[kixelated]

Problem

Iterator for Box<I> forwards only next, size_hint, nth, last, next_back, nth_back, len, and is_empty, so std reads every other consumer of a box by next. The translation erases the box and folds what it holds by that iterator's own fold. A differential probe in the lang fixture (on #437's head) shows where that diverges:

• A boxed Chain of mapped vec::IntoIters matches native across 750k cases for count, fold, for_each, find, position, max_by_key, rev().fold, and skip(1).count(). Chain's folds read and drop in next's order.
• Box::new(a.map(f).zip(b.map(g)).skip(1)) does not (98 differences): count and fold make and drop the skipped pair whole, as Skip's by-index fold does, while native skips it one side at a time through the zip's nth.

Approach

One quest, quest/m3/ts-box-folds.md [S]: read a Box<I> of a sized iterator, owned or behind a &mut, as rt.iterDyn reads a dyn Iterator, with last forwarded to the iterator's own as std's BoxIter specializes it. m3 because no consumer builds one: moq boxes only dyn Iterator, which #437 already reads this way.

Impact

None: quests only.

Alternatives

• Do nothing, as the Chain probe matches: the skip of an indexed zip shows that a box's iterator can fold in an order next doesn't.

Follow-ups

None. The probe also met rfold refused for any iterator, which quest/m3/ts-iter-back-folds.md already plans.

Review

• Codex (quest review), gpt-6.1-sol, medium effort, round 1 on e8af0d9f6aa57800b807ae68d4031bfefc7c81de: no findings.
• just check passes locally.

(written by Claude Opus 5.5)

🤖 Generated with Claude Code

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Reviewing files that changed from the base of the PR and between d9695a7 and 2a0a060.

Files selected for processing: 2

• quest/m3/README.md
• quest/m3/ts-box-folds.md

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