[kixelated]

Problem

rust2-ts erased a Box<I> and folded what it held with that iterator's own folds. std's Iterator for Box<I> forwards only next, size_hint, nth, last, next_back, nth_back, and len/is_empty, and reads everything else by next, advance_by and the folds included. So Box::new(a.map(f).zip(b.map(g)).skip(1)).count() made the skipped pair whole, where native skips it one side at a time through the zip's nth. It also read a zip's excess left item in a different order.

The same gap affected Box<dyn Iterator>, which went through rt.iterDyn and so forwarded advance_by. That is right for &mut dyn Iterator but not for a box, which reads advance_by by next (for example through Chain::nth).

Approach

• rt.iterBox(it): a runtime wrapper that forwards exactly what std's box forwards. Its last dispatches to the held iterator's own last, in both ownedLast and the non-dropping iterLast. Everything else falls back to the Iter defaults, which read by next. Dyn and Boxed now share a Forward base, and Dyn adds advanceBy/advanceBackBy.
• into_iter wraps by the box layers of the type it was given (boxes):
  • boxes before the first reference become one rt.iterBox;
  • a box past a reference becomes rt.iterBorrow(rt.iterBox(..)), so &mut Box<I> folds and takes last by next, as std's &mut I over a box does.
• call now passes the original self_ty and receiver to into_iter, so a consumer sees the box.

Proof: lang/dyns gains dyns::boxes. It runs a skip of a TRA zip whose items log as they are made and dropped, through every consumer: forwarded (size_hint, len, next, next_back, nth, nth_back, last, Box<Box<I>>::last) and by-next (count, fold, for_each, position, find, any, max_by_key, collect, rev().fold, sum, skip, step_by, Chain::nth's advance_by). Each is checked owned, behind &mut, as a Box<&mut I>, and as Box<dyn> vs &mut dyn. A non-dropping logging zip covers last in the iterLast path. Mutation checks pass: forwarding advanceBy in Boxed, dropping the last dispatch, or dropping the borrowed-box wrap each fails the differential test.

Impact

• Translator output: a boxed iterator, and a reference to one, is now rt.iterBox(..) where it is read. Snapshot churn in lang/dyns and lang/iters.
• Runtime: new export rt.iterBox. Nothing removed.

Alternatives

• Reporting the box layers from peel_iter, which is exact per layer. Collapsing them in boxes gives the same semantics with less code, since iterBorrow(iterBox(x)) reads alike whatever x holds.
• Wrapping only at Box<dyn> coercion. That misses Box<I> of sized iterators, and &mut *boxed_dyn would wrongly lose advance_by forwarding.

Follow-ups

• #463's box-reader commit (f1e9b4c) keeps a boxed crate iterator's by-value overrides out of rt.iterFrom except last. Once this lands, rt.iterBox already reads that box by next and forwards only last, so its boxed flag in own_methods becomes redundant and can be dropped when Folds from the back, and a crate's own folds #463 merges main. The call hunk here matches Folds from the back, and a crate's own folds #463's line for line, to keep that merge trivial.

Review

Codex gpt-6.1-sol, medium, via quest review.

• Round 1 (ff468ae): one finding. The non-dropping iterLast read a box by next instead of forwarding last. Fixed in f1f5ee6, with a fixture case that fails without the fix.
• Round 2 (602d16e): no findings.

🤖 Generated with Claude Code

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Commits

Reviewing files that changed from the base of the PR and between 1eb763b and 602d16e.

Files selected for processing: 11

• quest/m3/README.md
• quest/m3/ts-box-folds.md
• ts/README.md
• ts/fixtures/lang/dyns/expected/core/option.ts
• ts/fixtures/lang/dyns/expected/dyns.ts
• ts/fixtures/lang/dyns/expected/lib.ts
• ts/fixtures/lang/dyns/src/dyns.rs
• ts/fixtures/lang/dyns/src/lib.rs
• ts/fixtures/lang/iters/expected/iters.ts
• ts/runtime/rt/iter.ts
• ts/src/lower/iter.rs

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