# Eliminate the Implicit ownership default — Implementation Plan

> **Parent spec:** `docs/specs/0062-eliminate-implicit-mode.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement`
> skill to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Delete `ParamMode::Implicit` in a single cutover so
`ParamMode` becomes binary `{Own, Borrow}` and no fn-type slot —
authored or synthesised — carries a defaulted ownership mode.

**Architecture:** Four tasks. Tasks 1–2 add the two new
signature-level rejects (borrow-return, borrow-over-value) while
`Implicit` still exists — each leaves a green, independently
committable tree. Task 3 builds a throwaway `ail migrate-modes`
subcommand (parse → map `Implicit↦Own`, keep explicit `Own`/`Borrow`
→ print explicit → write back) and unit-tests it without touching the
corpus. Task 4 is the atomic cutover: run the migration tool over the
whole `.ail` corpus, hand-fix the read-only-heap params the
now-universal linearity check flags as over-consuming, then delete the
variant + all its compile sites (compiler-driven), add the parser
bare-slot reject, reset the five hash pins, flip the leak pin, update
both affected contracts, and remove the throwaway subcommand.

**Tech Stack:** `ailang-core` (`ast.rs` schema, `primitives.rs`
`is_value_type`), `ailang-surface` (`parse.rs` slot grammar, `print.rs`
slot printer), `ailang-check` (`lib.rs` signature checks + `CheckError`
registry, `linearity.rs`/`uniqueness.rs` activation gate), `ail`
(throwaway migration subcommand), the `.ail` corpus, the design ledger
contracts `0002`/`0008`.

---

## Recon corrections folded into this plan

The spec's enumeration drifted from the tree; the authoritative
figures the plan executes against (verified by `plan-recon`,
2026-06-01):

- **Kernel migration path:** the spec/issue cite
  `crates/ailang-kernel/src/kernel_stub/source.ail` — that path does
  **not exist** (kernel_stub retired in raw-buf.4 per CLAUDE.md). The
  live kernel-tier source is `crates/ailang-kernel/src/raw_buf/source.ail`.
- **Hash pins: FIVE, not two.** The spec named
  `crates/ailang-core/tests/hash_pin.rs` and
  `crates/ailang-surface/tests/prelude_module_hash_pin.rs`. Three more
  carry fn-type-bearing hex pins that the mode-serialisation change
  shifts: `crates/ailang-core/tests/embed_export_hash_stable.rs:32`,
  `crates/ail/tests/eq_ord_e2e.rs:139`,
  `crates/ail/tests/mono_hash_stability.rs` (the `eq__`/`compare__`/
  `show__` block, `:63-68,:107-110`).
- **Second contract.** The spec's acceptance criterion 7 names only
  `design/contracts/0008-memory-model.md`. `design/contracts/0002-data-model.md`
  also documents the `"implicit"` JSON form and `implicit ≡ own`, and is
  drift-anchored by `design_schema_drift.rs:design_md_anchors_every_parammode_variant`.
  The honesty rule forces it to update in the same iteration. **Both
  contracts update.**
- **Counts:** ~261 example fixtures carry `(fn-type` slots (spec said
  ~208); `fn_implicit` has 16 call sites + 1 def = 17 references (spec
  said 16). The compiler is the authoritative enumerator for the
  synthesis-site `Own` migration; do not hand-count.
- **Lambda annotations are NOT moded slots.** `parse_lam`
  (`parse.rs:1518`) reads `(typed name type)` params and `(ret type)`
  via `parse_type()`, not `parse_param_with_mode`. The bare-slot reject
  (Task 4) therefore hits `(fn-type …)` slots only; lambda type
  annotations stay bare and are untouched by the migration.

## Files this plan creates or modifies

- Create: `examples/borrow_return_reject.ail` — RED fixture, borrow-return reject (Task 1)
- Create: `examples/borrow_value_reject.ail` — RED fixture, borrow-over-value reject (Task 4 Step B2)
- Modify: `crates/ailang-codegen/src/subst.rs:165` — mono-coercion `(borrow value-type)→(own value-type)` (Task 4 Step B1)
- Create: `examples/bare_slot_reject.ail` — RED fixture, parser bare-slot reject (Task 4)
- Create: `examples/own_return_provenance_reject.ail` (+ `_let`, `_ctor` variants) — already-green `consume-while-borrowed` regression pins (Task 4)
- Create: `examples/ownership_total.ail` — post-cutover authoring-surface GREEN fixture (Task 4)
- Modify: `crates/ailang-check/src/lib.rs` — two new `CheckError` variants + `code()` entries + signature-check wiring (`:2240`); the ~50 `ret_mode: ParamMode::Implicit` synthesis literals → `Own` (Task 4)
- Modify: `crates/ailang-core/src/ast.rs:778-953` — `ParamMode` enum, `Type::Fn` serde, `fn_implicit`→`fn_owned`, helper deletion, `PartialEq` (Task 4)
- Modify: `crates/ailang-surface/src/parse.rs:1180-1222` — bare-slot reject + elision deletion (Task 4)
- Modify: `crates/ailang-surface/src/print.rs:357-360,:408` — delete `Implicit` arm (Task 4)
- Modify: `crates/ailang-codegen/src/{drop.rs:467-478,lambda.rs:164,lib.rs}` — `Implicit`→`Own` synthesis + comment (Task 4)
- Modify: `crates/ailang-core/src/{desugar.rs,pretty.rs}` — synthesis literals → `Own` (Task 4)
- Modify: `crates/ailang-prose/src/lib.rs:456,…` — delete `Implicit` arm + synthesis literals (Task 4)
- Modify: `crates/ailang-check/src/{linearity.rs:356,385,reuse_shape.rs:110,uniqueness.rs}` — dead activation-gate arm collapse (Task 4)
- Modify: `examples/*.ail` (261), `examples/prelude.ail`, `crates/ailang-kernel/src/raw_buf/source.ail` — corpus migration (Task 4)
- Modify: the five hash-pin test files (Task 4)
- Modify: `examples/rc_let_implicit_returning_app.ail` + `crates/ail/tests/print_no_leak_pin.rs` — leak flip (Task 4)
- Modify: `crates/ailang-core/tests/{schema_coverage.rs,design_schema_drift.rs}` — delete `ParamModeImplicit` tag + exemplar (Task 4)
- Modify: `design/contracts/0008-memory-model.md`, `design/contracts/0002-data-model.md` — binary-model prose (Task 4)
- Create (throwaway, removed in Task 4): `ail migrate-modes` subcommand in `crates/ail/src/main.rs` (Task 3)
- Test: `crates/ail/tests/migrate_modes.rs` — migration-tool unit test (Task 3, removed in Task 4)

