# Harden ownership part 2 — iteration 4 (final): partition_eithers double-consume rewrite (class 4) — Implementation Plan

> **Parent spec:** `docs/specs/0064-harden-ownership-analysis-part-2.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement`
> skill to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Rewrite `std_either_list.partition_eithers` to destructure
its recursive result once, removing the genuine double-consume that
universal linearity activation would reject — and pin the rejected
shape so the check's correctness is documented.

**Architecture:** `partition_eithers` projects both `(app Pair.fst
rest)` and `(app Pair.snd rest)` from one owned `rest`; `fst`/`snd` are
`own`-param projections that move a field out, so each consumes `rest`
→ a genuine double-consume. This is a real corpus bug, NOT a
false positive: the fix is a body rewrite (destructure `rest` once via
`(match rest (case (pat-ctor MkPair ls rs) …))`), not a check change.
The check is correct and unchanged. Two standalone fixtures pin the two
shapes: `c4_double_consume` (the rejected double-projection, must stay
RED) and `c4_rewrite` (the accepted destructure-once shape).

**Tech Stack:** `examples/std_either_list.ail` (the corpus fix),
`examples/` fixtures, `crates/ailang-check/tests/workspace.rs`
assertions, gated by the existing `std_either_list_demo` E2E
(`crates/ail/tests/e2e.rs`).

**Scope:** FOURTH and final iteration of spec 0064 — class 4 only.
Classes 1/2/3 shipped in iters 1-3. No check change, no schema/type
change, no codegen change.

**Latent-bug note.** `partition_eithers`'s param is a bare (Implicit)
slot today, so the linearity check is *skipped* on it — the
double-consume is not observable as RED on the real fixture until #55
activates the check universally. There is therefore no RED→GREEN flip
on `std_either_list.ail`; the rewrite is a pre-emptive corpus fix whose
correctness is proven by (a) the unchanged `2 3 2 3` E2E output and (b)
the `c4_double_consume` must-stay-RED fixture, which demonstrates — with
explicit modes, so the check is active today — that the
double-projection shape IS rejected. The orchestrator verified the
rewrite this session: `ail check examples/std_either_list.ail` → exit 0,
`ail run examples/std_either_list_demo.ail` → `2 3 2 3`.

---

**Files this plan creates or modifies:**

- Modify: `examples/std_either_list.ail:61-71` — `partition_eithers`
  Cons arm rewritten to destructure `rest` once.
- Create: `examples/c4_double_consume.ail` — must-stay-RED fixture (the
  rejected double-projection).
- Create: `examples/c4_rewrite.ail` — stays-clean fixture (the accepted
  destructure-once shape).
- Test: `crates/ailang-check/tests/workspace.rs:745-753` — generalise
  `harden_ownership_heap_double_consume_still_errors` to also assert
  `c4_double_consume` fires.
- Test: `crates/ailang-check/tests/workspace.rs:728` — add `c4_rewrite`
  to `harden_ownership_false_positives_are_clean`.

---

### Task 1: Rewrite partition_eithers and confirm behaviour unchanged

**Files:**
- Modify: `examples/std_either_list.ail:61-71`

- [ ] **Step 1: Rewrite the Cons arm**

In `examples/std_either_list.ail`, the `partition_eithers` Cons arm
(`:61-71`) is currently:

```text
        (case (pat-ctor Cons h t)
          (let rest (app partition_eithers t)
            (match h
              (case (pat-ctor Left l)
                (term-ctor Pair MkPair
                  (term-ctor List Cons l (app Pair.fst rest))
                  (app Pair.snd rest)))
              (case (pat-ctor Right r)
                (term-ctor Pair MkPair
                  (app Pair.fst rest)
                  (term-ctor List Cons r (app Pair.snd rest)))))))))))
```

Replace it with (destructure `rest` once; `ls` = lefts, `rs` = rights;
each consumed once per independent match-arm):

```text
        (case (pat-ctor Cons h t)
          (let rest (app partition_eithers t)
            (match rest
              (case (pat-ctor MkPair ls rs)
                (match h
                  (case (pat-ctor Left l)
                    (term-ctor Pair MkPair
                      (term-ctor List Cons l ls)
                      rs))
                  (case (pat-ctor Right r)
                    (term-ctor Pair MkPair
                      ls
                      (term-ctor List Cons r rs))))))))))))
```

(`MkPair` is the `Pair` ctor from the imported `std_pair`,
`examples/std_pair.ail:12`. The Nil arm `:57-60` is unchanged.)

- [ ] **Step 2: Confirm the module checks clean**

Run: `target/debug/ail check examples/std_either_list.ail`
Expected: `ok (65 symbols across 7 modules)`, exit 0. (Build the CLI
first if stale: `cargo build --bin ail`.)

- [ ] **Step 3: Confirm the E2E demo output is unchanged**

Run: `cargo test -p ail --test e2e std_either_list_demo`
Expected: PASS — `std_either_list_demo` still asserts stdout
`["2", "3", "2", "3"]`. The rewrite is semantically identical (same
lefts/rights partition), so the output is preserved.

