# str-concat — Implementation Plan

> **Parent spec:** `docs/specs/0026-fieldtest-form-a.md` §"[friction] No primitive str_concat / str_append in builtins" (lines 186-212).
>
> **For agentic workers:** REQUIRED SUB-SKILL: use `skills/implement` to run this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** Add `str_concat : (borrow Str, borrow Str) -> Str` as a runtime + checker + codegen primitive in the four-site-lockstep pattern established by `str_clone` / `int_to_str` / `bool_to_str` (iter 24.1), so the LLM-natural Show-MyType body `(app str_concat "label=" (app int_to_str x))` parses, checks, builds, and runs.

**Architecture:** Four-site lockstep registration mirroring `str_clone` (iter 24.1). The new primitive ships at: (1) `runtime/str.c` as a C helper `ailang_str_concat(const char *a, const char *b) -> char *` that slab-allocates a fresh heap-Str of size `a.len + b.len`, memcpys both payloads, terminates; (2) `crates/ailang-check/src/builtins.rs::install` as a `Type::Fn { params: 2× Str borrow, ret: Str own }` entry plus a `list()` row plus a `install_str_concat_signature` unit test; (3) `crates/ailang-codegen/src/lib.rs` as an `extern @ailang_str_concat(ptr, ptr)` declaration plus an arm in `lower_app` emitting the call plus an entry in `is_builtin_callable` plus an IR-pin unit test; (4) `examples/show_user_adt_with_label.ail` corpus fixture authored fresh (the LLM-natural shape the fieldtester first reached for) plus a `crates/ail/tests/str_concat_e2e.rs` E2E test asserting check + build + run produce the expected stdout. The plan also repairs one lockstep collision: `examples/bug_unbound_in_instance_method.ail` currently uses `str_concat` as its UNBOUND name (because that was the literal name the fieldtester tried); after str_concat ships as a builtin the fixture and its pin test `crates/ail/tests/unbound_in_instance_method_pin.rs` would silently turn into a false-OK. Task 5 renames the unbound name to `format_label` (LLM-author-realistic fn name) and updates the pin test accordingly, preserving the regression guard's intent (instance-method-body walked through unbound-var check) while substituting a name that will never become a builtin.

**Tech Stack:** Rust (`crates/ailang-check`, `crates/ailang-codegen`, `crates/ail/tests`), C (`runtime/str.c`), Markdown (`docs/DESIGN.md`, `docs/roadmap.md`), `.ail` Form-A (`examples/`).

---

**Files this plan creates or modifies:**

- Create: `examples/show_user_adt_with_label.ail` — Show user-ADT with str_concat-using body (Task 1)
- Create: `crates/ail/tests/str_concat_e2e.rs` — E2E pin asserting check + build + run on the new fixture (Task 1)
- Modify: `runtime/str.c:186+` — append `ailang_str_concat` C helper (Task 2)
- Modify: `crates/ailang-check/src/builtins.rs:232` — insert str_concat registration after str_clone (Task 3)
- Modify: `crates/ailang-check/src/builtins.rs:330` — insert list() row after str_clone (Task 3)
- Modify: `crates/ailang-check/src/builtins.rs:505-514` — insert signature unit test after install_str_clone_signature (Task 3)
- Modify: `crates/ailang-codegen/src/lib.rs:548` — append `@ailang_str_concat` extern declaration after str_clone declare (Task 4)
- Modify: `crates/ailang-codegen/src/lib.rs:1977` — insert str_concat arm in lower_app after str_clone arm (Task 4)
- Modify: `crates/ailang-codegen/src/lib.rs:2192` — extend is_builtin_callable name-list (Task 4)
- Modify: `crates/ailang-codegen/src/lib.rs:4255-4292` — append IR-pin test after str_clone IR-pin test (Task 4)
- Modify: `examples/bug_unbound_in_instance_method.ail:14` — rename `str_concat` to `format_label` (Task 5)
- Modify: `crates/ail/tests/unbound_in_instance_method_pin.rs` — rename str_concat references to format_label (Task 5)
- Modify: `docs/DESIGN.md:1992` — insert new §"Heap-Str primitives" subsection (Task 6)
- Modify: `docs/roadmap.md` — add struck-out `[x] [feature] str_concat primitive` line (Task 7)

