fix(rust): undefined is a declaration, not a value - #3341
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September 5, 2026 20:07
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Closes #3340
cannot find value \undefined`was the largest single name in the corpus's first-error census — **14 of 21** in its class — and every one of them sat in the same position:let mut x: T = undefined;`.The emitter already writes the right thing three lines up, for the no-initialiser case. So the repair is to treat
undefinedas no initialiser, not to invent a value.Why not
Default::default(). That was tried and withdrawn (#3223) because[usize; 256]has noDefault. The blocker was the mapping, not the defect. Deferred initialisation needs no bound, and it is the exact semantics: Zig says "uninitialised", Rust says "declared, assign before use".github/auth.t27now reportsE0381: partially assigned binding— a real defect surfaced where a default would have swallowed it.Two call sites, not one.
gen_fncarries its own copy of the local-emission logic. The first attempt patchedgen_rust_stmtalone and changed no output at all — the fix did not travel inside a single emitter, one scope smaller than the emitter-to-emitter gaps this corpus is full of. There is now one predicate,is_undefined_init, and two callers.Measured, two pinned binaries of distinct hashes over 651 specs: 336 OK both sides, 245 FAIL, 69 NOGEN — zero regressions. The column does not move.
What moves: the first rustc error changed on 13 of 14. The fourteenth is
vsa/similarity_search.t27, apub static mut xs: [T; N] = undefined;— named as untouched before the measurement, because a static cannot be uninitialised in Rust. The predicted exception is the only one.