Building on the Option 3 lvalue-borrow rule, the long-lived allocators
in `library/modules/allocators.sx` (GPA, Arena, TrackingAllocator) now
return their state by value instead of via a heap-allocated `*T`. The
caller binds the result to a local; the local IS the allocator state.
`xx local` borrows that storage under Option 3, so the `Allocator`
protocol value's `ctx` points at the local — no heap allocation for
the state struct, no `free` of the state needed.
```sx
gpa := GPA.init(); // GPA (value)
arena := Arena.init(xx gpa, 4096); // Arena (value)
tracker := TrackingAllocator.init(xx gpa); // TrackingAllocator (value)
push Context.{ allocator = xx tracker, data = null } { ... }
```
Why by-value:
- One fewer `libc_malloc` per allocator instance.
- No state-struct leak. The local is reclaimed at scope exit; `deinit`
only handles downstream resources (chunks, etc.) — not its own struct.
- Owning structs can embed allocators as value fields directly.
Callsite changes:
- `library/modules/ui/pipeline.sx`: `arena_a: Arena;` / `arena_b:
Arena;` (was `*Arena;`). The `build_arena: *Arena` local takes
`@self.arena_a` / `@self.arena_b`.
- `examples/126-xx-recover-then-dispatch.sx`: `recovered == @gpa`
instead of `recovered == gpa` (gpa is a value now).
- `examples/135-xx-lvalue-borrows.sx`: drop the `tracker_ptr.*`
deref — `init` already returns the value.
- `examples/50-smoke.sx`: Arena alloc counts dropped by 1 (no
state-struct allocation). Comments + snapshot updated.
`Arena.deinit` drops the trailing `parent.dealloc(xx a)` — the
caller's local owns the storage.
FFI IR snapshots regenerated to reflect the new signatures:
`@GPA.init` returns `i64` (was `ptr`); `@Arena.init` and
`@TrackingAllocator.init` use sret returns (was `ptr`).
CLAUDE.md "Allocator construction" rule rewritten around the
by-value convention. The forbidden caller-provides-storage and
redundant-pointer-rename patterns are still forbidden but for the
right reasons now (verbose, fragile) rather than as a workaround
for the old `init() -> *T` shape.
157/157 example tests pass; chess clean on macOS, iOS sim, and
Android via `tools/verify-step.sh`.
`xx <struct-typed local>` used to heap-copy the value through context.allocator.
The protocol value's `ctx` pointed at the heap copy; the original local was
left behind, untouched. Mutations through the protocol never reached the
original, and direct reads of the original never saw protocol mutations.
Two-fork bug, silent, easy to write by mistake.
New rule (Option 3 in the discussion):
- `xx <lvalue>` — identifier, field access, index expression, deref —
borrows the operand's storage. No heap copy, no `free` needed.
- `xx <rvalue>` — struct literal, function-call result, arithmetic, etc. —
heap-copies through context.allocator. Unchanged from today.
- `xx @ptr` and `xx <pointer-typed value>` — borrows the pointee. Unchanged.
Single switch in `buildProtocolErasure` ([lower.zig:10334](src/ir/lower.zig#L10334))
gated by a new `isLvalueExpr` helper ([lower.zig:10322](src/ir/lower.zig#L10322)).
Struct-typed operand: if the AST shape is identifier/field/index/deref,
emit `lowerExprAsPtr(operand_node)` and skip the heap-copy; otherwise
keep the alloca-store-heap_copy path.
specs.md §3 ownership table extended to three rows (rvalue, lvalue,
pointer) with examples and rationale per row.
Regressions:
- `examples/130-xx-value-routes-through-context-allocator.sx` — the
Phase 1.1 witness for heap-copy-via-context-allocator. Previous shape
(`xx <local-value>`) is now a borrow under Option 3 and no longer
exercises the heap-copy path. Rewritten to use a struct literal
(`xx ByValue.{...}`) which still heap-copies through context.allocator
— Tracer.count = 1 as before.
- `examples/135-xx-lvalue-borrows.sx` — new test. Dereferences a
TrackingAllocator into a stack value, does `xx tracker` inside a
push Context, and asserts alloc_count/dealloc_count on the LOCAL go
up. Under old semantics this would have stayed at 0 (heap copy got
the increments, local stayed stale).
157/157 example tests pass; chess clean on macOS / iOS sim / Android
(`tools/verify-step.sh` ran green immediately before this work).