---

## Task 1: New check — borrow-return rejection

`(ret (borrow T))` becomes a check error. Independent of `Implicit`;
the tree stays green and this task is independently committable.

**Files:**
- Create: `examples/borrow_return_reject.ail`
- Modify: `crates/ailang-check/src/lib.rs` (`CheckError` enum ~`:404`, `code()` ~`:837`, signature check ~`:2275`)
- Test: `crates/ail/tests/mode_signature_rejects.rs` (new, CLI-boundary E2E)

- [ ] **Step 1: Write the RED fixture**

Create `examples/borrow_return_reject.ail`:

```ail
(module borrow_return_reject

  (data Box
    (doc "Heap cell holding one Int.")
    (ctor Box (con Int)))

  (fn peek
    (doc "MUST FAIL post-cutover: borrow-return is refcount-invisible and can outlive its source; re-enabling needs the escape axis.")
    (type
      (fn-type
        (params (own (con Box)))
        (ret (borrow (con Box)))))
    (params b)
    (body b)))
```

- [ ] **Step 2: Write the failing test (CLI-boundary, the project's reject-pin pattern)**

Create `crates/ail/tests/mode_signature_rejects.rs` — modelled verbatim
on `crates/ail/tests/raw_buf_new_type_arg_pin.rs` (run `ail check` on
the fixture, assert non-zero exit + stderr carries the code):

```rust
//! Signature-level mode rejects added by spec 0062: borrow-return
//! (`(ret (borrow T))`) and borrow-over-value (`(borrow value-type)`).
//! CLI-boundary E2E (the project's reject-pin pattern): the diagnostic
//! is the observable exit behaviour of `ail check`.
use std::path::{Path, PathBuf};
use std::process::Command;

fn repo_root() -> PathBuf {
    Path::new(env!("CARGO_MANIFEST_DIR")).parent().unwrap().parent().unwrap().to_path_buf()
}

fn check_rejects_with(fixture: &str, code: &str) {
    let path = repo_root().join("examples").join(fixture);
    let out = Command::new(env!("CARGO_BIN_EXE_ail"))
        .arg("check").arg(&path).output().expect("ail check spawn");
    assert!(!out.status.success(),
        "expected `ail check {fixture}` to REJECT, but it passed: stdout={}",
        String::from_utf8_lossy(&out.stdout));
    let stderr = String::from_utf8_lossy(&out.stderr);
    assert!(stderr.contains(code), "expected code `{code}`, got: {stderr}");
}

/// `(ret (borrow T))` is rejected with code `borrow-return-not-permitted`.
#[test]
fn borrow_return_is_rejected() {
    check_rejects_with("borrow_return_reject.ail", "borrow-return-not-permitted");
}
```

- [ ] **Step 3: Run the test to verify it fails**

Run: `cargo test -p ail --test mode_signature_rejects borrow_return_is_rejected`
Expected: FAIL — today the fixture checks clean (no return check exists),
so `ail check` exits 0 and the `!out.status.success()` assert fires.

- [ ] **Step 4: Add the `CheckError` variant**

In `crates/ailang-check/src/lib.rs`, in `pub enum CheckError` (after the
last variant `Internal(_)`'s neighbours — place next to the other
signature rejects), add:

```rust
    /// A fn-type return slot is `(borrow T)`. Borrow-returns are
    /// refcount-invisible and can outlive their source; re-enabling
    /// them needs the escape/liveness axis (out of scope, spec 0062
    /// §"Out of scope"). Code: `borrow-return-not-permitted`.
    #[error("borrow-return not permitted for `{0}`: a (borrow …) return is refcount-invisible and can outlive its source; this language version forbids it (the escape/liveness axis that would make it sound is not yet built)")]
    BorrowReturnNotPermitted(String),
```

- [ ] **Step 5: Register the code**

In `fn code(&self)`, add before `CheckError::Internal(_) => "internal",`:

```rust
            CheckError::BorrowReturnNotPermitted(_) => "borrow-return-not-permitted",
```

- [ ] **Step 6: Fire the check at the fn signature**

In `check_def` (`crates/ailang-check/src/lib.rs`), the fn signature is
destructured at `:2240`:

```rust
    let (param_tys, ret_ty, declared_effs) = match &inner_ty {
        Type::Fn { params, ret, effects, .. } => {
            (params.clone(), (**ret).clone(), effects.clone())
        }
```

Change the pattern to also bind `ret_mode` (leave `param_modes` under
`..` — Task 1 does not read it; Task 4 Step B2 widens further), and
after the existing `check_type_well_formed(&ret_ty, &env)?;` line
(`:2275`) add the reject. First, widen the destructure:

```rust
    let (param_tys, ret_ty, ret_mode, declared_effs) = match &inner_ty {
        Type::Fn { params, ret, ret_mode, effects, .. } => {
            (params.clone(), (**ret).clone(), *ret_mode, effects.clone())
        }
        other => {
            return Err(CheckError::FnTypeRequired(
                f.name.clone(),
                ailang_core::pretty::type_to_string(other),
            ));
        }
    };
```

Then immediately after `check_type_well_formed(&ret_ty, &env)?;`:

```rust
    // spec 0062: borrow-returns are refcount-invisible — forbidden in
    // this language version (the escape/liveness axis is unbuilt).
    if matches!(ret_mode, ParamMode::Borrow) {
        return Err(CheckError::BorrowReturnNotPermitted(f.name.clone()));
    }
```

> Ensure `ParamMode` is in scope in `lib.rs` (it is — `param_modes`
> are already referenced elsewhere in the file). Task 1 binds only
> `ret_mode`, leaving `param_modes` under `..`, so there is no unused
> binding. **Task 4 Step B2** widens this same destructure to also bind
> `param_modes` (its borrow-over-value loop is the first reader).

