fix(bench): default to incremental GC under EOP, imply legacy persistence for legacy collectors

[Kamirus]

mops bench (no --gc flag) crashed on moc 0.15+ — a regression from 2.16.0 (#601), which made enhanced orthogonal persistence the default bench mode but left the default --gc as copying. moc fixes the GC to incremental under EOP and rejects the legacy collectors there, so the default run was an invalid flag combination.

Before — default mops bench:

moc-wrapper -c --idl canister.mo --force-gc --copying-gc --release ...
moc: Invalid compiler flag combination: --copying-gc is not supported with --enhanced-orthogonal-persistence
Error: benchmarks failed

After — default mops bench compiles under EOP with no collector flag (incremental is moc's fixed GC there):

moc-wrapper -c --idl canister.mo --force-gc --release ...

--gc copying|compacting|generational still works: those collectors only exist under legacy persistence, so selecting one now implies --legacy-persistence (instead of crashing with an opaque moc error). --gc incremental or omitting --gc keeps measuring under EOP.

Why the bug evaded detection

#601 switched bench to EOP-by-default and smoke-tested with --gc incremental, but the --gc default stayed copying, so the unflagged mops bench invocation everyone runs was never exercised. More fundamentally, there was no mops bench end-to-end test at all — and this repo's own mops.toml pins moc 0.14.14 (pre-EOP), so even the local bench never touched the EOP path. The crash only surfaced on a downstream project (motoko-core) running a newer moc.

This PR adds an e2e test (cli/tests/bench.test.ts + cli/tests/bench/ fixture) that pins moc 1.3.0 + pocket-ic 12.0.0 and runs mops bench for real, asserting exit 0 and no Invalid compiler flag combination. Verified it fails against the pre-fix state with the original moc error and passes with the fix.

Migration

Default benchmark numbers shift from copying (legacy) to incremental (EOP) — but 2.16.0 already shifted numbers by switching persistence modes, and incremental is the only GC consistent with EOP-by-default. To reproduce the old legacy-mode measurements: mops bench --gc copying (now auto-enables --legacy-persistence).

[github-actions]

Cursor AI review

👍 APPROVE — looks safe to merge

Category	Assessment	Details
Summary	✅	Fixes mops bench crashing on moc 0.15+ by defaulting --gc to incremental (EOP-compatible), omitting collector flags under EOP, and auto-enabling --legacy-persistence when a legacy collector is selected. Adds an EOP e2e test and syncs docs/changelog/help.
Code Quality	✅	Minimal, focused change; isLegacyGc and getMocArgs logic are clear and reuse existing SemVer/version checks.
Consistency	✅	Commander default, bench.ts defaults, docs, --help, and changelog all agree on the new behavior.
Security	✅	No auth, signing, or package-integrity paths touched.
Tests	✅	New e2e test pins moc 1.3.0 + pocket-ic 12.0.0 to exercise the EOP path that repo CI otherwise misses; targeted assertions are appropriate for a regression guard.
Maintainability	✅	Comments explain the moc 0.15+ EOP/GC constraint; verbose output surfaces the implicit legacy-persistence switch.

Verdict

Decision: APPROVE
Risk: Low
Reason: This is a targeted fix for a real 2.16.0 regression (invalid --copying-gc under EOP) with correct flag-generation logic, a documented migration path (--gc copying), and a meaningful e2e test. No correctness or production-risk issues found in the diff.

---

Generated for commit 91a2d45