Brummel
c96940ea96
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).
2 lines
2.5 KiB
JSON
2 lines
2.5 KiB
JSON
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