[kixelated]

Problem

vec![x; n] was refused when x drops ("passing T, which drops, to vec::from_elem"), since nothing dropped x for n == 0.

Approach

• vec::from_elem joins discards with its element, so the mapping hands rt.vecFromElem the element's dropper.
• rt.vecFromElem drops the value where n is 0. Otherwise it keeps std's order: n - 1 clones in order, then the original moved into the last slot.
• New differential case clones::repeats, checked against native Rust, logs every clone and drop of: a Logged; a type with its own Clone; an enum; an array; an Option; a tuple holding a String; a Vec (with capacities, so the original row is told apart from its exact-capacity clones); nested vec![vec![x; m]; n]; and a generic T: Clone. n comes from the input (0 to 4), plus a literal 0. One case logs while it evaluates the count, which shows x is made first.
• Verified the case fails with the n == 0 drop removed.

Consumer: std coverage planned in m3 (found writing clones::clone_froms); moq does not call it yet.

Impact

• Translator output: vec![x; n] of a dropping T now translates. Other vec![x; n] output is unchanged.
• Runtime: vecFromElem takes an optional trailing drop.

Alternatives

• Reusing vecResize on an empty vector, as the quest suggested. It reserves through size and always drops value when shrinking, so it saves nothing over the 4-line change here.

Follow-ups

None.

Review

Codex gpt-6.1-sol, medium effort, via quest review, round 2 on head b8201c9: no findings.

• Round 1 (d567303) reported one finding, ts/src/lower/collections.rs:574 (custom BuildHasher state). Refuted: this branch doesn't touch that file. The round diffed against a newer origin/main that held feat(ts): hold a crate's own BuildHasher in a hash map or set #509, so feat(ts): hold a crate's own BuildHasher in a hash map or set #509 looked like a revert here. Merging origin/main removed it from the diff.

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Files selected for processing: 9

• quest/m3/README.md
• quest/m3/ts-vec-from-elem-drops.md
• ts/fixtures/lang/clones/expected/clones.ts
• ts/fixtures/lang/clones/expected/core/option.ts
• ts/fixtures/lang/clones/expected/lib.ts
• ts/fixtures/lang/clones/src/clones.rs
• ts/fixtures/lang/clones/src/lib.rs
• ts/runtime/rt.ts
• ts/src/lower/library.rs

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