---

### Task 2: Pin the rejected and accepted shapes

**Files:**
- Create: `examples/c4_double_consume.ail`
- Create: `examples/c4_rewrite.ail`
- Test: `crates/ailang-check/tests/workspace.rs:745-753`, `:728`

- [ ] **Step 1: Create the must-stay-RED fixture**

Create `examples/c4_double_consume.ail` with exactly:

```ail
(module c4_double_consume
  (data Pair
    (doc "boxed pair of ints")
    (ctor MkPair (con Int) (con Int)))
  (fn fst (type (fn-type (params (own (con Pair))) (ret (own (con Int))))) (params p)
    (body (match p (case (pat-ctor MkPair a b) a))))
  (fn snd (type (fn-type (params (own (con Pair))) (ret (own (con Int))))) (params p)
    (body (match p (case (pat-ctor MkPair a b) b))))
  (fn both
    (doc "MUST STAY RED: rest projected by fst AND snd = consumed twice")
    (type (fn-type (params (own (con Pair))) (ret (own (con Pair)))))
    (params rest)
    (body (term-ctor Pair MkPair (app fst rest) (app snd rest)))))
```

- [ ] **Step 2: Create the stays-clean fixture**

Create `examples/c4_rewrite.ail` with exactly:

```ail
(module c4_rewrite
  (data Pair
    (doc "boxed pair of ints")
    (ctor MkPair (con Int) (con Int)))
  (fn both
    (doc "rewrite: destructure rest once via match")
    (type (fn-type (params (own (con Pair))) (ret (own (con Pair)))))
    (params rest)
    (body (match rest (case (pat-ctor MkPair a b) (term-ctor Pair MkPair a b))))))
```

- [ ] **Step 3: Generalise the heap-double-consume assertion to cover c4_double_consume**

In `crates/ailang-check/tests/workspace.rs`, the test
`harden_ownership_heap_double_consume_still_errors` (`:744-753`) is
currently single-fixture. Replace its doc comment and body:

```rust
/// #56 type-gating: the exemption is value-type-only. A heap param
/// consumed twice (`real_consume.dup`, `(term-ctor Pair Pair b b)`) MUST
/// still fire use-after-consume — proving the fix did not blanket-silence
/// genuine multi-consume.
#[test]
fn harden_ownership_heap_double_consume_still_errors() {
    let entry = examples_dir().join("real_consume.ail");
    let ws = load_workspace(&entry).expect("load real_consume");
    let diags = check_workspace(&ws);
    assert!(
        diags.iter().any(|d| d.code == "use-after-consume"),
        "real_consume.dup must still fire use-after-consume; got: {diags:#?}"
    );
}
```

with (loop over both genuine double-consume fixtures):

```rust
/// #56 type-gating + #57 class 4: a heap value consumed twice MUST still
/// fire use-after-consume — `real_consume.dup` (`(term-ctor Pair Pair b
/// b)`) and `c4_double_consume.both` (`rest` projected by both `fst` and
/// `snd`, the genuine double-consume the partition_eithers rewrite
/// removes). Proves the hardening did not blanket-silence genuine
/// multi-consume.
#[test]
fn harden_ownership_heap_double_consume_still_errors() {
    for name in ["real_consume", "c4_double_consume"] {
        let entry = examples_dir().join(format!("{name}.ail"));
        let ws = load_workspace(&entry).unwrap_or_else(|e| panic!("load {name}: {e:?}"));
        let diags = check_workspace(&ws);
        assert!(
            diags.iter().any(|d| d.code == "use-after-consume"),
            "{name} must still fire use-after-consume; got: {diags:#?}"
        );
    }
}
```

- [ ] **Step 4: Add c4_rewrite to the clean array**

In `crates/ailang-check/tests/workspace.rs`, the
`harden_ownership_false_positives_are_clean` array (`:728`). Change:

```rust
    for name in ["fp_value", "fp_hof", "fp_map", "c3_value_let", "c1_local_hof", "c2_let_alias"] {
```

to:

```rust
    for name in ["fp_value", "fp_hof", "fp_map", "c3_value_let", "c1_local_hof", "c2_let_alias", "c4_rewrite"] {
```

- [ ] **Step 5: Run both harden assertions**

Run: `cargo test -p ailang-check --test workspace harden_ownership`
Expected: PASS — both `harden_ownership_heap_double_consume_still_errors`
(now asserting `real_consume` AND `c4_double_consume` fire
use-after-consume) and `harden_ownership_false_positives_are_clean` (now
including `c4_rewrite`) green.

- [ ] **Step 6: Full crate + workspace regression**

Run: `cargo test -p ailang-check`
Expected: PASS — whole `ailang-check` suite green.

Run: `cargo test --workspace`
Expected: PASS — including `std_either_list_demo` (output `2 3 2 3`
unchanged). Per the typed-MIR re-synth strictness memory, run the whole
workspace, not just e2e.

Run: `python3 bench/check.py` then `python3 bench/compile_check.py`
Expected: both green — no check or compile-baseline change (a corpus
body rewrite + standalone fixtures only).