- [ ] **Step 7: Run the test to verify it passes**

Run: `cargo build -p ail && cargo test -p ail --test mode_signature_rejects borrow_return_is_rejected`
Expected: PASS

- [ ] **Step 8: Confirm no corpus fixture regresses**

Run: `cargo build -p ail && python3 bench/check.py`
Expected: 0 regressions (no checked-in fixture has a `(ret (borrow …))`
— the spec asserts none survive, and `consume-while-borrowed` already
rejects the body shapes that would force one).

---

## Task 2: New signature reject — borrow-over-value — FOLDED INTO TASK 4

> **Finding during implementation (2026-06-01), re-staged.** The
> borrow-over-value reject CANNOT land green in isolation before the
> cutover. The prelude's polymorphic comparison builtins are declared
> `(fn-type (params (borrow a) (borrow a)) (ret …))` (`eq`, `compare`,
> `lt`/`le`/`gt`/`ge`, …). When the mono pass specialises `a := Int`
> (or `Bool`/`Float`), `apply_subst_to_type` (`subst.rs:165`) produces
> `compare__Int(borrow Int, borrow Int)` — i.e. the language's OWN
> generated code is in the forbidden `(borrow value-type)` shape. The
> standalone check broke `compare_{int,bool}_mono_symbol_emits_branch_ladder`
> (`ailang-codegen` lib tests) with `borrow over value type in
> compare__Int`.
>
> **The spec's rule is universal ("value types always own", model
> §3.2), so the language's generated code must honour it too** — the
> principled fix is that the mono pass coerces `(borrow value-type) →
> (own value-type)` when specialising a polymorphic borrow param onto
> a value type (semantically a no-op: value types are never
> refcounted, so own vs borrow emits no inc/dec and the branch-ladder
> IR is unchanged). The check and the coercion are therefore a matched
> pair and land together in Task 4 (which touches the mono/codegen
> path anyway), not as a standalone pre-cutover task.
>
> Spec gap to record at the next brainstorm touch of 0062: the
> borrow-over-value rule needs the mono-coercion clause spelled out;
> the spec tested only the authored case.
>
> The borrow-over-value variant + check + fixture + the
> `borrow_over_value_is_rejected` test + the mono-coercion now live in
> **Task 4 Phase B Steps B1–B2** below.

---

## Task 3: Throwaway migration tool — `ail migrate-modes`

A parse → `Implicit↦Own` → print-explicit → write-back pass, run over
the corpus in Task 4. `Own`/`Borrow` are preserved verbatim. This is
semantically invisible today (`PartialEq` treats `Implicit == Own`,
`ast.rs:mode_eq`), so the migrated corpus still typechecks identically
— but it materialises the modes so the post-cutover parser (which
rejects bare slots) accepts the sources. Throwaway: removed in Task 4
once `Implicit` is deleted (the subcommand would not compile).

**Files:**
- Modify: `crates/ail/src/main.rs` (add `MigrateModes` subcommand)
- Test: `crates/ail/tests/migrate_modes.rs` (new)

- [ ] **Step 1: Write the failing tool test**

Create `crates/ail/tests/migrate_modes.rs`:

```rust
//! Throwaway migration-tool test (spec 0062). Asserts the
//! parse→Implicit↦Own→print pass materialises bare slots as `(own …)`
//! and leaves explicit `(borrow …)` untouched. Deleted in the cutover.
use ailang_surface::{parse, print};
use ailang_core::ast::ParamMode;

/// Re-implements the tool's core transform for the unit test so the
/// assertion does not depend on the CLI I/O wrapper.
fn migrate_text(src: &str) -> String {
    let mut m = parse(src).expect("parse");
    ailang_core::ast::for_each_fn_type_mut(&mut m, &mut |params_len, pm, rm| {
        pm.resize(params_len, ParamMode::Own);
        for x in pm.iter_mut() {
            if matches!(x, ParamMode::Implicit) { *x = ParamMode::Own; }
        }
        if matches!(rm, ParamMode::Implicit) { *rm = ParamMode::Own; }
    });
    print(&m)
}

#[test]
fn bare_slot_becomes_own_borrow_preserved() {
    let src = "(module m\n  (fn f\n    (doc \"d\")\n    (type (fn-type (params (con Int) (borrow (con Int))) (ret (con Int))))\n    (params x y)\n    (body x)))\n";
    let out = migrate_text(src);
    assert!(out.contains("(params (own (con Int)) (borrow (con Int)))"), "got: {out}");
    assert!(out.contains("(ret (own (con Int)))"), "got: {out}");
}
```

> `ailang_surface::parse` (`parse.rs:125`, `-> Result<Module, ParseError>`)
> and `ailang_surface::print` (`print.rs:17`, `-> String`) are the
> verified public entry points. `for_each_fn_type_mut` is introduced in
> Step 2.

