[kixelated]

Problem

rust2-ts refused String + &str ("unsupported call to <String as Add<&str>>::add") while += mapped. Cow<str>'s + and += (of a &str or another Cow<str>) were refused too, though a Cow<str> is the same JS string.

Approach

• library.rs: Add::add whose Self is a JS string (String or Cow<str>) is JS + of the moved text. Library mappings only see std's impls, so a crate's own Add on String still calls the crate's.
• call.rs: the push/push_str/add_assign arm now takes any std AddAssign on a JS string (Cow<str> included), and skips a crate's own impl AddAssign<Local> for String, which it used to swallow.
• Differential tests against native Rust (lang/strs and lang/cows fixtures):
  • strs::edit: +=, mem::take(s) + &piece, a chain with "" and &&str, and String::with_capacity(a) + s through deref of the &mut String, over empty and multi-width text. The moved-from string's capacity reuse is unobservable, as capacity is refused.
  • strs::edits: + of the moved local itself.
  • cows: + and += of &str and Cow, including an empty left side, where std borrows the right side.
• ts/README.md notes +.

Impact

• Translator output: String + &str, Cow<str> + &str, Cow<str> + Cow<str>, and Cow<str> += &str | Cow<str> translate instead of being refused. No public Rust API change.

Alternatives

• String only, per the quest's letter. Dropped: Cow<str>'s + is the same gap with the same representation, so leaving it would split one problem across PRs.

Follow-ups

• Merging main removed the Related link in ts-operator-paths.md (quest: plan operator trait methods called by path #481) to the quest this PR deletes.
• Popping a String that is reassigned elsewhere: found while writing the local test. s.pop() on a local that another arm assigns whole (s = String::from("x"), no + involved) writes through an undeclared rt.Place, so tsc fails. Pre-existing and independent of this change; the test shadows the local instead of reassigning it.

Checks

• just ci passed on 0f46b6c, merged with main at 3d37327.
• Mutation check: swapping the + operands fails the native differential of lang::strs (1746 differences) and lang::cows (13).

Review

• Reviewer: Codex, gpt-6.1-sol, medium effort (quest review), round 2.
• Head: 0f46b6c
• Findings: none.

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Commits

Reviewing files that changed from the base of the PR and between 3d37327 and 0f46b6c.

Files selected for processing: 11

• quest/m3/README.md
• quest/m3/ts-operator-paths.md
• quest/m3/ts-reassigned-pop.md
• quest/m3/ts-string-add.md
• ts/README.md
• ts/fixtures/lang/cows/expected/cows.ts
• ts/fixtures/lang/cows/src/cows.rs
• ts/fixtures/lang/strs/expected/strs.ts
• ts/fixtures/lang/strs/src/strs.rs
• ts/src/lower/library.rs
• ts/src/lower/mir/call.rs

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