fix(hir): hoist top-level var entry slot in function-expression bodies

[proggeramlug]

Summary

A var is function-scoped and hoisted: a declaration textually after a statement that reads/writes it is still bound (as undefined) from function entry. Perry honored this for function declarations and arrows, but not for function expressions — so a var used before its own late declaration folded to a constant undefined at every earlier read.

Root cause

The fn-expr lowering has a top-level var pre-pass (issue #838) that pre-registers the hoisted local and adds it to hoisted_id_set. That only fixes the forward-capture case (a closure created before the declaration reads the var through a prealloc box). A var merely read/written before its declaration but not captured by any closure got no box and — crucially — no undefined-initialised entry slot, so its storage first materialised at the late var x = … statement. The fn-decl/arrow path (predefine_var_bindings_in_function_body) always emits that entry slot; the fn-expr path did not.

HIR was already correct (one shared LocalId for both the loop use and the late redeclaration); only the entry Let was missing, so codegen resolved earlier LocalGets to a folded undefined.

Real-world impact: React cloneElement drops all props

React 19's cloneElement/createElement are exactly this shape:

exports.cloneElement = function (element, config, children) {
  var props = assign({}, element.props), key = element.key;
  if (null != config)
    for (propName in (void 0 !== config.key && (key = "" + config.key), config))
      !hasOwnProperty.call(config, propName) || "key" === propName || … ||
        (props[propName] = config[propName]);
  var propName = arguments.length - 2;   // hoisted; declared AFTER the loop
  …
};

Compiled as a function expression, the loop body's hasOwnProperty.call(config, propName) saw propName === undefined, so the !hasOwn(...) || … chain short-circuited and props[propName] = config[propName] never ran — cloneElement returned the element's original props with none of config merged in. Radix Slot (shadcn <Button asChild>) merges its props onto its child via cloneElement, so <Button asChild><Link/></Button> rendered a bare <a href> with no data-slot/data-variant/className.

Fix

The top-level var pre-pass now also pushes the undefined-initialised entry Let into nested_var_prologue, exactly like the nested-var pre-pass a few lines below (and the fn-decl/arrow path).

Testing

Reproduces at -O0, confirming a lowering bug rather than an LLVM-optimisation artifact. Added test-files/test_gap_var_hoist_forin_fn_expr.ts (byte-identical to node --experimental-strip-types) covering the cloneElement shape plus forward-var reads in function expressions, object methods, arrows, and if-branch vars. Verified end-to-end: a real Next.js 16 app's shadcn <Button asChild> buttons now render with their full class list, matching Node byte-for-byte in the visible DOM.

Summary by CodeRabbit

• Bug Fixes

  • Fixed var hoisting in function expressions so variables are available with undefined values before their declarations.
  • Corrected hoisting behavior in for-in loops, arrow functions, object methods, and conditional branches.

• Tests

  • Added regression coverage for forward var references and related function-expression scenarios.

[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: e76d0d28-d89f-4590-ac76-0fc66e413428

📥 Commits

Reviewing files that changed from the base of the PR and between 885a8f5 and b53f3fb.

📒 Files selected for processing (2)

• crates/perry-hir/src/lower/expr_function.rs
• test-files/test_gap_var_hoist_forin_fn_expr.ts

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📝 Walkthrough

Walkthrough

Function-expression lowering now creates undefined-initialized storage for newly hoisted top-level var bindings. A regression test covers for-in loops, forward reads, arrow functions, object methods, and conditional declarations.

Changes

Function-expression var hoisting

Layer / File(s)	Summary
Materialize hoisted var slots crates/perry-hir/src/lower/expr_function.rs	The top-level var pre-pass reuses the binding name and emits an undefined-initialized Stmt::Let for each newly hoisted binding.
Validate hoist behavior test-files/test_gap_var_hoist_forin_fn_expr.ts	Regression cases verify hoisting across for-in loops, forward reads, arrow functions, object-literal methods, and conditional branches.

Estimated code review effort: 3 (Moderate) | ~15 minutes

Possibly related PRs

• PerryTS/perry#5270: Modifies function-expression lowering and top-level var hoisting.
• PerryTS/perry#5474: Updates the same function-expression hoisting pre-pass and storage registration.
• PerryTS/perry#6038: Represents hoisted bindings with Stmt::Let and undefined initializers.

Suggested reviewers: andrewtdiz

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely matches the main change: hoisting the entry slot for top-level var bindings in function expressions.
Description check	✅ Passed	Covers summary, root cause, fix, and testing well, but omits the template's explicit Changes and Related issue sections.
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 fix/var-hoist-forin-fn-expr

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