- [ ] **Step 2: Add the AST fn-type walker**

In `crates/ailang-core/src/ast.rs`, add a public mutator that visits
every `Type::Fn` reachable from a module's defs (signatures incl.
`Forall` bodies, and nested fn-types inside param/ret/arg types). The
callback receives the param count, a `&mut Vec<ParamMode>`, and a
`&mut ParamMode`:

```rust
/// Visit every `Type::Fn` in `m`, letting `f` rewrite its modes.
/// `f(params_len, param_modes, ret_mode)`. Used by the throwaway
/// `migrate-modes` tool (spec 0062); has no other caller and is
/// removed if the migration machinery is retired.
pub fn for_each_fn_type_mut(
    m: &mut Module,
    f: &mut impl FnMut(usize, &mut Vec<ParamMode>, &mut ParamMode),
) {
    fn walk_ty(t: &mut Type, f: &mut impl FnMut(usize, &mut Vec<ParamMode>, &mut ParamMode)) {
        match t {
            Type::Fn { params, param_modes, ret, ret_mode, .. } => {
                let n = params.len();
                for p in params.iter_mut() { walk_ty(p, f); }
                walk_ty(ret, f);
                f(n, param_modes, ret_mode);
            }
            Type::Con { args, .. } => { for a in args.iter_mut() { walk_ty(a, f); } }
            Type::Forall { body, .. } => walk_ty(body, f),
            Type::Var { .. } => {}
        }
    }
    for def in m.defs.iter_mut() {
        if let Def::Fn(fd) = def {
            walk_ty(&mut fd.ty, f);
        }
    }
}
```

> The implementer confirms the `Module`/`Def`/`FnDef` field names
> (`m.defs`, `Def::Fn(fd)`, `fd.ty`) against the actual `ast.rs`; the
> recon-confirmed shape is `Def::Fn(f) => f.ty` (`lib.rs:1427`).

- [ ] **Step 3: Run the tool test to verify it fails**

Run: `cargo test -p ail --test migrate_modes`
Expected: FAIL — `for_each_fn_type_mut` is new; before Step 2 it does
not compile, after Step 2 the test should compile and pass. (If it
passes immediately after Step 2, that is the GREEN; the RED was the
compile failure pinning the missing walker.)

- [ ] **Step 4: Add the throwaway CLI subcommand**

In `crates/ail/src/main.rs`, add a `MigrateModes { path: PathBuf }`
arm to `enum Cmd` and a handler that reads the file, runs the same
transform as `migrate_text` (factored into a shared fn or duplicated —
this is throwaway), and writes the printed result back in place:

```rust
    /// THROWAWAY (spec 0062): rewrite every bare fn-type slot in a
    /// `.ail` file as `(own …)`, preserving explicit `(own)`/`(borrow)`.
    /// Removed in the Implicit-deletion cutover.
    MigrateModes { path: std::path::PathBuf },
```

Handler (in the match on `Cmd`):

```rust
        Cmd::MigrateModes { path } => {
            let src = std::fs::read_to_string(&path)?;
            let mut m = ailang_surface::parse(&src)?;
            ailang_core::ast::for_each_fn_type_mut(&mut m, &mut |n, pm, rm| {
                pm.resize(n, ailang_core::ast::ParamMode::Own);
                for x in pm.iter_mut() {
                    if matches!(x, ailang_core::ast::ParamMode::Implicit) {
                        *x = ailang_core::ast::ParamMode::Own;
                    }
                }
                if matches!(rm, ailang_core::ast::ParamMode::Implicit) {
                    *rm = ailang_core::ast::ParamMode::Own;
                }
            });
            std::fs::write(&path, ailang_surface::print(&m))?;
            Ok(())
        }
```

> Match the handler's error type / `Ok(())` shape to the sibling arms
> in `main.rs`; the recon confirms `enum Cmd` at `main.rs:63`.

- [ ] **Step 5: Build the tool**

Run: `cargo build -p ail`
Expected: 0 errors. `target/debug/ail migrate-modes <file>` now exists.

- [ ] **Step 6: Full-suite gate (no corpus touched yet)**

Run: `cargo test --workspace`
Expected: green — Task 3 is purely additive; no corpus file or schema
changed.

---

## Task 4: The atomic cutover

Everything that cannot land except together: run the migration over the
corpus, delete the variant + all compile sites, parser reject, hash
resets, leak flip, contract updates, drift-pin updates, new fixtures.
A deleted variant makes the whole workspace fail to compile until every
site is migrated, and the parser reject breaks every bare slot until
the corpus is migrated — so all of it is one task with a single green
gate at the end. The compiler is the authoritative site enumerator.

**Files:** all under "Files this plan creates or modifies" tagged
(Task 4), plus removal of the Task 3 throwaway.

### Phase A — migrate the corpus (Implicit still present, tree green)

- [ ] **Step 1: Run the migration tool over the whole corpus**

Run:
```
cargo build -p ail
for f in (find examples -name '*.ail') examples/prelude.ail crates/ailang-kernel/src/raw_buf/source.ail
  ./target/debug/ail migrate-modes $f
end
```
(fish syntax; the implementer adapts to the available shell. The
`find` already includes `examples/prelude.ail`, so the explicit repeat
is harmless — dedupe if preferred.)
Expected: every `.ail` file rewritten with explicit modes on every
fn-type slot; bare slots gone.

- [ ] **Step 2: Verify the migrated corpus parses and round-trips**

Run: `cargo build -p ail && python3 bench/check.py`
Expected: 0 regressions. (Sources are now explicit `own`/`borrow`;
bare slots are still legal under the current parser, so anything the
tool missed still parses — but `check.py` must stay green. The
round-trip invariant is enforced by the build deriving JSON via
`parse`.)

- [ ] **Step 3: Activate the strict check early via the migration, fix over-consuming params**

Migrating a fn to explicit modes turns on the (today gated-off)
strict linearity check for it — exactly the universal activation #56
hardened. A bare heap param the tool set to `(own …)` that the body
only reads, and which a caller passes while retaining, now trips
`consume-while-borrowed` / `use-after-consume`.

