Phase 29. Runtime matrix CI + dead-code cull (audit-4)

Field	Value
MEP	MEP-56 §Phases
Status	LANDED
Started	2026-05-29 14:01 (GMT+7)
Landed	2026-05-29 14:01 (GMT+7)
Tracking issue	—
Tracking PR	#22510
Commit	e0956a4238

Gate

Two orthogonal gates, both audit-driven (no new user-visible Mochi feature, but the CI matrix gate moves the umbrella phase coverage rule forward):

1. transpiler3-ruby-test.yml exists and is green on the required CRuby versions. The workflow runs go test -v -timeout 600s ./transpiler3/ruby/... on a three-entry matrix: CRuby 3.2 on ubuntu (blocking), CRuby 3.4 on ubuntu (blocking), CRuby 3.4 on macos (non-blocking). Each job uses ruby/setup-ruby@v1 to pin the toolchain, exports MOCHI_RUBY=$(which ruby) so resolveToolchain picks up the matrix-installed binary instead of falling through to Homebrew slots, and runs the full Ruby transpiler test suite against that interpreter.
2. go build ./transpiler3/ruby/... and go test ./transpiler3/ruby/... still pass after removing Lambda and BlockLit from rtree/nodes.go. The audit-4 dead-code sweep removed two rubyExpr-implementing types that lower.go never instantiates: Lambda (intended ->(p) { … } lambda emission, never reached because MethodDecl is preferred) and BlockLit plus its MethodCall.Block field (block syntax for HOF calls is emitted via RawExpr strings instead, as documented in the new MethodCall comment). Build- and vet-clean confirms no caller drift.

The MEP-56 umbrella phase coverage rule (feedback_umbrella_phase_targets.md) requires every target in §6's matrix to be exercised before the MEP can close. CRuby 3.2 / 3.4 are now exercised on every PR; JRuby 10, TruffleRuby 33, and mruby 4 remain unexercised and are tracked as sub-phases 29.1, 29.2, 29.3 below.

Build target / audit decisions

This phase does not change the transpiler pipeline, the runtime gem, or any build target. Two surface changes:

1. .github/workflows/transpiler3-ruby-test.yml added. Modeled after transpiler3-jvm-test.yml: same paths: filter shape (the new workflow only runs when transpiler3/ruby/**, mochi-runtime/**, tests/transpiler3/ruby/**, or the workflow itself changes), same Go setup, same continue-on-error: ${{ !matrix.blocking }} semantics. The matrix entries are deliberately narrow at first: CRuby 3.2 / 3.4 on ubuntu blocking, 3.4 on macos non-blocking. CRuby 4.0 is not in the matrix because ruby/setup-ruby@v1 does not publish a 4.0 image yet; 4.0 coverage lives in the locally-run developer suite via the existing Homebrew slot detection in build.go.
2. transpiler3/ruby/rtree/nodes.go shrunk. Removed Lambda (lines 509 to 519 of the pre-change file) and BlockLit (lines 433 to 438 of the pre-change file). Removed the Block *BlockLit field on MethodCall and the block-rendering branch in MethodCall.RubyExprString() (lines 467 to 480 of the pre-change file). The MethodCall comment now explicitly says block-using call sites are emitted via RawExpr (and that is true: every .map { … } / .select { … } / .inject { … } call site is built as a RawExpr in lower.go lines 911 / 922 / 937).

Three audit decisions are baked into this phase:

1. Delete unused code rather than wire it up. The project rule "Avoid backwards-compatibility hacks. If you are certain that something is unused, you can delete it completely" applies here. Both Lambda and BlockLit had complete RubyExprString implementations but zero call sites in lower.go. Wiring them up would mean reshaping working RawExpr-based block emission for no user-visible delta. Deleting them is the cheaper choice and removes the risk of a future contributor mistakenly believing the rtree supports first-class lambdas when in fact every lambda flows through RawExpr.
2. Block emission stays in RawExpr. A future audit may want to re-introduce a typed Block node so that block bodies can be lowered structurally instead of by string formatting. That is deliberately out of scope for phase 29: the spec change would have to land first (MEP-56 §3 would need a "block syntax" row), and the lower pass would have to migrate ListMapExpr / ListFilterExpr / ListFoldlExpr / AsyncExpr lowering. None of that is justified by an existing bug.
3. Non-CRuby runtimes are explicit sub-phases. index.md now lists JRuby 10 / TruffleRuby 33 / mruby 4 as sub-phases 29.1 / 29.2 / 29.3 with a note that container-based toolchain detection in build.go is the prerequisite. This makes the umbrella phase rule operational: future contributors can pick up 29.1 (JRuby) without re-reading the audit thread that surfaced the gap.

Files changed

File	Purpose
.github/workflows/transpiler3-ruby-test.yml	New CI workflow, matrix CRuby 3.2 / 3.4 on ubuntu (blocking) and CRuby 3.4 on macos (non-blocking)
transpiler3/ruby/rtree/nodes.go	Removed dead Lambda and BlockLit types plus MethodCall.Block field and block-rendering branch
website/docs/implementation/0056/index.md	Runtime matrix updated to mark CRuby 3.2 / 3.4 as LANDED via CI; JRuby / TruffleRuby / mruby renamed as sub-phases 29.1 / 29.2 / 29.3
website/docs/implementation/0056/phase-29-runtime-matrix.md	This tracking page
website/sidebars.js	Phase 29 entry appended after phase 28

Sub-phases (not yet landed)

Sub-phase	Target	Status	Prerequisite
29.1	JRuby 10 (CRuby-compatible subset)	NOT STARTED	Container image entry in transpiler3-ruby-test.yml plus $MOCHI_JRUBY env var detection in build.go resolveToolchain
29.2	TruffleRuby 33	NOT STARTED	Container image entry plus $MOCHI_TRUFFLERUBY env var detection
29.3	mruby 4 (language subset only, no threads / agents / streams)	NOT STARTED	MRUBY_HOME and MOCHI_MRBC env var detection plus a sub-corpus marker on phases 10 / 11 / 12 / 13 / 21 to opt out

Each sub-phase keeps the same gate template: the new runtime appears as a continue-on-error matrix entry first (non-blocking), and only promotes to blocking once all per-phase tests pass on it.

Test set

No new Go test functions are added. The phase is validated by:

• go build ./transpiler3/ruby/... succeeds after dead-code removal.
• go vet ./transpiler3/ruby/... succeeds.
• go test -v ./transpiler3/ruby/... succeeds locally and in CI on CRuby 3.2 / 3.4 / 3.4-macos via transpiler3-ruby-test.yml.

Closeout notes

Phase 29 is the audit-4 follow-up to phase 28's gap closure. Phase 28 added tests for Driver.Build error paths, edge-case coverage, and emitted-syntax checks; phase 29 closes the umbrella phase coverage rule (CRuby 3.2 / 3.4 in CI) and the dead-code residue (Lambda, BlockLit) that phase 28's emit audit surfaced. The MEP-56 §6 runtime matrix now matches reality: CRuby 3.2 / 3.4 are CI-green; JRuby / TruffleRuby / mruby are sub-phases waiting on a toolchain detection refactor.