---

### Task 1: RED — fixture + E2E test for str_concat

**Files:**
- Create: `examples/show_user_adt_with_label.ail`
- Create: `crates/ail/tests/str_concat_e2e.rs`

- [ ] **Step 1: Author the fresh `.ail` fixture exercising str_concat in a Show body**

Create `examples/show_user_adt_with_label.ail` with content:

```
(module show_user_adt_with_label
  (data Item
    (ctor MkItem (con Int)))
  (instance
    (class prelude.Show)
    (type (con Item))
    (method show
      (body (lam (params (typed it (con Item))) (ret (con Str)) (body (match it
        (case (pat-ctor MkItem n) (app str_concat "Item " (app int_to_str n)))))))))
  (fn main
    (type (fn-type (params) (ret (con Unit)) (effects IO)))
    (params)
    (body (app print (term-ctor Item MkItem 42)))))
```

This is the LLM-natural shape: a single-ctor ADT with an integer payload, a Show instance whose body produces a labelled string (`"Item 42"`) by concatenating a static-Str prefix with the int_to_str rendering. The output via `print` (polymorphic from iter 24.3) goes through the prelude's `print : forall a. Show a => (a borrow) -> Unit !IO`.

- [ ] **Step 2: Author the E2E test file**

Create `crates/ail/tests/str_concat_e2e.rs` with content:

```rust
//! E2E pin for the `str_concat` heap-Str primitive shipped in iter
//! `str-concat`. Asserts that `ail check` + `ail build` + run on
//! `examples/show_user_adt_with_label.ail` produce stdout
//! `Item 42\n` — i.e. that the Show body's
//! `(app str_concat "Item " (app int_to_str n))` evaluates correctly
//! and `print` emits the concatenated heap-Str.
//!
//! Without `str_concat` registered as a builtin (pre-iter state), the
//! fixture fails `ail check` with `[unbound-var]: str_concat`. After
//! the iter ships, both check and build succeed and the binary
//! produces the expected stdout.

use std::path::Path;
use std::process::Command;

fn ail_bin() -> &'static str {
    env!("CARGO_BIN_EXE_ail")
}

#[test]
fn str_concat_e2e_show_user_adt_with_label() {
    let manifest_dir = env!("CARGO_MANIFEST_DIR");
    let workspace = Path::new(manifest_dir).parent().unwrap().parent().unwrap();
    let src = workspace
        .join("examples")
        .join("show_user_adt_with_label.ail");
    assert!(src.exists(), "fixture missing: {}", src.display());

    let check = Command::new(ail_bin())
        .args(["check", src.to_str().unwrap()])
        .output()
        .expect("ail check failed to spawn");
    assert_eq!(
        check.status.code(),
        Some(0),
        "ail check must succeed; got stdout={} stderr={}",
        String::from_utf8_lossy(&check.stdout),
        String::from_utf8_lossy(&check.stderr)
    );

    let tmp = tempfile::tempdir().expect("tempdir");
    let bin_path = tmp.path().join("a.out");
    let build = Command::new(ail_bin())
        .args([
            "build",
            src.to_str().unwrap(),
            "-o",
            bin_path.to_str().unwrap(),
        ])
        .output()
        .expect("ail build failed to spawn");
    assert_eq!(
        build.status.code(),
        Some(0),
        "ail build must succeed; got stdout={} stderr={}",
        String::from_utf8_lossy(&build.stdout),
        String::from_utf8_lossy(&build.stderr)
    );

    let run = Command::new(&bin_path)
        .output()
        .expect("produced binary failed to spawn");
    assert_eq!(run.status.code(), Some(0), "binary must exit 0");
    let stdout = String::from_utf8_lossy(&run.stdout);
    assert_eq!(
        stdout, "Item 42\n",
        "expected `Item 42\\n` from str_concat + int_to_str + print; got {stdout:?}"
    );
}
```

