[kixelated]

Problem

A Cow<str> is a plain JS string (#105), so matching its Borrowed or Owned variant is refused, and to_mut never changes the variant. Found in #537, where the variant of from_utf8_lossy's result can't be tested. The maintainer chose to plan it on 2026-11-03.

Approach

Adds quest/m3/ts-cow-str-variant.md [L], linked from quest/m3/README.md:

• Goal: matching the variant, plus into_owned and to_mut, translate as std's do, proven by differential tests that include from_utf8_lossy's variant. is_borrowed/is_owned are unstable (cow_is_borrowed) and stay refused.
• Plan: names where translation refuses today. Lays out the representation options: a wrapper only where some match observes the variant, found whole-program, or a wrapper everywhere. Each is to be measured against the plain string on the TS bench and moq's codec before choosing.

Impact

• Public API: none (quest only).

Alternatives

• Leaving Cow<str> without variants for good: it conflicts with the plan to cover all of std.

Follow-ups

None.

Review

Skipped: a quest-only plan with no code. just check passed on 922e4c451553f9422f127f01dc3e689de9dadd63.

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Reviewing files that changed from the base of the PR and between 090291b and 922e4c4.

Files selected for processing: 2

• quest/m3/README.md
• quest/m3/ts-cow-str-variant.md

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[pullfrog]

ℹ️ No critical issues — one minor suggestion inline.

Reviewed changes

• New quest doc quest/m3/ts-cow-str-variant.md — plans matching a Cow<str>'s Borrowed/Owned and into_owned/to_mut's variant semantics, noting what refuses today and laying out three representation options to measure on the TS bench and moq's codec.
• Index row in quest/m3/README.md — links the new quest from m3's list.

Quest-only plan, no code. I verified the plan's claims against the source: the library::cow_variant refusal sites match (ts/src/lower/mir/place.rs:644, ts/src/lower/mir/place.rs:1245, ts/src/lower/mir/rvalue.rs:562, ts/src/lower/library.rs:2590); into_owned indeed returns the text unchanged and to_mut is same_place while nothing reads the variant; Cow<[T]> splits on rt.Borrowed/rt.ByteSlice as the shape hint says; and the lang/cows, lang/utf8, and reject fixtures all exist as named. Format matches neighboring m3 quest docs.

(Written by glm-5.3-flash)

  ^{｜ Fix all ➔ ｜ Fix 👍s ➔ ｜ View workflow run ｜ Using glm-5.3-flash ｜ 𝕏}

[pullfrog]

Not every Cow<str> case in reject/src/strs.rs can move: cow_borrowed asks is_borrowed on a Cow<str>, which this quest keeps refused. Worth scoping this to the cases the quest resolves (the match), leaving cow_borrowed in place.

Technical details

# reject fixture's Cow<str> cases can't all move

## Affected sites
- quest/m3/ts-cow-str-variant.md:42 — "The `reject` fixture's `Cow<str>` cases move to them" sweeps in `cow_borrowed`, which stays refused.

## Required outcome
- The plan should say only the variant-matching case (`cow_matched`) moves to `lang/cows`; `cow_borrowed` (`is_borrowed` on a `Cow<str>`) stays in `reject` because `is_borrowed` remains refused per the Goal.

## Suggested approach
- "The `reject` fixture's matching case moves there" or naming `cow_matched` directly.