[kixelated]

Problem

A whole write through a &mut to a LazyCell, LazyLock, PollFn, or LocalKey (*r = v, mem::replace, mem::swap, Cell::from_mut(r).set, a mapped RefMut) was refused since #386, because the runtime keeps their state in #private fields that the Object.assign field copy misses.

Approach

• rt.LazyCell (so rt.LazyLock) and rt.PollFn each own a $move(to?) method that moves their private state into to, or into a new object. ptrWrite, replaceIn, objectCopy (so cellIn and partPlace) use it when an object has it. *r = v on these types lowers to rt.ptrWrite instead of Object.assign.
• A LocalKey is a handle: every copy of the const names the same storage, so moving its state into another key would change that key for every user. A &mut LocalKey is now an rt.Place, as an Arc's is, so a write changes which key the place holds. A key's const used as a value, not only borrowed, now maps to its module value. Without that, a stock crate can't get an owned key to write.
• The method is a plain name ($move), not a symbol, because esbuild keeps any class with a computed key and that broke the codec tree-shake test. No Rust field can be named $move.

Tests, each differential against native Rust:

• lang/statics writes: a LazyCell and a LazyLock whose function owns a value with a Drop impl. Each write way, with neither, either, or both forced first. It checks Debug, the value, how many times the function ran, and drop order.
• lang/locals keys: a copy of one key written with another by each way. It checks which key the copy names after the write and that neither key's storage changed.
• task futures::boxed: poll_fns written by each way, plus a Pin<Box<PollFn>> written whole. It checks drop order and which function polls.

I confirmed the statics and task tests fail when the runtime move is disabled. The four reject cases these replace are removed.

Impact

• Translator output: these writes now translate instead of being refused. A &mut LocalKey is an rt.Place<rt.LocalKey<T>>.
• Runtime: objectCopy and ptrWrite move $move classes. rt.PollFn's fields are no longer readonly. Nothing is breaking.

Alternatives

• A symbol key (rt.MOVE): this broke tree-shaking (above).
• Static dispatch in the translator for every runtime helper: this adds a variant of replaceIn, cellIn, and partPlace per class, for the same result.
• Moving a LocalKey's state like a lazy cell's: this is wrong, because the key's other copies share that storage.

Follow-ups

None.

Review

Codex gpt-6.1-sol, medium, round 1 on d90460f: no findings. Since then, a conflict-free merge of origin/main into 26361c3.

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Commits

Reviewing files that changed from the base of the PR and between 0bebbec and 26361c3.

Files selected for processing: 26

• quest/m3/README.md
• quest/m3/ts-lazy-writes.md
• ts/README.md
• ts/fixtures/lang/locals/expected/lib.ts
• ts/fixtures/lang/locals/expected/locals.ts
• ts/fixtures/lang/locals/expected/std/thread/local.ts
• ts/fixtures/lang/locals/src/lib.rs
• ts/fixtures/lang/locals/src/locals.rs
• ts/fixtures/lang/statics/caller.ts
• ts/fixtures/lang/statics/expected/lib.ts
• ts/fixtures/lang/statics/expected/statics.Dropped.ts
• ts/fixtures/lang/statics/expected/statics.ts
• ts/fixtures/lang/statics/expected/std/thread/local.ts
• ts/fixtures/lang/statics/src/lib.rs
• ts/fixtures/lang/statics/src/statics.rs
• ts/fixtures/reject/census.txt
• ts/fixtures/reject/expected.txt
• ts/fixtures/reject/src/places.rs
• ts/fixtures/reject/src/statics.rs
• ts/fixtures/reject/ts.toml
• ts/fixtures/task/expected/futures.ts
• ts/fixtures/task/src/futures.rs
• ts/runtime/rt.ts
• ts/runtime/rt/task.ts
• ts/src/lower/mir/place.rs
• ts/src/lower/mod.rs

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