[kixelated]

Problem

rust2-ts refused a closure that owns a value that drops when it was given to String::retain or as a str search's Pattern (contains, split, and the rest), as the reject fixture's string_retained and pattern_closure showed. Completes quest/m3/ts-string-closures.md.

Approach

• called_by reads a Pattern bound as calling its closure by FnMut, so called_closures decides its drop as it does for any other closure std calls: rt.dropAfter for a search that returns a value, and the returned iterator for one that returns an iterator.
• holds_closure lists str's search iterators. drops counts one as dropping when its closure does, because std nests the closure inside internal searcher structs that no rt.Iter stands for.
• Runtime: str.ts's Searcher drops its pattern once, and Split, Matches, and MatchIndices drop their searcher. rsplit and the other r forms are iterRev of these, which already drops what it wraps.
• String::retain, lowered in mir/call.rs, drops a closure it owns after assigning the edited string. That is where std's parameter drops, after the length guard.
• Proof: a new searched step in the task fixture, differential against native Rust:
  • Every value search, matched and unmatched.
  • Every iterator search, read partly, ended by a for with break, collected, counted, last, nth, and wrapped in rev().map().
  • retain with an owned closure, a borrowed &mut one that its owner drops later, and an empty string that never calls its closure.

Impact

• Translator output: these calls now translate where they were refused. No runtime API is exported or changed.

Alternatives

• Recursing drops into all of std's internal structs, which would also reach the searcher. Rejected: it changes how every std type drops. The holds_closure arm reaches only the iterators the runtime drops closures for.

Follow-ups

• A reference to a closure as a pattern (s.contains(&f), s.split(&mut f)) is still refused as a pattern other than a char, a &str, a predicate, or chars, as the README lists. That gap is separate from drops. It is not a quest yet: suggested for /quest-plan.
• #463 edits the same reject fixture, census, and task.ts. Whichever lands second regenerates the line-numbered expected.txt and census.md. The code changes don't overlap.

Review

• Reviewer: Codex, gpt-6.1-sol, medium effort (quest review), round 3.
• Reviewed head: 7803e3d, which merges origin/main at ccf8d33. The reject fixture's line-numbered expected.txt and census.md conflicted and were regenerated.
• Findings: none, in this round or in rounds 1 and 2.
• f586641 and 4c9afea merge origin/main at fdb9bff and 94c22e0, a quest change and an ffi change, without conflicts. just ci passed on each.

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Commits

Reviewing files that changed from the base of the PR and between 94c22e0 and 4c9afea.

Files selected for processing: 19

• quest/m3/README.md
• quest/m3/ts-string-closures.md
• ts/README.md
• ts/fixtures/reject/census.md
• ts/fixtures/reject/expected.txt
• ts/fixtures/reject/src/lib.rs
• ts/fixtures/reject/ts.toml
• ts/fixtures/task/expected/drops.ts
• ts/fixtures/task/expected/drops/searched.ts
• ts/fixtures/task/src/drops.rs
• ts/fixtures/task/src/drops/searched.rs
• ts/fixtures/task/src/harness.rs
• ts/fixtures/task/ts.toml
• ts/runtime/rt/str.ts
• ts/src/lower/drop.rs
• ts/src/lower/iter.rs
• ts/src/lower/library.rs
• ts/src/lower/mir/call.rs
• ts/tests/task.ts

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