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The final Ruby 3.3+ runtime stage doesn’t enable YJIT — Ruby’s bundled JIT compiler.
PropertyValue
SeverityInfo
CategoryPerformance
DefaultEnabled
Auto-fixYes (FixSuggestion)

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Description

YJIT is Ruby’s bundled JIT compiler. As of Ruby 3.3, it’s production-ready and delivers a 15–30% CPU win on most Rails workloads at near-zero cost. But it’s opt-in: you have to set RUBY_YJIT_ENABLE=1, pass --yjit to the Ruby/Rails binary, or include --yjit in RUBYOPT. The corpus shows only 3 of 196 Rails Dockerfiles enable YJIT. For projects on Ruby 3.3+ this is a near-free performance gain that’s missing from almost every production deployment. This rule fires when:
  • The final stage’s effective Ruby version is 3.3 or later (resolved from the base image tag, or from .ruby-version / .tool-versions / Gemfile.lock’s RUBY VERSION block when the base is ARG-templated).
  • The stage looks like a long-running Ruby server (ENTRYPOINT/CMD references rails, puma, unicorn, thrust, rackup, sidekiq, falcon, thin, passenger, or iodine).
  • None of these YJIT signals is present:
    • ENV RUBY_YJIT_ENABLE=... (any truthy value).
    • ENV RUBYOPT="--yjit" (or contains --yjit).
    • ENTRYPOINT/CMD passes --yjit to the binary.

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Suppressions

The rule skips:
  • Ruby < 3.3 — YJIT existed but was experimental and had Rails-specific regressions.
  • Non-final stages (only the production runtime image matters for YJIT).
  • CLI-only Ruby images (no rails/puma/sidekiq/etc. ENTRYPOINT) — JIT warmup dominates short-lived processes.
  • Stages explicitly named dev, development, test, testing, ci, or debug.
  • Non-Ruby and Windows stages.

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Examples

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Before

FROM ruby:3.3-slim
COPY . .
CMD ["bin/rails", "server"]

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After

FROM ruby:3.3-slim
ENV RUBY_YJIT_ENABLE="1"
COPY . .
CMD ["bin/rails", "server"]
Equivalent forms also satisfy the rule: 
ENV RUBYOPT="--yjit"

CMD ["bin/rails", "server", "--yjit"]

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Auto-fix

FixSuggestion. Inserts ENV RUBY_YJIT_ENABLE="1" on the line immediately after the final stage’s FROM. The fix is suggestion-level because performance changes — even good ones — are best applied with the user’s explicit consent.

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References