error: expect(err).to include('ACTUAL:true')

       expected "/Users/runner/hostedtoolcache/Ruby/4.0.0-preview2/arm64/lib/ruby/4.0.0+0/bundler/rubygems_integratio.../4.0.0-preview2/arm64/lib/ruby/4.0.0+0/rubygems.rb:233:in 'Gem.bin_path'\n\tfrom -:10:in '<main>'\n" to include "ACTUAL:true"
       Diff:
       @@ -1 +1,3 @@
       -ACTUAL:true
       +/Users/runner/hostedtoolcache/Ruby/4.0.0-preview2/arm64/lib/ruby/4.0.0+0/bundler/rubygems_integration.rb:236:in 'block in Gem.replace_bin_path': can't find executable cucumber for gem cucumber (Gem::Exception)
       +	from /Users/runner/hostedtoolcache/Ruby/4.0.0-preview2/arm64/lib/ruby/4.0.0+0/rubygems.rb:233:in 'Gem.bin_path'
       +	from -:10:in '<main>'
     # ./spec/datadog/core/environment/execution_spec.rb:196:in 'block (7 levels) in <top (required)>'
     # ./spec/datadog/core/environment/execution_spec.rb:186:in 'Dir.chdir'
     # ./spec/datadog/core/environment/execution_spec.rb:186:in 'block (6 levels) in <top (required)>'
     # ./spec/datadog/core/environment/execution_spec.rb:185:in 'block (5 levels) in <top (required)>'
     # ./spec/datadog/core/environment/execution_spec.rb:36:in 'block (4 levels) in <top (required)>'
     # ./spec/datadog/core/environment/execution_spec.rb:11:in 'block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:274:in 'block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:154:in 'block (2 levels) in <top (required)>'
     # ./spec/support/execute_in_fork.rb:32:in 'ForkableExample#run'

  3) Datadog::Core::Environment::Execution.development? when not in an RSpec test when in a Rails Spring process returns true
     Failure/Error: expect(err).to end_with('ACTUAL:true')
       expected "-:10: warning: already initialized constant ARGV\n/Users/runner/hostedtoolcache/Ruby/4.0.0-preview2/.../4.0.0-preview2/arm64/lib/ruby/4.0.0+0/rubygems.rb:233:in 'Gem.bin_path'\n\tfrom -:11:in '<main>'\n" to end with "ACTUAL:true"
     # ./spec/datadog/core/environment/execution_spec.rb:132:in 'block (5 levels) in <top (required)>'
     # ./spec/datadog/core/environment/execution_spec.rb:36:in 'block (4 levels) in <top (required)>'
     # ./spec/datadog/core/environment/execution_spec.rb:11:in 'block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:274:in 'block (2 levels) in <top (required)>'
     # ./spec/spec_helper.rb:154:in 'block (2 levels) in <top (required)>'
     # ./spec/support/execute_in_fork.rb:32:in 'ForkableExample#run'

[ivoanjo]

Update for profiling: Once these PRs are in

• [PROF-13115] Disable heap profiling on Ruby 4 preview due to incompatibility #5090
• [PROF-13115] Fix profiler support for Ruby 4.0.0-preview2 #5091
• Add Ruby 4.0.0-preview2 headers datadog-ruby_core_source#18
  • ➕ we need to release the latest datadog-ruby_core_source in rubygems.org OR temporarily manually pull it in on the 4.0 gemfile

...the profiler should be green on Ruby 4. So skipping profiling specs probably won't be needed on this branch (just need to rebase once the above have landed)

[lloeki]

Rebased, but we still need a release for DataDog/datadog-ruby_core_source#18

Or we could hack something to temporarily point to git instead.

[ivoanjo]

Or we could hack something to temporarily point to git instead.

I think it's fine to point to git specifically in the gemfile-4.0.rb? Possibly with a check that we're in a preview/head Ruby build (so it starts failing once we try to upgrade to 4.0.0 stable and we need to remove it)

[datadog-datadog-prod-us1]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage
• Patch Coverage: 50.00%
• Total Coverage: 95.17% (-0.01%)

View detailed report

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