chore(gc): delete the GcCallEffect::NeverReturns variant that is never constructed

[proggeramlug]

Never constructed since #7314 added it, so its three match arms in precise_roots.rs were dead and rustc warned on every build — one of the warnings currently failing the Warnings gate on main.

Deleted rather than wired up, because the classifier's own comment already records why it can never be enabled:

The audit it rested on is already false: js_throw_reference_error_tdz, js_throw_not_a_constructor and others are declared -> f64, not -> !. And since #7302 a throw UNWINDS rather than longjmps, so the call site is an invoke whose unwind edge needs relocations — while these helpers allocate the Error they raise and can therefore collect.

I went looking at this as a root-density lever (throw helpers take 5 statepoints in one probe alone, and a call that never returns can't consume a relocation). The comment is the answer: it's a closed dead-end, not an unfired optimization. The rationale is preserved at the site in the past tense so the next person doesn't re-derive it.

CLAUDE.md's kill-policy is the rule: an unexercised mode is a decision nobody has made, and the losing mode should stop compiling.

No behaviour change, asserted not assumed: 01_nursery_churn reports 62 statepoints / 88 relocations before and after. cargo test -p perry-codegen --lib green.

Summary by CodeRabbit

• Bug Fixes
  • Improved garbage-collection safety around calls that may throw or allocate.
  • Ensured calls that never return are handled conservatively unless explicitly approved as non-collecting.
  • Preserved safepoint handling for throwing calls that may allocate or unwind.

[coderabbitai]

📝 Walkthrough

Walkthrough

The change removes the NeverReturns GC call effect. Throwing helpers now use Unknown, and precise-root lowering processes affected invokes and ordinary calls through normal safepoint handling.

Changes

GC call classification

Layer / File(s)	Summary
Remove NeverReturns effect crates/perry-codegen/src/gc_call_effects.rs	GcCallEffect::NeverReturns is removed. js_throw* calls now use Unknown.
Update precise-root handling crates/perry-codegen/src/function/precise_roots.rs	NeverReturns calls are no longer marked as leaf calls or skipped during invoke and ordinary-call safepoint lowering.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the primary change: removal of the unused GcCallEffect::NeverReturns variant.
Description check	✅ Passed	The description explains the change, rationale, preserved behavior, and test results, although it does not use all template headings.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/delete-dead-neverreturns

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[coderabbitai]

🧹 Nitpick comments (1)

crates/perry-codegen/src/gc_call_effects.rs (1)

108-113: 🩺 Stability & Availability | 🔵 Trivial | ⚡ Quick win

Add direct regression tests for js_throw* classification.

The fallback classifies js_throw* helpers as GcCallEffect::Unknown, and no prefix arm remains. Add assertions for representative throw helpers such as js_throw_reference_error_tdz and js_throw_not_a_constructor so a future prefix rule cannot reintroduce unsafe safepoint suppression.

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@crates/perry-codegen/src/gc_call_effects.rs` around lines 108 - 113, Add
direct regression tests for the js_throw* helper classification, covering
representative names such as js_throw_reference_error_tdz and
js_throw_not_a_constructor. Assert that each resolves to GcCallEffect::Unknown
through the existing classification test path, preserving the fallback behavior
and preventing a future prefix rule from suppressing safepoints.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@crates/perry-codegen/src/gc_call_effects.rs`:
- Around line 108-113: Add direct regression tests for the js_throw* helper
classification, covering representative names such as
js_throw_reference_error_tdz and js_throw_not_a_constructor. Assert that each
resolves to GcCallEffect::Unknown through the existing classification test path,
preserving the fallback behavior and preventing a future prefix rule from
suppressing safepoints.

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: ca31a5f1-46b6-4645-a278-722b102a3098

📥 Commits

Reviewing files that changed from the base of the PR and between 494083f and f2c6f0b.

📒 Files selected for processing (2)

• crates/perry-codegen/src/function/precise_roots.rs
• crates/perry-codegen/src/gc_call_effects.rs