[kixelated]

Problem

Since #437, a dyn Iterator, dyn DoubleEndedIterator, or dyn ExactSizeIterator translates behind &, &mut, and Box. Several forms were still refused:

• An Arc, Rc, or Weak of one, refused as "a trait object of another crate's trait".
• Arc::get_mut of one, an "unsupported call".
• A dyn FusedIterator behind any pointer.

The trait objects quest names Arc<dyn Iterator> and dyn FusedIterator. moq builds neither; this is std coverage the quest calls for.

Approach

• dyn FusedIterator joins the family: an rt.Iter, made by rt.iterDyn as the others are. Its vtable is Iterator's.
• An Arc or Rc unsized to one of these is an rt.ArcDyn over rt.iterDyn of the iterator the Arc holds, built as dyn Future's is. A Weak coerced to one, and Arc::from(Box<dyn Iterator>), follow from #450. The last Arc drops the iterator, and so the items it still owns.
• Arc::get_mut and Rc::get_mut of a trait object lend its object while no other Arc or Weak shares it, as they do for a sized value. This covers the crate's own traits too.
  • A dyn Any's stays refused. Its object may hold a copy of a value JS copies, and Arc::downcast gives back the Arc<T>, which would not see a write made through get_mut.

Also, as the maintainer decided, quest/m3/ts-refcell-debug.md now covers Debug of a Cell too.

Tests: dyns::iters in the lang fixture's differential test against native wasm32 now also covers:

• Fused: through &mut dyn FusedIterator, a fuse of the crate's own iterator that resumes after its end, a Box<dyn FusedIterator> of the crate's own fused iterator, and the resuming one unfused, through dyn Iterator, for contrast.
• An Arc<dyn Iterator<Item = Tagged>> made from an Arc<vec::IntoIter<_>>:
  • size_hint through a clone, and get_mut refused while a clone or a Weak shares it.
  • next through get_mut once they drop, dropping that item.
  • The rest of the items, which drop with the last Arc.
• An Rc<dyn ExactSizeIterator> and its upcast to Rc<dyn Iterator>: len, get_mut refused while shared, then next and size_hint.
• Arc::from(Box<dyn Iterator>) of the crate's own iterator, read by get_mut and then dropped.
• A Weak coerced from an Rc<Countdown>, upgraded before and after it drops.
• Pin<Box<dyn Iterator + Unpin>>, read and dropped.

Census at the moq pin, before → after: unchanged (542 / 86 / 70 for moq-net / model / kio), since moq builds none of these.

Impact

• The translator's output: these trait objects translate instead of failing.
• No breaking change.

Alternatives

• Arc::get_mut of dyn Iterator alone: it is the same mapping for every trait object, so limiting it would leave the crate's own traits refused for no reason. Only dyn Any's has a way back to the concrete value.

Follow-ups

• quest/m3/ts-dyn.md now holds only dyn Error's downcasts, resized from [L] to [M]. Its goal no longer names &mut dyn Debug, which landed in #453.
• Arc::get_mut of a dyn Any stays refused, as above.

Review

Codex gpt-6.1-sol at medium effort, via quest review, round 2 on head 95458959956d7ccc24a98e47e65b39144422a197: no findings.

Round 1 (head 402db123) reported one finding, refuted: callback ownership in ffi/src/dart.rs, which is #472's code. #472 landed on main during the round and is outside this branch's diff.

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Reviewing files that changed from the base of the PR and between 070208c and cf0607f.

Files selected for processing: 11

• quest/m3/README.md
• quest/m3/ts-dyn.md
• quest/m3/ts-refcell-debug.md
• ts/README.md
• ts/fixtures/lang/expected/core/option.ts
• ts/fixtures/lang/expected/dyns.Resuming.ts
• ts/fixtures/lang/expected/dyns.ts
• ts/fixtures/lang/src/dyns.rs
• ts/src/lower/iter.rs
• ts/src/lower/mir/rvalue.rs
• ts/src/lower/task.rs

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