[kixelated]

Problem

BinaryHeap<u8>::from(h.into_vec()), and VecDeque<u8>::from(Vec<u8>), convert a Vec<u8> (rt.ByteVec) to a JS array with rt.byteVecToVec. That conversion sets the array's capacity with capSet, which records nothing when the capacity equals what the length implies. Once the heap is cleared, its capacity falls back to what the new length implies, below what an earlier reserve guaranteed. Rust keeps it:

let mut h = BinaryHeap::<u8>::new();
h.extend(...10 items...);   // capacity 16
h.reserve(0);               // guarantees at least 10
h = BinaryHeap::from(h.into_vec());
h.clear();
h.capacity()                // native 16; translated 0

The lang differential found this once #414's two-byte selector moved random inputs onto capacity::heaps (reduced from input 695dff6e…).

Approach

byteVecToVec holds the capacity with capHold, as reserve does, so it survives the length dropping. No other capSet makes a fresh array from a container whose capacity may be held: the others either keep an existing hold (c.cap !== undefined) or start a fresh floor.

Impact

• Output: unchanged; the runtime's byteVecToVec now keeps a u8 heap's or deque's capacity as Rust does. No API change.

Tests: a named capacity::heaps case of each element type: extended past its least capacity, reserve(0), remade from its Vec, and cleared. It differs from native on the u8 heap without the fix, and agrees with it.

Stacks on #414 (its two-byte op in caller.ts); land after it.

Alternatives

• Make capSet always record the capacity. Rejected: it would put a cap on every cloned Vec, where the derived one already meets std's guarantee.

Review

• Codex gpt-6.1-sol at medium effort (quest review --base origin/ts-harness/selector), round 1 on 7e814171: no findings.
• just ci passes locally on 7e814171.

(written by Claude Opus 5.5)

🤖 Generated with Claude Code

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