- [ ] **Step 3: Run the RED test to confirm it fails today**

Run: `cargo test --manifest-path=Cargo.toml -p ail --test str_concat_e2e 2>&1 | tail -15`
Expected: FAIL with `ail check must succeed` panic, because `str_concat` is unbound. Specifically the diagnostic surface should mention `[unbound-var]` and `str_concat`. The test must fail because of this RED, not because the fixture parse-fails or the binary spawn fails — verify by reading the panic output.

- [ ] **Step 4: Full workspace test as a no-regression smoke**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6} END {print "passed:",passed,"failed:",failed}'`
Expected: `passed: 559 failed: 1` (or 560 passed 1 failed depending on whether the new pin counts; the only failing test must be `str_concat_e2e_show_user_adt_with_label`).

### Task 2: Runtime C helper

**Files:**
- Modify: `runtime/str.c` — append after `ailang_str_clone` (currently ends at line 186)

- [ ] **Step 1: Append the C helper implementation**

After the closing brace of `ailang_str_clone` at line 186 of `runtime/str.c`, append:

```c

/*
 * `ailang_str_concat(a, b)` — heap-Str concatenation primitive
 * shipped in iter str-concat (2026-05-13). Reads `len` headers from
 * both source Str payloads (offset 0 of each), allocates a fresh
 * heap-Str slab sized for the combined bytes via `str_alloc`,
 * memcpys both payloads in order, NUL-terminates, returns the new
 * payload pointer. Like `str_clone`, this works uniformly on static-
 * Str and heap-Str inputs because the consumer ABI is identical
 * between realisations (see DESIGN.md §"Heap-Str primitives").
 *
 * Used by Show bodies that want labelled output (e.g.
 * `"Item " ++ int_to_str n`) and by any caller that needs to
 * combine two existing Str values into a new owned Str.
 *
 * Returns the payload pointer of a fresh heap-Str slab. The rc
 * header is initialised to 1 by `str_alloc`.
 */
char *ailang_str_concat(const char *a, const char *b) {
    uint64_t la = *(const uint64_t *)a;
    uint64_t lb = *(const uint64_t *)b;
    char *payload = str_alloc(la + lb);
    memcpy(payload + 8, a + 8, la);
    memcpy(payload + 8 + la, b + 8, lb);
    payload[8 + la + lb] = '\0';
    return payload;
}
```

- [ ] **Step 2: Verify the C file compiles**

Run: `clang -O2 -c runtime/str.c -o /tmp/str.o 2>&1 | head -20`
Expected: no output (clean compile).

- [ ] **Step 3: Full workspace test as a no-regression smoke**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6} END {print "passed:",passed,"failed:",failed}'`
Expected: `passed: 559 failed: 1` (str_concat_e2e still RED — check still rejects, can't reach build).

### Task 3: ailang-check builtin registration

**Files:**
- Modify: `crates/ailang-check/src/builtins.rs:232` — insert install entry after the `str_clone` block (currently ends at line 232)
- Modify: `crates/ailang-check/src/builtins.rs:330` — append list() row after `("str_clone", ...)`
- Modify: `crates/ailang-check/src/builtins.rs:514` — append signature test after `install_str_clone_signature`

- [ ] **Step 1: Insert str_concat install entry**

Immediately after the closing `);` of the `str_clone` `env.globals.insert(...)` block at `crates/ailang-check/src/builtins.rs:232`, insert:

```rust
    env.globals.insert(
        "str_concat".into(),
        Type::Fn {
            params: vec![Type::str_(), Type::str_()],
            ret: Box::new(Type::str_()),
            effects: vec![],
            param_modes: vec![
                ailang_core::ast::ParamMode::Borrow,
                ailang_core::ast::ParamMode::Borrow,
            ],
            ret_mode: ailang_core::ast::ParamMode::Own,
        },
    );
```