Run: `cargo test --workspace 2>&1 | grep -iE 'consume-while-borrowed|use-after-consume|over-strict' `
For each flagged fn: change the offending param's `(own …)` to
`(borrow …)` in its `.ail` source (the `suggested_mode: "borrow"`
diagnostic names the param). Re-run until the grep is empty.
Expected (final): no linearity errors; the corpus encodes the derived
modes (consumed ⇒ own, read-only-heap ⇒ borrow, value ⇒ own).

> This is the spec's "derived from the existing uniqueness/consume
> analysis … reviewed by the orchestrator" — the tool drafts blanket
> `own`, the universal check flags the read-only-heap params, and the
> fix is mechanical and check-gated. The orchestrator reviews the full
> mode diff before commit (the derivation is a drafting aid, not a
> surviving default).

### Phase B — delete the variant (compiler-driven site set)

> Phase B order: Steps 4–8 (schema + compiler-driven migration + parser
> + printer + drift tag) first, THEN Steps B1–B2 (mono-coercion +
> borrow-over-value), because B1 relies on Step 4's length invariant
> (`param_modes.len() == params.len()`, the elision gone) and B2's check
> must not fire until B1 has coerced the generated value-type-borrow
> instances. Final Phase-B gate is at B2.

- [ ] **Step 4: Edit `ParamMode` and `Type::Fn` serde in `ast.rs`**

In `crates/ailang-core/src/ast.rs`:

Replace the `Type::Fn` field attributes (`:780`, `:783`) — drop the
elision:

```rust
    Fn {
        params: Vec<Type>,
        param_modes: Vec<ParamMode>,
        ret: Box<Type>,
        ret_mode: ParamMode,
        #[serde(default)]
        effects: Vec<String>,
    },
```

Replace `fn fn_implicit` (`:837`) with `fn_owned`:

```rust
    /// Build a `Type::Fn` with every parameter mode and the return
    /// mode set to `ParamMode::Own`. The synthesis form for every
    /// typechecker / desugar / codegen site that builds a fn-type;
    /// `Own` is correct by construction (spec 0062 Data flow: the old
    /// typechecker made `Implicit ≡ Own`, so synthesised fn-types were
    /// already semantically `Own`).
    pub fn fn_owned(params: Vec<Type>, ret: Type, effects: Vec<String>) -> Type {
        let n = params.len();
        Type::Fn {
            params,
            param_modes: vec![ParamMode::Own; n],
            ret: Box::new(ret),
            ret_mode: ParamMode::Own,
            effects,
        }
    }
```

Replace the `ParamMode` enum (`:860`) — drop `Default` + `Implicit`:

```rust
/// Per-parameter / return mode marker on a [`Type::Fn`]. Full
/// contract lives in `design/contracts/0008-memory-model.md`.
/// Ownership has no default: every fn-type slot carries an explicit
/// `Own` or `Borrow` (spec 0062).
#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "lowercase")]
pub enum ParamMode {
    /// `(own T)` — caller transfers ownership; callee consumes.
    Own,
    /// `(borrow T)` — caller retains ownership; callee may not consume.
    Borrow,
}
```

Delete `impl ParamMode { fn is_implicit }` (`:872-878`), `fn all_implicit`
(`:880-887`), `fn mode_eq` (`:889-901`), and `fn mode_slices_eq`
(`:903-919`).

In `impl PartialEq for Type`, replace the `Type::Fn` arm's mode
comparison (`:946-947`):

```rust
                ap == bp
                    && ar == br
                    && apm == bpm
                    && arm == brm
                    && {
                        let mut a = ae.clone();
                        let mut b = be.clone();
                        a.sort();
                        b.sort();
                        a == b
                    }
```

- [ ] **Step 5: Compiler-driven migration of all remaining sites**

Run: `cargo build --workspace 2>&1 | rg 'ParamMode::Implicit|fn_implicit|mode_eq|all_implicit|is_implicit|mode_slices_eq'`

Apply the deterministic decision rule to every reported site (no
judgement calls):

- `ret_mode: ParamMode::Implicit` / `param_modes: vec![ParamMode::Implicit; n]`
  / `.insert(_, ParamMode::Implicit)` — a **synthesis** site → replace
  `Implicit` with `Own`.
- `.unwrap_or(ParamMode::Implicit)` — modes are always present now →
  the `.unwrap_or(...)`/`.get(i).copied().unwrap_or(...)` becomes a
  direct index (`param_modes[i]`) or `.unwrap_or(ParamMode::Own)` if a
  fallback is structurally still needed; prefer the direct index where
  the length invariant (`param_modes.len() == params.len()`) holds.
- `matches!(m, ParamMode::Implicit)` in an **activation gate**
  (`linearity.rs:356`, `reuse_shape.rs:110`) — the `any(Implicit)`
  disjunct is now always false → delete that disjunct (gate reduces to
  its other condition, e.g. `params.is_empty()`).
- `ParamMode::Own | ParamMode::Implicit => …` match arm
  (`linearity.rs:385`) → collapse to `ParamMode::Own => …`.
- `ParamMode::Implicit => write_type(out, t)` (printer `print.rs:360`,
  prose `lib.rs:456`) → delete the arm (the `match mode` is now
  exhaustive over `{Own, Borrow}`).
- `fn_implicit(` callers (17 references) → `fn_owned(`.
- `mode_eq` / `mode_slices_eq` callers → already handled in Step 4
  (only caller was `PartialEq`).

Repeat `cargo build --workspace` until 0 errors.
Expected (final): `cargo build --workspace` 0 errors, and
`git grep -nE 'ParamMode::Implicit|fn_implicit|is_implicit|all_implicit|mode_eq|mode_slices_eq' crates/` is empty (acceptance criterion 1).

