[kixelated]

Problem

rust2-ts refused BuildHasherDefault<H>, std's BuildHasher of a Default Hasher, which is how rustc-hash's FxHashMap and nohash-hasher's IntMap build theirs: "unsupported type std::hash::BuildHasherDefault from another crate". Quest: quest/m3/ts-build-hasher-default.md (all of std; no moq consumer yet).

Approach

• BuildHasherDefault holds only a PhantomData, so it joins library::nothing: its value is undefined, made by new, Default, Clone, a constant, or an aggregate, every value ==, and its Debug writes BuildHasherDefault, as std's debug_struct(..).finish() does.
• build_hasher is H::default(). hash_one builds that, hashes, finishes, and drops the hasher then the value: through the crate's own H as feat(ts): hold a crate's own BuildHasher in a hash map or set #509's own_hash_one does, or std's DefaultHasher as RandomState's does.
• A map or set holding one already hashes keys around it (the hash-around decision of feat(ts): hold a crate's own BuildHasher in a hash map or set #509); hasher lends undefined, which hashes through H as above.
• Root-cause fix found by the tests: an aggregate spilled a shared reference to a value that holds nothing into a const part that nothing read once Debug ignored it, which tsc refuses (also for PhantomData, e.g. format!("{:?}", &pair.1)). Such a reference is now undefined in place. A &mut is left alone, as it may be an rt.Place.

Tests, differential against native Rust:

• lang/hashes hashes::defaults: a logging Hasher (Tally) behind BuildHasherDefault: new, Default, Clone, a const, ==/!=, Debug ({:?}, {:#?}, inside Option), a struct deriving Default/Clone/Debug/PartialEq over it, build_hasher, and hash_one of several key kinds, logging each Default, write, finish, and drop. Then std HashMap/HashSet over BuildHasherDefault<Fnv>: default, with_capacity_and_hasher, entry API, lookups, hasher().hash_one, clone, collect, |, is_subset.
• stock maps::default_hasher: hashbrown maps over BuildHasherDefault<DefaultHasher>, whose hasher().hash_one must match DefaultHasher::new byte for byte, and their ==.
• Both cases also check == of a map over BuildHasherDefault against its clone, now that feat(ts): translate == of hash maps and sets as std's #543 has landed.
• Snapshot churn in lang/cells, lang/results, lang/shapes: a shared &() or &.. part now reads as undefined in place (for example f(self) becomes f(undefined)).

Impact

• Translator output only: BuildHasherDefault<H> translates as undefined. No public API change.

Alternatives

• Lower std's own impls from MIR: std items are not translated, and every other std type is mapped by library, so this follows RandomState and PhantomData.
• held_state returning None for a nothing state: changes how maps pick their key hasher; keeping it held needs no special case.

Follow-ups

• None.

Review

• Codex quest review, gpt-6.1-sol, medium effort.
• Round 1 (head c1611cb): one finding, refuted. It flagged rt/collections.ts:474's lookup-key eq receiver, which this branch never touched; the base already had feat(ts): translate == of hash maps and sets as std's #543 and the branch hadn't merged it yet. Merged since; git diff origin/main...HEAD -- ts/runtime is empty.
• Round 2 (head 4463a25): no findings.
• eca1139 only merges origin/main (feat(ts): translate Debug of std's iterator sources, Option's, Result's, and an array's iterators, and a slice's parts #542) cleanly on top of the reviewed head; just ci passed on it.

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Commits

Reviewing files that changed from the base of the PR and between aff50d0 and eca1139.

Files selected for processing: 26

• quest/m3/README.md
• quest/m3/ts-build-hasher-default.md
• ts/README.md
• ts/fixtures/lang/cells/expected/cells.ts
• ts/fixtures/lang/cells/expected/core/result.ts
• ts/fixtures/lang/hashes/expected/hashes.Fnv.ts
• ts/fixtures/lang/hashes/expected/hashes.Key.ts
• ts/fixtures/lang/hashes/expected/hashes.Tallied.ts
• ts/fixtures/lang/hashes/expected/hashes.Tally.ts
• ts/fixtures/lang/hashes/expected/hashes.ts
• ts/fixtures/lang/hashes/expected/lib.ts
• ts/fixtures/lang/hashes/src/hashes.rs
• ts/fixtures/lang/hashes/src/lib.rs
• ts/fixtures/lang/results/expected/core/option.ts
• ts/fixtures/lang/results/expected/results.ts
• ts/fixtures/lang/shapes/expected/fmt.ts
• ts/fixtures/stock/expected/harness.ts
• ts/fixtures/stock/expected/maps.ts
• ts/fixtures/stock/src/harness.rs
• ts/fixtures/stock/src/maps.rs
• ts/src/lower/collections.rs
• ts/src/lower/consts.rs
• ts/src/lower/fmt.rs
• ts/src/lower/library.rs
• ts/src/lower/mir/rvalue.rs
• ts/src/lower/mod.rs

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