- [ ] **Step 2: Append list() row**

Find the row `("str_clone", "(Str) -> Str"),` near `crates/ailang-check/src/builtins.rs:330`. Insert the new row immediately after it:

```rust
        ("str_concat", "(Str, Str) -> Str"),
```

Maintain the comma + indentation pattern of the surrounding rows.

- [ ] **Step 3: Append signature unit test**

Find `install_str_clone_signature` around `crates/ailang-check/src/builtins.rs:505-514`. Immediately after its closing `}`, append:

```rust
    #[test]
    fn install_str_concat_signature() {
        // Iter str-concat: `str_concat : (Str borrow, Str borrow) -> Str
        // own`. Codegen lowers via the runtime C glue `ailang_str_concat`
        // from `runtime/str.c`.
        let mut env = Env::default();
        install(&mut env);
        let ty = env
            .globals
            .get("str_concat")
            .expect("str_concat must be installed");
        match ty {
            Type::Fn {
                params,
                ret,
                effects,
                param_modes,
                ret_mode,
            } => {
                assert_eq!(params.len(), 2);
                assert!(matches!(params[0], Type::Con { ref name, .. } if name == "Str"));
                assert!(matches!(params[1], Type::Con { ref name, .. } if name == "Str"));
                assert!(matches!(**ret, Type::Con { ref name, .. } if name == "Str"));
                assert!(effects.is_empty(), "str_concat must be effect-free");
                assert_eq!(
                    param_modes,
                    &vec![
                        ailang_core::ast::ParamMode::Borrow,
                        ailang_core::ast::ParamMode::Borrow,
                    ]
                );
                assert_eq!(*ret_mode, ailang_core::ast::ParamMode::Own);
            }
            other => panic!("expected Type::Fn; got {other:?}"),
        }
    }
```

- [ ] **Step 4: Run the new unit test**

Run: `cargo test --manifest-path=Cargo.toml -p ailang-check builtins::tests::install_str_concat_signature 2>&1 | tail -5`
Expected: `test result: ok. 1 passed`.

- [ ] **Step 5: Full workspace test**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6} END {print "passed:",passed,"failed:",failed}'`
Expected: `passed: 560 failed: 1` (str_concat_e2e moves further but still RED — check passes now, build fails because codegen doesn't yet emit the call).

### Task 4: ailang-codegen — extern declare + lower_app arm + is_builtin_callable + IR pin

**Files:**
- Modify: `crates/ailang-codegen/src/lib.rs:548` — append extern declaration
- Modify: `crates/ailang-codegen/src/lib.rs:1977` — insert lower_app arm after str_clone arm
- Modify: `crates/ailang-codegen/src/lib.rs:2192` — extend is_builtin_callable list
- Modify: `crates/ailang-codegen/src/lib.rs:4292` — append IR-pin test after str_clone IR-pin

- [ ] **Step 1: Append extern declaration**

Find the line `out.push_str("declare ptr @ailang_str_clone(ptr)\n");` at `crates/ailang-codegen/src/lib.rs:548`. Immediately after that push_str line, insert:

```rust
    out.push_str("declare ptr @ailang_str_concat(ptr, ptr)\n");
```

