[kixelated]

Problem

TS loses a Vec's capacity when the length drops, if that capacity came only from pushes. Six pushes of 1280-byte elements, then clear: native Rust reports 8, TS reports 0. std documents that clear and truncate leave the capacity unchanged, and a Vec never shrinks on its own. So the guarantee-only capacity model decided on 2026-11-01 must keep it.

Approach

• Adds the [S] quest quest/m3/ts-vec-clear-capacity.md, linked from quest/m3/README.md just above the exact-capacity quest.
• It is scoped to what moq-dev/rust2#527 leaves. feat(ts): clone_from through containers of values that drop, in std's order #527 records the capacity only inside rt.vecCloneFrom, before its own truncate, and leaves rt.vecTruncate and the other steps unchanged. moq-dev/rust2#522 already fixed a Vec made from a deque.
• I retested on feat(ts): clone_from through containers of values that drop, in std's order #527's head (3c1fab74) with a scratch crate: a Vec<[u64; 160]>, six pushes, then one step.
  • Native Rust reports 8 after every step.
  • TS reports 0 after clear, popping until empty, drain(..), retain of none, and resize(0, _); 1 after truncate(1) and dedup to one; 2 after split_off(2).
  • TS reports 8 after remove and swap_remove only because five elements still imply 8.
  • clone_from reports 8, which feat(ts): clone_from through containers of values that drop, in std's order #527 fixed.
• The quest notes that the capacity fixture's Known checks capacity only against a floor that with_capacity or reserve set, so the fixture misses this.
• It recommends recording the implicit capacity once, when the length first drops below it, and measuring on push/stack first.
• ts-vec-exact-capacity.md and the new quest each link the other under ## Related.
• The new quest names ts-clone-from-capacity.md through feat(ts): clone_from through containers of values that drop, in std's order #527's URL. That file is not on main yet, and quest check refuses a path link that does not resolve.

Impact

• Public API: none, quests only.

Alternatives

• Fold it into ts-vec-exact-capacity.md: that quest is about std's exact capacity, which the maintainer chose to leave open. This one is a guarantee.

Follow-ups

• Once feat(ts): clone_from through containers of values that drop, in std's order #527 lands, turn the reference to ts-clone-from-capacity.md into a path link.

Review

Skipped: a quest-only change of planning prose. just check passed on head 5693d2aa36cc1cf3f186c6aff014f5216436897d.

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Files selected for processing: 3

• quest/m3/README.md
• quest/m3/ts-vec-clear-capacity.md
• quest/m3/ts-vec-exact-capacity.md

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