fix(lint): unbreak the 2000-line file-size gate on main

[proggeramlug]

What

lint has been red on main since the 2026-08-11 batch: two files crossed the 2000-line cap that scripts/check_file_size.sh enforces.

file	before	after	landed in
crates/perry-hir/src/lower/pre_scan.rs	1985	2011	#7828
crates/perry-runtime/src/gc/layout.rs	1982	2023	#7809 → 2010, #7812 → 2023

pre_scan.rs is my own regression from #7828 — CLAUDE.md warns about exactly this cap ("adding a long doc comment can trip it") and I did not run the script before pushing.

The split

Both are pure code moves. No logic changes, no behaviour changes.

pre_scan_weakref_locals → lower/pre_scan/weakref_locals.rs (1653 lines left behind). It is the cohesive unit to carve out: one of the file's three top-level pre-scans, owning its own five local sets, and the one #7828 grew.

LayoutSlotMask (enum + its whole impl) → gc/layout/slot_mask.rs (1807 lines left behind). One self-contained data type with its methods. Its visibility widens from pub(super) to pub(in crate::gc) purely because the type now sits one module deeper and layout_tables.rs / hot_tls.rs still name it — the reachable set is identical.

Validation

• scripts/check_file_size.sh → OK: no Rust source files exceed 2000 lines. (was exit 1 on both files)
• cargo check -p perry-hir -p perry-runtime clean; zero warnings introduced (the 19 that cargo check prints are pre-existing and all in files this PR does not touch)
• cargo fmt --all -- --check clean
• test_gap_proxy_local_name_collision_7775.ts (the fix(hir): a local named like a proxy local is not a proxy #7828 test whose code moved) still matches Node byte-for-byte

Summary by CodeRabbit

• Refactor
  • Improved internal handling of weak-reference analysis and garbage-collection layout operations.
  • Maintained existing application behavior while improving code organization and maintainability.
• Documentation
  • Added a changelog entry confirming compliance with the 2,000-line source-file size limit.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: f918164b-9a9c-4e42-9cc1-3b498849671e

📥 Commits

Reviewing files that changed from the base of the PR and between a0e4505 and 8028773.

📒 Files selected for processing (1)

• changelog.d/7830-file-size-gate.md

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📝 Walkthrough

Walkthrough

The PR moves the weak-reference pre-scan and GC slot-mask implementations into dedicated Rust modules. It preserves existing behavior and internal access through re-exports. It also adds a changelog entry for the file-size lint gate.

Changes

Weak-reference local pre-scan

Layer / File(s)	Summary
Weak-reference pre-scan extraction crates/perry-hir/src/lower/pre_scan.rs, crates/perry-hir/src/lower/pre_scan/weakref_locals.rs	The pre-scan moves to weakref_locals.rs. The existing crate-visible function is re-exported. The module retains constructor tracking, recursive scanning, alias handling, shadowing detection, and ambiguity poisoning.

GC slot-mask extraction

Layer / File(s)	Summary
GC slot-mask module and layout wiring crates/perry-runtime/src/gc/layout.rs, crates/perry-runtime/src/gc/layout/slot_mask.rs, changelog.d/7830-file-size-gate.md	LayoutSlotMask moves to slot_mask.rs. The module supports inline, heap-backed, and all-pointer masks with mutation, queries, iteration, counting, and bounded lookup. layout.rs provides crate-visible access. The changelog records the file-size gate update.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• PerryTS/perry#7444: Splits oversized Rust files into focused submodules while preserving behavior and internal visibility.
• PerryTS/perry#7812: Modifies GC layout mask handling, including LayoutSlotMask-related behavior.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the primary change: fixing the 2,000-line file-size lint gate.
Description check	✅ Passed	The description explains the cause, code moves, behavior impact, and validation results, although it does not use the repository template headings.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
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 unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/pre-scan-file-size

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[coderabbitai]

Actionable comments posted: 1

🤖 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.

Inline comments:
In `@changelog.d/7830-file-size-gate.md`:
- Around line 3-23: Replace the narrative in the changelog entry with one
concise release-note statement describing the shipped fix: resolve the file-size
lint gate by moving the GC layout slot-mask implementation and weak-reference
pre-scan implementation into dedicated modules. Remove CI history, PR
references, line counts, and development guidance.

🪄 Autofix

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 475c3418-d304-4d88-81c3-6dc0dcd78847

📥 Commits

Reviewing files that changed from the base of the PR and between 1ee158d and a0e4505.

📒 Files selected for processing (5)

• changelog.d/7830-file-size-gate.md
• crates/perry-hir/src/lower/pre_scan.rs
• crates/perry-hir/src/lower/pre_scan/weakref_locals.rs
• crates/perry-runtime/src/gc/layout.rs
• crates/perry-runtime/src/gc/layout/slot_mask.rs