jit: #1093 review follow-ups on the operation dump, the slice-index match, and the closure-select fixtures - #1110

Follow-up to #1093, which merged before its review tail was closed. One commit, majit-translate only.

Seven findings were open on #1093. Five are fixed here, one was already moot, one is declined with evidence.

Fixed

tool/error.rs — the operation dump could omit the failing operation.
no_source_lines listed the first 40 operations of a block. A block whose failure sits past operation 40 therefore showed neither the failing op nor the ops that produced its operands, which is the entire point of the dump. It now takes the failing operation's index and ends the window there, reporting the elided prefix as … N earlier operation(s). All five call sites pass the index.

A 60-operation regression test with operindex = 55 asserts op[55] and op[54] are listed, op[56] is not, the listing stays exactly 40 long, and both elision lines read … 16 earlier operation(s) / … 4 more operation(s). The fixture sets graph.func = None so every source accessor errors and the dump path is the one under test.

front/slice_index.rs — nested if let + inner match.
Collapsed into a single match, which removes the SliceIndexBounds::MinusOne => unreachable!() arm. The two unreachable!()s at :214 and :673 are unrelated (they re-destructure an already-matched OpKind::Call) and are left alone.

front/mir.rs — the anchor's name contradicted its body.
call_function_impl_result_has_no_residual_array_index asserted that the residual index call remains. Renamed to call_function_impl_result_keeps_residual_array_index.

The review also asked for an index_mut assertion. I probed the real graph rather than adding it blind: call_function_impl_result contains no index_mut at all. Its array/slice calls are core::array::<Impl>::index ×1, __array_repeat ×1, core::slice::iter ×4 and Vec::extend_from_slice ×2. The request and the pre-existing doc comment were both unfounded, so the comment is corrected and the verifiable paired negative __getslice_rangeto == 0 is asserted instead — the fold is declined, so no site may be rewritten.

front/option_closure_select.rs — the fixtures could not distinguish the two owners.
Receiver and result shared one owner root, so an assertion about "the result enum" would have passed against the receiver's. The fixtures now use test::recv::Option and test::result::Option, and the tests assert FieldWrite::owner_root for __discriminant and __pos_0.

Mutation-proven rather than asserted-and-hoped: swapping result_option_owner back to option_owner fails the test with left: ["test::recv::Option", "test::result::Option"] vs right: ["test::result::Option"].

The __pos_0 owner list has two entries — the second __pos_0 write is the closure's (x,) Args tuple from emit_call_once, a real Tuple unrelated to either Option instantiation. That is asserted explicitly so the extra entry cannot be mistaken for leakage.

front/option_closure_select.rs — result_niche: true was never exercised.
Two tests added for map and and_then, asserting no synthetic constructor, no __discriminant write, and exactly one core::ptr::null_mut call. They cannot pass on the non-niche path.

Moot

slice_index.rs:920's == false predicate was already removed by e1fc067eae2.

Declined

flowspace_adapter.rs:4740 — the suggestion was to change the fixture's ValueType::Int. front::mir.rs's Rvalue::Repeat really does emit result_ty: ValueType::Int for __array_repeat, and git show 128590c675f confirms that declaration predates this work, so the fixture mirrors what it models. Changing it would make the fixture disagree with the lowering. The graph-level contract is anchored by the sibling real-LLBC test directly below it.

Verification

• cargo test --release -p majit-translate --lib — 3132 passed, 0 failed
• cargo test --all --no-default-features --features dynasm — 102 suites, 7521 passed, 0 failed (run at base fd658cb0723; the base has since advanced by jit(wasm): drop the wasm32 arm of the self-recursive root-bridge inline #1106, which this commit does not touch)
• The renamed real-corpus anchor re-run against a freshly extracted corpus
• cargo fmt --check clean

— authored by Claude