Closes 18d.2's per-field-dec scope cut without mutating the
source. Codegen Emitter gains a per-fn-body side table:
moved_slots: BTreeMap<binder_name, BTreeSet<field_idx>>
When lower_match destructures a pattern and binds a non-wildcard
pointer-typed slot, codegen records (scrutinee_binder, slot_idx)
in moved_slots — but does NOT mutate the source's slot. The
source box stays untouched; the bookkeeping carries the
move-out information through codegen.
Two consumer sites:
(1) Term::Let scope close. If moved_slots[binder] is non-empty,
codegen inlines a per-field dec sequence: load each pointer-
typed field whose index is NOT in moved_slots, dispatch through
field_drop_call, then ailang_rc_dec on the outer cell. If
moved_slots[binder] is empty (the common case for every shipping
fixture today), the call to drop_<m>_<T>(ptr) is unchanged from
18c.4 — no IR shape regression.
(2) lower_reuse_as_rc reuse arm. Per-field dec is now safe — the
moved-out slots are skipped via moved_slots lookup; non-moved
slots are dec'd via field_drop_call's null-guarded drop fns.
Replaces the 18d.2 punt rationale block with the new invariant.
drop_<m>_<T> itself is unchanged: it operates on a fully-owned
pointer (the cascade has no notion of "partially moved"). The
partial-move complexity is confined to top-level emission sites
that have the static info.
Why static tracking, not source-mutation: the previous attempt
(null-out the source's slot at pattern-bind point) ran into two
structural issues: (i) borrowed scrutinees must not be mutated —
null-out violates 18a's borrow contract; (ii) Desugarer creates
chains that re-scrutinise the same scrutinee across arms, and
mutation in arm 1 makes arm 2's re-load see garbage. Static
tracking sidesteps both: the source box stays bit-identical
across the entire match.
Tests:
- new fixture examples/pat_extract_partial_drop — Pair(IntList,
IntList) destructured (a, _) — exercises moved + wildcard
pointer slots.
- new e2e alloc_rc_partial_drop_skips_moved_keeps_wildcarded
asserts stdout 6 under both gc and rc, plus IR shape: main's
let-close inlines drop on the wildcarded slot but NOT on the
moved slot.
- updated reuse_as_demo_under_rc_uses_inplace_rewrite IR-shape
assertion — the canonical fixture moves the only pointer
slot, so the reuse arm should contain neither @drop_ nor
@ailang_rc_dec for fields.
Test deltas: e2e 56 -> 57 (+1). All other buckets unchanged.
cargo test --workspace green.
Known regression — narrow, deliberate, queued for 18d.4:
The prior alloc_rc_borrow_only_recursive_list_drop fixture from
18c.4 had a pattern (Cons h t) where t was bound but never
consumed downstream. Under 18c.4, drop_<m>_<IntList>(xs) at
let-close cascaded through xs.tail and freed the entire 5-cell
chain. Under 18d.3 with strategy (b), xs.tail is in
moved_slots, so the cascade is interrupted there — but t is
never consumed, so its 4-cell tail now leaks. The fixture
stdout is unchanged (still "11"), but memory hygiene under
--alloc=rc is strictly worse.
The fix is symmetric to 18c.4's known fn-parameter-dec debt:
pattern-bound binders whose consume_count is 0 at arm close
should also get a drop call emitted. Same emission seam as
let-close-drop. Queued as 18d.4. Until 18d.4 ships, fixtures
with pattern-extracted unused pointer binders leak under
--alloc=rc; this is documented and bounded (shipping fixtures
beyond the test suite generally consume every binder they
bind).
Brummel
c96940ea96