- [ ] **Step 6: Parser — reject bare slots, delete elision**

In `crates/ailang-surface/src/parse.rs`, replace the bare fallthrough
in `parse_param_with_mode` (`:1220-1221`):

```rust
        // bare type in a fn-type slot is no longer permitted (spec 0062):
        // every slot must carry (own …) or (borrow …).
        let tok = self.peek().cloned();
        Err(self.error_at(tok, "fn-type slot requires a mode: write (own T) or (borrow T)"))
```

> The implementer uses the crate's actual `ParseError` constructor
> (the helper the sibling parse errors in this file already use); the
> load-bearing behaviour is "bare slot → `Err`", verified by Step 12's
> fixture.

In `parse_fn_type`, delete the all-Implicit elision (`:1180-1187`) —
store `param_modes` directly:

```rust
        Ok(Type::Fn {
            params,
            ret: Box::new(ret),
            effects,
            param_modes,
            ret_mode,
        })
```

- [ ] **Step 7: Printer — drop the `Implicit` arm**

In `crates/ailang-surface/src/print.rs`, `write_fn_type_slot` (`:357`)
becomes exhaustive over `{Own, Borrow}` (Step 5 already deleted the
`Implicit` arm); change the `param_modes.get(i).copied().unwrap_or(ParamMode::Implicit)`
at `:408` to a direct index `param_modes[i]` (length invariant holds
post-cutover).

- [ ] **Step 8: Delete the `ParamModeImplicit` drift tag + exemplar**

- `crates/ailang-core/tests/schema_coverage.rs:70,114,345` — delete the
  `VariantTag::ParamModeImplicit` enum value and its exhaustive-`match`
  arm so the coverage test no longer expects an `"implicit"` form.
- `crates/ailang-core/tests/design_schema_drift.rs:385,392` — delete the
  `"implicit"` exemplar row and the `match` arm that asserts it
  serialises and is doc-anchored.

- [ ] **Step B1: Mono-coercion — `(borrow value-type) → (own value-type)`**

The prelude's polymorphic comparison builtins are `(params (borrow a) …)`
(`eq`/`compare`/`lt`/…); specialising `a` to a value type would emit
`compare__Int(borrow Int, …)`, which Step B2's check forbids. Coerce at
the mono specialisation site so the language's generated code honours
"value types always own". In `crates/ailang-codegen/src/subst.rs`,
`apply_subst_to_type`'s `Type::Fn` arm (`:165`), recompute the modes
after substituting the slot types:

```rust
        Type::Fn { params, ret, effects, param_modes, ret_mode } => {
            let new_params: Vec<Type> = params.iter().map(|p| apply_subst_to_type(p, subst)).collect();
            let new_ret = apply_subst_to_type(ret, subst);
            // spec 0062: a polymorphic (borrow a) specialised onto a
            // value type becomes (own value-type) — borrow-over-value is
            // forbidden and is a no-op for unboxed types (no RC).
            let coerce = |ty: &Type, m: &ParamMode| -> ParamMode {
                if matches!(m, ParamMode::Borrow) {
                    if let Type::Con { name, args } = ty {
                        if args.is_empty() && ailang_core::primitives::is_value_type(name) {
                            return ParamMode::Own;
                        }
                    }
                }
                *m
            };
            let new_param_modes: Vec<ParamMode> = new_params.iter().zip(param_modes.iter())
                .map(|(t, m)| coerce(t, m)).collect();
            let new_ret_mode = coerce(&new_ret, ret_mode);
            Type::Fn {
                params: new_params,
                ret: Box::new(new_ret),
                effects: effects.clone(),
                param_modes: new_param_modes,
                ret_mode: new_ret_mode,
            }
        }
```

> Runs after Step 4, so the length invariant `param_modes.len() ==
> params.len()` is in force (the elision is gone) and the `.zip` is
> total. `ParamMode` is already in scope in `subst.rs` (the arm binds
> `param_modes`/`ret_mode`).

- [ ] **Step B2: borrow-over-value reject (corpus re-mode + check + fixture + test)**

First re-mode any authored `(borrow value-type)` left in the corpus
(the migration tool left explicit `Borrow` untouched in Phase A, so e.g.
`examples/mq3_class_eq_vs_fn_eq_fnmod.ail`'s `(params (borrow (con Int))
(borrow (con Int)))` is still present):

Run: `git grep -lE '\(borrow \(con (Int|Bool|Float|Unit)\)' examples crates/ailang-kernel`
For each hit, replace `(borrow (con <Value>))` with `(own (con <Value>))`.
Re-run the grep until empty.

Add the `CheckError` variant to `pub enum CheckError`
(`crates/ailang-check/src/lib.rs`):

```rust
    /// A fn-type slot is `(borrow V)` where `V` is an unboxed value
    /// type (`Int`/`Bool`/`Float`/`Unit`). Borrow is meaningless over
    /// a value type — it has no refcount and is copied by value
    /// (model §3.2). Code: `borrow-over-value`.
    #[error("borrow over value type in `{def}`: `{ty}` is an unboxed value type and cannot be borrowed; use `(own {ty})`")]
    BorrowOverValueType { def: String, ty: String },
```

Register the code in `fn code(&self)` before `CheckError::Internal(_)`:

```rust
            CheckError::BorrowOverValueType { .. } => "borrow-over-value",
```

In `check_def`, first widen Task 1's destructure to also bind
`param_modes` (it was left under `..` in Task 1 Step 6):

```rust
    let (param_tys, param_modes, ret_ty, ret_mode, declared_effs) = match &inner_ty {
        Type::Fn { params, param_modes, ret, ret_mode, effects } => (
            params.clone(), param_modes.clone(), (**ret).clone(), *ret_mode, effects.clone(),
        ),
        other => {
            return Err(CheckError::FnTypeRequired(
                f.name.clone(),
                ailang_core::pretty::type_to_string(other),
            ));
        }
    };
```

