Brummel
b129af1363
The strict linearity check accepted consuming a heap-typed pattern sub-binder extracted from a `(borrow)`-mode match scrutinee. The sub-binder aliases the caller-owned interior, so moving it into an `(own)` slot double-frees at runtime (SIGSEGV) — `ail check` returned exit 0, `ail build`+run crashed. A pre-existing hole the corpus never exercised; the #55 cutover's universal activation of the strict check exposed it (sibling to the #57 / spec 0064 class-2 let-alias-of-borrow hardening — this is the pattern-sub-binder analog). Root: `walk_arm` seeded every pattern sub-binder with `borrow_count = 0` (freely consumable), bypassing the `Borrow`-param seeding in `check_fn` that starts a borrowed param at `borrow_count = 1`. The whole-param consume was caught (cut55_2d, correctly rejected); the moved-out heap sub-binder was not. Fix: in `Term::Match`, resolve the scrutinee through `resolve_alias` and record whether it is a borrowed binder; thread that into `walk_arm`, which now seeds a heap-typed (`!is_value`) sub-binder of a borrowed scrutinee with `borrow_count = 1`. Value-typed sub-binders (e.g. the `Int` head) stay at `0` — copied out, no heap alias. Nested matches fall out for free: the inner match re-derives borrowed status from the borrow-carrying sub-binder. Rejected alternative: forcing fieldtest fixture cut55_2b to also reject. 2b is a clean borrow-traversal — `bump`'s recursive param is `(borrow)`, so the tail `t` is only borrowed, never consumed into an `(own)` slot. Distorting the fix to reject it would be unsound (false positive on legitimate borrow-rebuild). The fixture's own comment misdescribed its recursive param as `(own)`; the code says `(borrow)`. Verification: - RED test linearity::tests::consume_heap_sub_binder_of_borrow_scrutinee_is_rejected: was left:0 right:1 (check returned []), now green. - Full workspace: 732 passed / 0 failed (was 731; +1 the new test). No committed corpus newly rejected by the sharper check. - fieldtest repro cut55_2c: exit 0 (wrongly accepted) -> exit 1 (consume-while-borrowed on `t`). closes #58