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
Brummel
b129af1363