Then, after the borrow-return check, add the borrow-over-value loop:

```rust
    // spec 0062: borrow over an unboxed value type is meaningless.
    // Fired on the signature, before body dataflow. The mono-coercion
    // (subst.rs, Step B1) guarantees no generated value-type-borrow
    // instance reaches here.
    for (ty, mode) in param_tys.iter().zip(param_modes.iter()) {
        if matches!(mode, ParamMode::Borrow) {
            if let Type::Con { name, args } = ty {
                if args.is_empty() && ailang_core::primitives::is_value_type(name) {
                    return Err(CheckError::BorrowOverValueType {
                        def: f.name.clone(),
                        ty: name.clone(),
                    });
                }
            }
        }
    }
```

Create `examples/borrow_value_reject.ail`:

```ail
(module borrow_value_reject

  (fn ignore
    (doc "MUST FAIL post-cutover: borrow over a value type is meaningless.")
    (type
      (fn-type
        (params (borrow (con Int)))
        (ret (own (con Int)))))
    (params n)
    (body 0)))
```

Add to `crates/ail/tests/mode_signature_rejects.rs` (the
`check_rejects_with` helper is created in Task 1 Step 2):

```rust
/// `(borrow value-type)` is rejected with code `borrow-over-value`.
#[test]
fn borrow_over_value_is_rejected() {
    check_rejects_with("borrow_value_reject.ail", "borrow-over-value");
}
```

Gate: `cargo test -p ailang-codegen --lib compare` — the
`compare_{int,bool}_mono_symbol_emits_branch_ladder` tests PASS (the
mono-coercion makes their value-type params `own`; branch-ladder IR is
unchanged — value types emit no RC ops). And `cargo test -p ail --test
mode_signature_rejects` PASS (all three: borrow-return, borrow-over-value,
provenance pins).

### Phase C — pins, fixtures, contracts

- [ ] **Step 9: Reset all five hash pins**

For each pin, regenerate the expected hex from the migrated module and
re-assert (do not hand-edit blindly — recompute):

- `crates/ailang-core/tests/hash_pin.rs` — `sum`, `IntList`, and `:257`/`:266`/`:275`.
- `crates/ailang-surface/tests/prelude_module_hash_pin.rs:50`.
- `crates/ailang-core/tests/embed_export_hash_stable.rs:32`.
- `crates/ail/tests/eq_ord_e2e.rs:139`.
- `crates/ail/tests/mono_hash_stability.rs:63-68,107-110`.

Procedure per pin: run the test, read the `assertion failed: left == right`
actual value, paste the new hex into the pin, re-run.

Run: `cargo test --workspace 2>&1 | rg 'hash|pin' ` until the pin tests pass.
Expected: each pin holds the post-migration hash; this is the one-time
corpus-wide reset (spec §6, "the single irreversible step").

- [ ] **Step 10: Flip the leak pin**

- `examples/rc_let_implicit_returning_app.ail` — its return is now
  `(own …)` (migrated in Phase A). Rewrite the file header and the
  property-3 assertion from "Implicit leaks by design" / `live=1` to
  "own-return frees correctly" / `live=0`.
- `crates/ail/tests/print_no_leak_pin.rs` — change the pinned
  `AILANG_RC_STATS` expectation for this fixture from `live=1` to
  `live=0`; update the doc comment (`:26`) that mentions
  `ParamMode::Implicit`.

Run: `cargo test -p ail --test print_no_leak_pin`
Expected: PASS with `live=0` (acceptance criterion 5).

- [ ] **Step 11: Add the regression-pin + authoring-surface fixtures**

Create these (each verified to `ail check` at the stated exit code —
see "Spec fixture parse-gate" below):

`examples/own_return_provenance_reject.ail` (exit 1, `consume-while-borrowed`):
```ail
(module own_return_provenance_reject
  (data Box
    (doc "Heap cell holding one Int.")
    (ctor Box (con Int)))
  (fn passthrough
    (doc "Already rejected today (consume-while-borrowed): own-return aliases a borrowed binder.")
    (type
      (fn-type
        (params (borrow (con Box)))
        (ret (own (con Box)))))
    (params b)
    (body b)))
```

`examples/own_return_provenance_let.ail` (exit 1) — body `(let y b y)`;
`examples/own_return_provenance_ctor.ail` (exit 1) — adds
`(data Wrap (doc "Wraps a Box.") (ctor Wrap (con Box)))` and body
`(term-ctor Wrap Wrap b)` with ret `(own (con Wrap))` (regime A).

`examples/ownership_total.ail` (exit 0) — the post-cutover
authoring-surface module verbatim from spec 0062 §"User-facing"
(`list_length` borrows, `sum_list` owns, `main` builds `[1,2,3]`).

Add a test asserting each provenance fixture still exits 1 under the
post-cutover check (extend `crates/ail/tests/mode_signature_rejects.rs`,
reusing the `check_rejects_with` helper from Task 1 Step 2):

```rust
/// The own-return-provenance / regime-A shapes stay rejected by
/// `consume-while-borrowed` after the schema deletion (spec 0062
/// already-green pins — these are NOT new checks).
#[test]
fn own_return_provenance_still_rejected() {
    for f in ["own_return_provenance_reject.ail",
              "own_return_provenance_let.ail",
              "own_return_provenance_ctor.ail"] {
        check_rejects_with(f, "consume-while-borrowed");
    }
}
```

- [ ] **Step 12: Add the parser bare-slot RED fixture + test**

Create `examples/bare_slot_reject.ail`:
```ail
(module bare_slot_reject

  (fn id
    (doc "MUST FAIL post-cutover: bare `(con Int)` slot carries no mode.")
    (type
      (fn-type
        (params (con Int))
        (ret (con Int))))
    (params x)
    (body x)))
```

Add a parser test (in `crates/ailang-surface/tests/`) asserting this
source now fails to **parse**:

```rust
#[test]
fn bare_fn_type_slot_is_a_parse_error() {
    let src = include_str!("../../../examples/bare_slot_reject.ail");
    assert!(ailang_surface::parse(src).is_err(), "bare slot must not parse");
}
```

> A CLI-boundary variant is also acceptable (assert `ail check
> bare_slot_reject.ail` exits non-zero with a parse-error stderr) if
> the implementer prefers consistency with the other reject pins; the
> load-bearing behaviour is "bare slot → error".

