[kixelated]

Problem

std::iter::empty::<&mut T>() for a T JS copies, as given to zip or chain, was refused as "a mutable reference to a value JS copies, which its mapping would copy", though it yields nothing to write through. Default::default() of an empty iterator of places (iter::Empty, a LinkedList's IterMut or IntoIter) was refused by the same check, as #362 noted when it left IterMut::<u8>::default() in the reject fixture.

Approach

• std_places treats iter::empty and Default::default of an iterator whose default yields nothing as lending places, like a &mut collection given to into_iter.
• library::empty_default names those iterators, sharing a new list_iter matcher with default_value's list arm.
• refs::adapted in the lang fixture chains and zips iter::empty on either side of a &mut Vec and Option of bytes, and chains Empty::default() and a list's IterMut and IntoIter defaults, differentially against native Rust. The reject fixture drops IterMut::<u8>::default(), which now translates.

Impact

• Translator output only: these calls now translate instead of being refused. No API change.

Alternatives

• Accepting any call that yields places but takes none: too broad, since iter::from_fn and similar make places in closures the check exists for.

Follow-ups

None.

Review

Codex, gpt-6.1-sol, medium effort, round 1 on 18ab8cb: no findings. Since then, only conflict-free merges of origin/main (#492, #494, #463).

🤖 Generated with Claude Code

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No actionable comments were generated in the recent review.

Walkthrough

The lowering code now recognizes empty iterators and default-empty linked-list iterators as lending no places. It also represents default linked-list iterators as iterators over an empty list. Language fixtures add chain and zip cases with empty iterators. Rejection fixtures remove the Default::default case for a mutable reference to a copied value. The deferred-task entry and its plan document are removed.

Merge Risk: Minimal · up to 4caa4

No actionable merge-blocking issue is identified in the empty-iterator handling changes. Merge after normal checks pass.

Pre-merge checks: 5

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: accepting empty iterators of mutable references as place sources.
Docstring Coverage		Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The description directly explains the empty-iterator handling changes, the affected fixtures, the implementation approach, and the translator-only impact. It matches the changeset.

Full details: Docstring Coverage

Explanation

Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 5 functions across 5 files.

Finishing Touches

Simplify code

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Recent review info

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• Source mode: Auto
• Repository: Unavailable
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• Layers: Organization UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: 510f1630-f49b-4510-90fd-075ff21ee210

Commits

Reviewing files that changed from the base of the PR and between ab8f549 and 4caa41f.

Files selected for processing: 9

• quest/m3/README.md
• quest/m3/ts-empty-places.md
• ts/fixtures/lang/refs/expected/refs.ts
• ts/fixtures/lang/refs/src/refs.rs
• ts/fixtures/reject/census.txt
• ts/fixtures/reject/expected.txt
• ts/fixtures/reject/src/values.rs
• ts/src/lower/library.rs
• ts/src/lower/mir/call.rs

Files with no reviewable changes: 1

• quest/m3/ts-empty-places.md

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