fix(hir): suppress Annex B.3.3 legacy var across for-head/destructuring-catch lexicals (#5346)

[proggeramlug]

Summary

Closes the dominant cluster from #5346 (follow-up to #5297/#5319): 94× An initialized binding is not created in test262 annexB/language.

These are the B.3.3 skip cases. A sloppy block-nested function f(){} gets an enclosing-scope legacy var f only when replacing it with var f would not be an early error. #5319's collector seeded the forbidden set with the body's own top-level lexicals and each descended block's let/const/class, but missed lexicals introduced by intermediate scopes between the body and the nested declaration:

• a let/const for-head — for (let f; ;) { { function f(){} } }, for (let f in/of …) { … } — lexically scopes the loop body, so an enclosing var f collides → early error → var suppressed;
• a destructuring catch parameter — catch ({ f }) { { function f(){} } } — lexically binds its names. A simple catch (f) is exempt: Annex B.3.4 lets var f alias it, so it does not gate B.3.3.

Perry created the legacy var regardless, so f was observable (and a function after the block ran) where the spec keeps it unbound — reading f must throw ReferenceError and typeof f stay "undefined".

Fix

Seed the for-head (let/const only) and destructuring-catch lexical names into the forbidden set as the B.3.3 traversal descends into each of those bodies (annexb_nested_stmt in lower_decl/block.rs). Pure addition to the forbidden-set computation — no other lowering path is touched. The switch skip variant already worked (case lexicals were collected).

Validation

• New behavior matches node v26 exactly for the for / for-in / for-of / switch / try shapes (f throws ReferenceError, typeof f === "undefined" before and after the block).
• test262 annexB/language (--all-features, node v26 differential): all 112 function-code/global-code *skip-early-err-{for,for-in,for-of,switch,try} cases now pass; overall annexB parity 645/970 → 776/1026 (75.6%).
• The eval-code variants of the same shape still fail — they need eval enablement (separate item in test262 annexB/language tail — 303 fails after #5319 (B.3.3 residual + missed-negatives + dynamic eval) #5346).
• No regressions in the positive B.3.3 cases. cargo test -p perry-hir green.

Remaining #5346 clusters (out of scope here): 72× dynamic eval (AOT limit), 73× Annex B missed-negatives, and the existing-fn-no-init hoisting-precedence SameValue cluster.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved scoping correctness in for loops and try-catch blocks to ensure consistent behavior with JavaScript standards.

[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: 9c081f93-9c5f-44f3-9a40-fb2eed989ef2

📥 Commits

Reviewing files that changed from the base of the PR and between 2b30f9f and 7542a4c.

📒 Files selected for processing (1)

• crates/perry-hir/src/lower_decl/block.rs

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📝 Walkthrough

Walkthrough

A new internal helper annexb_forhead_lexical_names is added to collect let/const binding names from for-loop variable declaration heads. annexb_nested_stmt is updated to clone and extend the Annex B forbidden set with those names before traversing for, for..in, and for..of bodies, and to extend the forbidden set with destructured binding names when a catch parameter is an array/object pattern.

Changes

Annex B B.3.3 forbidden set refinements

Layer / File(s)	Summary
annexb_forhead_lexical_names helper and for-loop forbidden set extension crates/perry-hir/src/lower_decl/block.rs	Adds annexb_forhead_lexical_names to collect lexical binding names from let/const for-heads (returning early for var), and updates annexb_nested_stmt to clone and extend the forbidden set with those names before descending into for, for..in, and for..of bodies.
Destructuring catch parameter forbidden set extension crates/perry-hir/src/lower_decl/block.rs	Modifies the try/catch branch of annexb_nested_stmt to extend the forbidden set with destructured binding names when the catch parameter is an object or array pattern; non-destructuring catch params leave the forbidden set unchanged.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related issues

• test262 annexB/language tail — 303 fails after #5319 (B.3.3 residual + missed-negatives + dynamic eval) #5346: The forbidden-set fixes for for-loop let/const heads and destructuring catch parameters directly address the residual B.3.3 failures identified in that issue as the source of 94 test262 failures.

Possibly related PRs

• PerryTS/perry#5319: Introduced the Annex B B.3.3 block-function name collection and forbidden-set traversal machinery in block.rs that this PR directly extends.
• PerryTS/perry#5356: Targets the same annexb_nested_stmt forbidden-set logic in block.rs for for-head lexical bindings and destructuring catch parameters, directly overlapping with this PR's changes.

Poem

🐇 Hop through the for-loop, quick as a hare,
Let-bindings forbidden — now properly there!
A catch with destructuring? No var shall sneak past,
The Annex B rulebook is tight to the last.
This bunny enforces each scope with great care! 🌿

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and specifically describes the main fix: suppressing Annex B.3.3 legacy var declarations across for-head and destructuring-catch lexicals.
Description check	✅ Passed	The description includes all required sections (Summary, Changes, Related issue, Test plan) with substantive content demonstrating thorough testing and validation.
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.

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✨ Finishing Touches

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• Commit unit tests in branch fix/5346-annexb-b33-residual

⚔️ Resolve merge conflicts

• Resolve merge conflict in branch fix/5346-annexb-b33-residual

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[proggeramlug]

Closing as a duplicate — superseded by #5356 (commit 6964193), already merged to main, which fixes this exact #5346 B.3.3 skip-case cluster with a functionally identical approach (seed for-head + destructuring-catch lexicals into the forbidden set, with the Annex B.3.4 simple-catch exemption). My branch was based on a stale local main at the #5319 commit, so I didn't see #5356 had already landed. Rebased onto current main, this commit produces no net diff.