chore(lint): allowlist guarded GcHeader probe in collection_proto_thunks (#6659)

[proggeramlug]

Problem

The lint gate is red on main and on every open PR (verified: #6721, #6720, #6717, #6712, #6711 all fail lint identically). The failing step is the address-classification audit (scripts/addr_class_inventory.py):

crates/perry-runtime/src/object/collection_proto_thunks.rs:329: [gcheader-cast]
  (*((ptr as *const u8).sub(crate::gc::GC_HEADER_SIZE) as *const crate::gc::GcHeader))

This GcHeader obj_type probe was added by #6659 (fac2bbead, merged 2026-07-19) without an allowlist entry, so the ratchet has been failing lint for every PR branched from that point.

Why an allowlist entry is the right fix

The cast is already correctly magnitude-classified — it runs only inside:

if crate::value::addr_class::is_above_handle_band(ptr)
    && crate::object::is_valid_obj_ptr(ptr as *const u8)
{
    let obj_type = unsafe {
        (*((ptr as *const u8).sub(crate::gc::GC_HEADER_SIZE) as *const crate::gc::GcHeader)).obj_type
    };

So ptr is never a handle-band small integer at the deref — exactly the invariant the ratchet exists to enforce. This is the same shape as the sibling object/* probes already in the allowlist, so I added one line with a follow-up note to migrate it to addr_class::try_read_gc_header.

Change

One line in scripts/addr_class_allowlist.txt. No functional change.

Verification

All five lint steps pass locally on this branch: cargo fmt --all -- --check, benchmark-freshness, check_file_size.sh, GC store-site inventory, and the address-classification audit (self-test + full run, exit 0). Restores lint to green, which unblocks all currently-open PRs.

No version bump / changelog — maintainer folds metadata at merge.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Updated audit exemptions to document a guarded runtime object-type check.
  • Recorded a follow-up migration for improved address validation.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds one address-classification allowlist entry documenting a guarded GcHeader probe in collection_proto_thunks.rs and its planned migration to addr_class::try_read_gc_header.

Changes

Address classification

Layer / File(s)	Summary
Add guarded probe exemption scripts/addr_class_allowlist.txt	Adds an allowlist entry for the guarded GcHeader object-type probe and records the planned migration path.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Suggested reviewers: thehypnoo

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly states the PR adds a guarded GcHeader probe allowlist entry in collection_proto_thunks.rs.
Description check	✅ Passed	The description covers the problem, change, and verification, though it doesn't follow the repo's template exactly.
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

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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 `@scripts/addr_class_allowlist.txt`:
- Line 139: Update the allowlist entry for collection_proto_thunks.rs to replace
the broad “*” match with the exact GcHeader cast/probe substring being
grandfathered. Preserve the existing guard context and comment, while ensuring
future GcHeader casts in that file are not automatically allowlisted.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

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

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 95fd6449-ccac-4704-be3e-dcd68000f2bc

📥 Commits

Reviewing files that changed from the base of the PR and between 53909ce and f4169b2.

📒 Files selected for processing (1)

• scripts/addr_class_allowlist.txt

[coderabbitai]

🔒 Security & Privacy | 🟠 Major | ⚡ Quick win

Narrow the allowlist match to the specific probe.

Using * suppresses every future GcHeader-cast finding in collection_proto_thunks.rs, not just the guarded probe being grandfathered. Replace the wildcard with the exact cast substring so newly added unsafe probes cannot bypass the audit.

🤖 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 `@scripts/addr_class_allowlist.txt` at line 139, Update the allowlist entry for
collection_proto_thunks.rs to replace the broad “*” match with the exact
GcHeader cast/probe substring being grandfathered. Preserve the existing guard
context and comment, while ensuring future GcHeader casts in that file are not
automatically allowlisted.