mir.4 closes bug #49 (a heap-Str loop binder replaced across `recur`
leaks every superseded str_concat slab, live=3) structurally rather
than with a fourth leg-by-leg patch.
Design calls made in planning (folded into the spec as the mir.4
refinement note):
- Mechanism: producer-side representation, codegen reads it — the
milestone thesis. lower_to_mir sets rep = StrRep::Heap on every
MTerm::Str literal that flows into a Str loop-binder alloca (seed
inits AND recur args at Str-binder positions); codegen's MTerm::Str
arm gains a Heap leg that promotes the static literal via
ailang_str_clone (fresh ailang_rc_alloc slab, rc_header refcount 1).
No special-casing in the codegen Loop/Recur store arms — the clone is
emitted automatically when the Str{Heap} node is lowered.
- Recur args are promoted too, not just seeds: `(recur "reset" …)` is
well-typed (verified: `ail check` accepts it), and a seed-only
promotion would leave a static literal under the now-unconditional
dec — a constructible UB hole. Soundness over minimalism on the
highest-risk RC/drop surface.
- Only the recur dec gate (lib.rs:2231) loses !is_str. The drop.rs
loop-RESULT trackability gate (drop.rs:493) STAYS conservative:
deleting it needs a broader "every Str a loop can return is
owned-heap" guarantee (a static exit-arm literal breaks it) and is
not required by the #49 acceptance (the #49 loop result is consumed
by print). drop.rs is untouched.
- Boundary (out of scope, asserted ill-typed): a borrowed static-Str
Var seeded/recur'd into a consumed Str loop binder is rejected
upstream by uniqueness (a consumed binder position is owned), so it
is not constructible.
Plan 0119 carries exact code per step: producer rep promotion + a
loop-frame-driven Str-binder mask for recur args (reusing the existing
ctx.loop_stack, no new ctx field), the codegen Heap leg, the gate
deletion, the #[ignore] lift on the #49 pin, a recur-literal witness
fixture + runtime pin proving the recur-arg leg, and the full-suite +
ir_snapshot verification. The existing lower_to_mir_ty Static-seed pin
flips to assert Heap.
Brummel
2536b5f535