Three sibling sites previously fell back to shallow `ailang_rc_dec`
when a binder had non-empty `moved_slots` and a dynamic runtime ctor
tag: Iter B Own-param dec at fn-return (lib.rs), Iter A arm-close
pattern-binder dec (match_lower.rs), and emit_inlined_partial_drop's
non-Ctor branch (drop.rs). Shallow freed the outer cell but did not
walk the unmoved ptr fields — silent leak.
Adds `emit_partial_drop_fn_for_type`: a per-ADT helper structurally
parallel to `emit_drop_fn_for_type` but taking an i64 moved-slots
bitmask. Each ptr-field's dec is gated on the corresponding mask
bit; the outer box is dec'd at join. Emitted alongside the per-type
drop fn for every ADT under --alloc=rc.
Three call sites rewritten to resolve the partial_drop symbol from
the binder's static type and build the mask from `moved_slots`.
Three RED-then-GREEN fixtures (rc_own_param_/rc_match_arm_/
rc_app_let_partial_drop_leak) pin the carve-outs in regression
coverage; live cell counts go from 3/1/3 (pre-fix) to 0/0/0.
e2e count: 66 → 69.
The pre-existing `alloc_rc_own_param_dec_at_fn_return` IR-shape
assertion is widened to accept `partial_drop_<m>_<T>(%arg_xs,
i64 mask)` as a third valid drop shape (the canonical fu2 case).
Closes the JOURNAL queue's only remaining iter; the "wait for
organic fixture" stance from the 18g tidy is retracted — known
leaks are bugs, CLAUDE.md's TDD rule covers them autonomously.
Third slice of the codegen split. Pulls the three big lowering
methods that share the ADT runtime layout into a dedicated
module. No behaviour change.
Methods moved (all pub(crate)):
- lower_ctor (135 lines): allocation + tag store + field
stores. Iter 17a alloca-vs-heap routing through escape
analysis lives here.
- lower_reuse_as_rc (261 lines): Iter 18d.2 in-place rewrite
optimisation under --alloc=rc. Reuse arm + fresh arm,
Lean-4-shaped runtime refcount-1 dispatch.
- lower_match (497 lines): switch + per-arm pattern
destructure + per-arm phi + 18d.4 Iter A pattern-binder
drops + 18g.1 outer-cell shallow dec at tail-call sites.
lookup_ctor_in_pattern stays in lib.rs (sits next to its
lookup-family siblings).
Visibility: four parent-module helpers that match_lower.rs
calls back into were upgraded to pub(crate): start_block,
lower_term, collect_owner_local_types, fresh_id.
lookup_ctor_in_pattern likewise pub(crate).
lib.rs drops from 4139 → 3237 lines. match_lower.rs is 935
lines (the lower_match body alone is ~500 of those — the
single-largest method in the codebase, but moving it gives
the biggest navigability win). cargo test --workspace clean.
Brummel