Also extend the round-trip test suite with a case proving a previously
round-tripping bare-slot input now fails to parse (spec Testing
§"Round-trip invariant").

- [ ] **Step 13: Update both contracts**

`design/contracts/0008-memory-model.md`:

Replace `:57-59`:
```
`Own` and `Borrow` are the two modes; every fn-type slot carries one
explicitly. Ownership has no default — there is no bare/unannotated
mode (spec 0062 deleted the legacy `Implicit` state).
```

Replace `:62-65`:
```
JSON canonical form: `param_modes` and `ret_mode` are always present
(one mode per slot). The pre-0062 elision (skipping all-`Implicit`
vectors for hash stability) is gone with the variant; the
corpus-wide hash reset that accompanied the deletion is a one-time
event recorded in `git log`.
```

Replace the `:249-251` bullet (Iter B):
```
  param itself. `Borrow` parameters are skipped: `Borrow` retains the
  caller's ownership by contract. (There is no `Implicit` parameter
  any longer — every param is `Own` or `Borrow`.)
```

Replace the `:260` clause (Iter A):
```
  the dec — only `Own`-mode scrutinees enable it; a `Borrow` scrutinee
  would let the arm dec memory the caller still references.
```

Replace the `:297` clause (ret_mode):
```
  holds a view, not an own ref). Every `Term::App` callee now carries
  an explicit `Own`/`Borrow` `ret_mode`; an `Own`-returning call is
  trackable for scope-close drop, a `Borrow`-returning call is not
  (and is in any case rejected at the signature — borrow-returns are
  out of scope, spec 0062).
```

`design/contracts/0002-data-model.md`:

Replace the comment `:272-273`:
```
// `paramModes` and `retMode` are always present (one mode per slot).
// Full mode contract lives in contracts/0008-memory-model.md.
```

Replace the `ParamMode` block `:297-302`:
```
"own"        — (own T) — caller transfers ownership; callee consumes.
"borrow"     — (borrow T) — caller retains ownership; callee may not consume.
```
and the trailing prose (`:302-…`):
```
Every fn-type slot carries `own` or `borrow`; ownership has no default
(spec 0062 deleted the legacy `implicit` state). The full mode contract
(codegen consequences, the over-strict-mode lint, the `Suppress`
mechanism) lives in [memory model](0008-memory-model.md); the four
language-design preconditions that make RC sound live in
[language constraints](0015-language-constraints.md).
```

### Phase D — remove throwaway, final gate

- [ ] **Step 14: Remove the throwaway migration tool**

Delete the `MigrateModes` arm + handler from `crates/ail/src/main.rs`,
delete `crates/ail/tests/migrate_modes.rs`, and delete
`for_each_fn_type_mut` from `ast.rs` (it has no other caller). (It
references `ParamMode::Implicit` and would not compile anyway.)

- [ ] **Step 15: Final workspace gate**

Run: `cargo build --workspace && cargo test --workspace`
Expected: 0 errors, all tests green (acceptance criterion 6).

- [ ] **Step 16: Regression scripts + grep-clean gate**

Run:
```
python3 bench/check.py
python3 bench/compile_check.py
python3 bench/cross_lang.py
git grep -nE 'ParamMode::Implicit|fn_implicit|is_implicit|all_implicit|mode_eq|mode_slices_eq' crates/
git grep -nE 'ParamModeImplicit' crates/
```
Expected: 0 regressions on all three scripts; both greps empty
(acceptance criteria 1).

---

## Spec fixture parse-gate (planner self-review item 9 attestation)

Every surface-language fixture inlined above was run through
`ail check` against current HEAD (2026-06-01, `target/debug/ail`):

```text
ownership_total.ail                  : exit 0   ok (36 symbols)
borrow_return_reject.ail             : exit 0   (legal today; check error post-cutover)
borrow_value_reject.ail              : exit 0   (legal today; sig error post-cutover)
bare_slot_reject.ail                 : exit 0   (legal today; parse error post-cutover)
own_return_provenance_reject.ail     : exit 1   [consume-while-borrowed] passthrough: `b` …
own_return_provenance_let.ail        : exit 1   [consume-while-borrowed] passthrough: `b` …
own_return_provenance_ctor.ail       : exit 1   [consume-while-borrowed] escape: `b` …  (regime A)
```

All seven match the spec's claimed exit codes. The migration-tool unit
test fixture (Task 3 Step 1) is a Rust string literal, not a corpus
fixture, and is exercised by `cargo test -p ail --test migrate_modes`.

## Commit shape (orchestrator note, not an implement step)

Task 1 (borrow-return reject) and Task 3 (throwaway migration tool)
each leave a green, independent tree and may be committed separately
before the cutover (`feat(check): borrow-return reject`, `chore:
throwaway mode-migration tool`). The borrow-over-value reject is NOT a
standalone pre-cutover task — it is folded into Task 4 (Steps B1–B2)
because it is entangled with the mono pass (see the Task 2 finding
note). Task 4 is the single irreversible cutover commit (`feat: delete
ParamMode::Implicit — binary ownership modes (#55)`) carrying the
one-time hash reset and the borrow-over-value reject + mono-coercion.
main only ever advances on a green tree; nothing half-migrated is
committed.
```
