[kixelated]

Problem

== and != of two HashMaps or two HashSets, std's or hashbrown's, were refused as an unsupported call, for a RandomState map too, and so was a derived PartialEq of a type that holds one. Completes quest/m3/ts-hash-map-eq.md. No consumer yet; planned because rust2 aims to support all of std.

Approach

• rt.hashEq(eq?) does what std's and hashbrown's PartialEq do: equal lengths, then each entry of self looked up in other, and self's value == the one found. A set passes no value eq.
• eq_fn maps a HashMap or HashSet to it, so maps inside a Vec, an Option, or a derived PartialEq compare too.
• A lookup now calls eq of the key looked up by, given the held key, as hashbrown's Equivalent does (self == key.borrow()). The runtime called the held key's eq instead, which a logging Eq showed for every lookup, not only map equality.
• Hashing stays around the crate's own BuildHasher, as feat(ts): hold a crate's own BuildHasher in a hash map or set #509 decided: the runtime never calls it, so maps whose hashers differ (seeded apart on each side) compare by their keys alone, as std's do.

Tests, differential against native Rust:

• lang/hashes hashes::equal: std maps and sets with a crate BuildHasher seeded apart on each side, of a key and value that log each Hash and eq with the side they are on. Each compares both ways, against the same bytes in another order, all but the last, and one byte flipped. The crate hasher puts the key in hashbrown's tag bits so native calls eq only on the matching key; logs are written in call order for maps of at most one entry, sorted for equal maps, and dropped otherwise, as std's order decides which entries it visits before one differs. Then RandomState maps of primitive and string keys, maps in a Vec, and a derived PartialEq holding a map and a set.
• stock maps::equal: hashbrown's maps and sets with a crate BuildHasher seeded apart on each side, and the default hasher.
• Reverting the lookup's eq order fails hashes::equal.

Impact

• rt.hashEq is new in the runtime; the translator's output for ==/!= of hash maps and sets.
• A lookup in a translated hash map calls the looked-up key's eq rather than the held key's, matching std. No API change.

Alternatives

• Keep slot's argument order and swap only in hashEq: leaves every other lookup calling the wrong side's eq.
• Call the map's own BuildHasher during comparison: contradicts feat(ts): hold a crate's own BuildHasher in a hash map or set #509's hash-around decision, and lookups elsewhere don't.

Follow-ups

• quest/m3/ts-build-hasher-default.md is a separate problem (making BuildHasherDefault), left for its own PR.

Review

Codex (quest review), gpt-6.1-sol, medium effort, round 1 on 8a32ee10d551546d8a5e330a4911e7c7719336f6: no findings. c4de918b992ed29e5b45f3f4a1705c58dc9a4fca adds only conflict-free merges of origin/main on top, after which just ci passed.

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Reviewing files that changed from the base of the PR and between 7e7e420 and c4de918.

Files selected for processing: 16

• quest/m3/README.md
• quest/m3/ts-hash-map-eq.md
• ts/README.md
• ts/fixtures/lang/hashes/expected/hashes.Index.ts
• ts/fixtures/lang/hashes/expected/hashes.Sided.ts
• ts/fixtures/lang/hashes/expected/hashes.Spread.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/stock/expected/harness.ts
• ts/fixtures/stock/expected/maps.ts
• ts/fixtures/stock/src/harness.rs
• ts/fixtures/stock/src/maps.rs
• ts/runtime/rt/collections.ts
• ts/src/lower/collections.rs

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