[kixelated]

Problem

An instance of a generic function can compare two constants, such as size_of::<T>() == 1, which u64's instance writes as 8 === 1. tsc refuses an equality of two literal types that differ (TS2367), so the crate fails to translate. Found translating the deque's slices fixture (moq-dev/rust2#443).

It reaches every kind of constant: integers (8 === 2, -1 === -2), chars (99 === 98), floats (1.5 === 0), bools (false === true), a wide integer cast from a narrow one (T::I32 as i128), and a constant folded into a temporary that a comparison then reads (T::U8 / 2 == 1 as 4 === 1).

Approach

The translator already folds comparisons of two number literals, through fold in lower/mir, but only for switch tests and asserts. Now:

• A comparison rvalue folds as it is lowered, and again where a pending temporary substitutes into it.
• fold also folds two boolean literals, and reads literals through a source location, as a size_of call's result has one.
• Expr::not negates a boolean literal to the other, so a folded test reads if (true), not if (!false).

The fold computes what JS would compute at run time from the same two literals, so it keeps the output's semantics. The differential test checks the result against native Rust.

A new lang/constants fixture runs each of the six operators on each kind of constant: size_of, u8, i32, u64, i128, bool, char, f32, f64, casts, and temporaries, plus two ifs. It runs for a T below, equal to, and above the one it compares with, and for one at the limits, NaN, and -0.0. Without the fix, tsc rejects that output with 40+ TS2367 errors.

Other snapshot changes are the same fold showing up in std's code, such as const isSignedTy: boolean = 0 > -128 becoming true. Std instances named by a hash of their text get new names.

Impact

• The translator's output: a comparison of two literals is written as its value. No API changes.

Alternatives

• Widen one side, as in (8 as number) === 1. Rejected: noisier output, and it would leave if (8 === 1) for later code to reason about. Folding reuses the existing fold.
• Fold on the MIR, by evaluating constant operands. Rejected: it misses literals that come from the translator's own arithmetic folds and from temporaries it substitutes.

Follow-ups

• std::mem::needs_drop::<T>() is refused as an unsupported call, which I found while reproducing this. No quest covers it yet.

Review

mechanical fix by codex / Codex: Merged base, preserving both generated symbol renames and error wrappers; regenerated errors snapshot. Passed in ts/ on nightly-2026-10-15: cargo build --locked -p rust2-ts; UPDATE_EXPECT=1 cargo test --locked -p rust2-ts --test fixtures lang::errors -- --exact; cargo test --locked -p rust2-ts (97 tests). (head 33eca9f)

• Reviewer: Codex, gpt-6.1-sol, medium effort, via quest review --base origin/main.
• Round 1 at b4ad4bdd: no findings.
• Round 2 at a7689cef, after merging main's per-module split of the lang fixture (moq-dev/rust2#474): no findings.
• Since then, only conflict-free merges of main up to c8e03978. Snapshots were regenerated with no changes, and just ci passed at f8439ed0.

🤖 Generated with Claude Code

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[moq-bot]

quest land watch: the merge queue removed this pull request at f8439ed because it conflicts with main (merge_conflict).

The land fixer works on it in the background, and the watcher re-enqueues it at the back of the queue once it is fixed or judged flaky, or hands it back here with what it needs.

(written by quest land watch)

[moq-bot]

quest land watch: armed auto-merge on this pull request at 33eca9f, so it joins the back of the merge queue once its required checks pass, with the land fixer's fix (codex / Codex): Merged base, preserving both generated symbol renames and error wrappers; regenerated errors snapshot. Passed in ts/ on nightly-2026-10-15: cargo build --locked -p rust2-ts; UPDATE_EXPECT=1 cargo test --locked -p rust2-ts --test fixtures lang::errors -- --exact; cargo test --locked -p rust2-ts (97 tests)..

(written by quest land watch)