[kixelated]

Problem

A crate's own iterator whose JS value is copied, as an enum's is, failed to type-check when taken by value: toggle.map(f) gave rt.iterFrom the enum's value where its next takes an rt.Place (found by #515). The same held for every by-value use (filter, zip, chain, collect, extend, sum, rev, cycle, IntoIterator of a crate type returning one), and for std's methods taking &mut self (nth, any, find, position), whose mapping got the enum's value, so writes landed in a copy and a value that drops dropped twice. A borrowed one ((&mut toggle).map(f)) also passed the place to its own &self overrides (size_hint, len), which take the value.

Approach

• into_iter of an owned one moves it into a place of its own, { v: value }; its dropper and its overrides that take self or &self read it.v, and its clone (for cycle and clone) makes a new place.
• std's Iterator and DoubleEndedIterator methods of such an iterator take the &mut's place (pointer::takes_place), and a std method taking &mut self of the crate's own iterator borrows it, so into_iter gives no dropper and only the overrides std's &mut I forwards. For a struct, this drops the by-value fold, count, and last from what try_fold, rposition, and the like may call (the ends and stock snapshot changes); std's never call them there.
• New iters::by_value differential case: a fieldless enum with its own size_hint, count, and next_back, an enum holding a value that logs as it drops, and a struct that logs as it drops, through adaptors, consumers, for, extend, cycle, IntoIterator of a crate type, adaptors dropped partway, and std's &mut self methods on a local.
• Records three std gaps fix(ts): keep a nested Vec's capacity where clone_from writes over it #533 found as quest/m3/ts-std-stragglers.md.

Impact

• Translator output only: such iterators now translate, by value and through std's &mut self methods. No public API change.

Alternatives

• A runtime rt.iterFrom variant that knows which Own methods take &mut self: duplicates the trait's signatures in the runtime, where the translator reads them from rustc.

Follow-ups

• Cow's clone_from, Reverse's clone, and Option::as_slice, from fix(ts): keep a nested Vec's capacity where clone_from writes over it #533's follow-ups.

Consumer: none yet, planned as part of std (m3).

Review

Codex (quest review), gpt-6.1-sol, medium effort, round 1 on head 5d8d8a5 against origin/main f0b2a31: no findings. Head 3c0fc4b adds only a conflict-free merge of origin/main.

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Commits

Reviewing files that changed from the base of the PR and between 7e7e420 and 3c0fc4b.

Files selected for processing: 18

• quest/m3/README.md
• quest/m3/ts-own-iter-value.md
• quest/m3/ts-std-stragglers.md
• ts/README.md
• ts/fixtures/lang/ends/expected/ends.ts
• ts/fixtures/lang/iters/expected/core/option.ts
• ts/fixtures/lang/iters/expected/iters.Bag.ts
• ts/fixtures/lang/iters/expected/iters.Logged.ts
• ts/fixtures/lang/iters/expected/iters.Noted.ts
• ts/fixtures/lang/iters/expected/iters.Phase.ts
• ts/fixtures/lang/iters/expected/iters.Ticks.ts
• ts/fixtures/lang/iters/expected/iters.ts
• ts/fixtures/lang/iters/expected/lib.ts
• ts/fixtures/lang/iters/src/iters.rs
• ts/fixtures/lang/iters/src/lib.rs
• ts/fixtures/stock/expected/maps.ts
• ts/src/lower/iter.rs
• ts/src/lower/pointer.rs

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