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captured variable reassigned inside a labeled block is not boxed — closure reads stale snapshot #5869
Description
Summary
A captured variable that is reassigned inside a labeled block is not boxed: closures capture the creation-time value and never observe the reassignment. Reads through the closure see a stale snapshot while direct reads in the same scope see the live value.
Repro (vs node --experimental-strip-types)
function factory() {
let arr: any = [];
const read = () => arr.length;
outer: {
arr = [1, 2, 3, 4];
break outer;
}
console.log("len:", read());
console.log("json:", JSON.stringify(arr));
}
factory();node:
len: 4
json: [1,2,3,4]
perry (main):
len: 0
json: [1,2,3,4]
The closure reads .length === 0 on the stale empty array while JSON.stringify in the same scope prints all four elements.
Root cause
The boxing analysis in crates/perry-codegen/src/boxed_vars.rs is inconsistent on Stmt::Labeled: collect_write_ids_in_stmt (~1179) HAS a Labeled arm, but five sibling walkers do not —
collect_closure_refs_and_writes_in_stmt(_ => {}at ~737)collect_outer_writes_in_stmt(~970)collect_for_init_ids(~641)collect_self_recursive_closure_ids(~596)collect_let_types_in_stmts(~1362-1367)
A reassignment inside a labeled block is invisible to outer_writes (and a closure inside a labeled block contributes no closure_refs), so the captured+mutated variable is never boxed → the closure keeps a creation-time snapshot.
Why this matters
Minified bundles emit labeled blocks (e: { … break e }) pervasively; hand-written repros don't — which is exactly why this class of bug reproduces only in large bundles. The failure signature (a captured array reading .length === 0 while JSON.stringify in the same scope shows the elements, and a fresh local array "fixing" it) matches the long-standing unexplained misbehavior seen in a 13.8MB Next.js app-page bundle's webpack factory.
Suggested fix
Add Stmt::Labeled arms (recurse into the body) to all five walkers, mirroring the existing arm at ~1179.
Activity
proggeramlug commentedon Jul 2, 2026
PR #5871 fixes the codegen half (all five
boxed_vars.rswalkers now descend intoStmt::Labeled); the write-inside-label repro passes.Residual (this issue stays open): the closure-created-INSIDE-the-label variant still misbehaves, one layer up:
--trace hir --focus factory2shows perry-hir lowers the labeled block toLabeled { body: DoWhile { body: [LocalSet(1, Closure { captures: [0], mutable_captures: [], … }), LabeledBreak], condition: false } }— notemutable_captures: []: the HIR mutable-capture classifier doesn't see closures nested under theLabeled/DoWhile(false)wrapper, so it classifiesnas a non-mutable capture. With #5871 the codegen creation site now stores the box pointer (boxing analysis sees the capture), but the closure body still reads the capture raw (HIR says non-mutable) — the call returns the box-pointer bits as a denormal number, the classic unboxed-capture signature.Fix direction: teach the perry-hir mutable-capture classification walk to descend
Stmt::Labeled(and the synthesizedDoWhile(false)wrapper) when pairing closures with later writes. A regression test is already in the tree,#[ignore]d:crates/perry/tests/issue_5869_labeled_block_capture_boxing.rs::closure_inside_labeled_block_counts_as_capture— un-ignore it when the HIR side lands.fix(codegen): #5869 — boxing analysis must descend into labeled blocks
fix(codegen): boxing analysis must descend into labeled blocks (#5871)
fix(codegen): #5869 residual — boxed locals must not appear in module…
fix(codegen): #5869 residual — boxed locals must not feed the typed-A…
fix(codegen): #5982 — module-global captures must not feed the typed-…
proggeramlug commentedon Jul 13, 2026
Fixed by #5871 (boxing analysis descends into labeled blocks) and #5949 (the residual: boxed locals must not feed the typed-ABI closure specializations).
Re-verified on current main: both the original repro (write inside the label) and the residual from the comment thread (closure created inside the label) are byte-identical to node — the latter previously printed a denormal (
2.64e-311) instead of42.Both regression tests in
crates/perry/tests/issue_5869_labeled_block_capture_boxing.rsare live (no longer#[ignore]d).perf(codegen): keep the numeric-array specialization when the array i…
perf(codegen): keep the numeric-array specialization when the array i…