- [ ] **Step 2: Insert str_concat arm in lower_app after the str_clone arm**

Find the str_clone arm at `crates/ailang-codegen/src/lib.rs:1960-1977`. Immediately after its closing brace (the `}` at line 1977), insert:

```rust
        if name == "str_concat" {
            // Iter str-concat: lowers to the runtime C glue
            // `ailang_str_concat(ptr, ptr) -> ptr` defined in
            // `runtime/str.c`. Reads `len` from offset 0 of each
            // source Str payload and allocates a fresh heap-Str
            // slab sized for the combined bytes; works uniformly
            // on static-Str and heap-Str inputs because the
            // consumer ABI is identical. Common shape in Show
            // bodies: `(app str_concat "label=" (app int_to_str x))`.
            if args.len() != 2 {
                return Err(CodegenError::Internal("str_concat arity".into()));
            }
            let (a, _) = self.lower_term(&args[0])?;
            let (b, _) = self.lower_term(&args[1])?;
            let dst = self.fresh_ssa();
            self.body.push_str(&format!(
                "  {dst} = call ptr @ailang_str_concat(ptr {a}, ptr {b})\n"
            ));
            return Ok((dst, "ptr".to_string()));
        }
```

- [ ] **Step 3: Extend is_builtin_callable name list**

Find the `matches!` block at `crates/ailang-codegen/src/lib.rs:2183-2193`:

```rust
        if matches!(
            name,
            "neg"
                | "int_to_float"
                | "float_to_int_truncate"
                | "is_nan"
                | "float_to_str"
                | "int_to_str"
                | "bool_to_str"
                | "str_clone"
        ) {
```

Change to:

```rust
        if matches!(
            name,
            "neg"
                | "int_to_float"
                | "float_to_int_truncate"
                | "is_nan"
                | "float_to_str"
                | "int_to_str"
                | "bool_to_str"
                | "str_clone"
                | "str_concat"
        ) {
```

- [ ] **Step 4: Append IR-pin unit test**

Find the `str_clone_emits_call_to_ailang_str_clone` test at `crates/ailang-codegen/src/lib.rs:4255-4292`. Immediately after its closing `}`, append:

```rust
    #[test]
    fn str_concat_emits_call_to_ailang_str_concat() {
        // Iter str-concat: `(app str_concat "x" "y")` must lower to
        // `call ptr @ailang_str_concat(ptr, ptr)` in the emitted IR.
        // Pins the extern declaration + the lower_app arm together.
        let src = r#"
(module ir_pin_str_concat
  (fn produce_label
    (type (fn-type (params) (ret (con Str))))
    (params)
    (body (app str_concat "x" "y"))))
"#;
        let module: Module = serde_json::from_str(
            &ailang_surface::parse(src).expect("parse").to_string(),
        )
        .expect("module from parse");
        let mut ws = Workspace::default();
        ws.modules
            .insert(module.name.clone(), module.clone());
        let ir = lower_workspace_to_ir(&ws, &CodegenOptions::default())
            .expect("codegen");
        assert!(
            ir.contains("declare ptr @ailang_str_concat(ptr, ptr)"),
            "IR must declare the extern; got {ir}"
        );
        assert!(
            ir.contains("call ptr @ailang_str_concat(ptr"),
            "IR must contain the call to @ailang_str_concat; got {ir}"
        );
    }
```

(If the imports in the surrounding test module differ from what's used by the str_clone IR pin, lift them — read 4255-4292 first and mirror the pattern exactly. The above is a structural sketch; the exact `Module` / `Workspace` / `lower_workspace_to_ir` imports must match the sibling test.)

- [ ] **Step 5: Run the new IR-pin test**

Run: `cargo test --manifest-path=Cargo.toml -p ailang-codegen str_concat_emits_call_to_ailang_str_concat 2>&1 | tail -5`
Expected: `test result: ok. 1 passed`.

- [ ] **Step 6: Run the str_concat E2E test**

Run: `cargo test --manifest-path=Cargo.toml -p ail --test str_concat_e2e 2>&1 | tail -5`
Expected: `test result: ok. 1 passed`. The RED from Task 1 is now GREEN: check passes, build passes, run produces `Item 42\n`.

- [ ] **Step 7: Full workspace test**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6} END {print "passed:",passed,"failed:",failed}'`
Expected: `passed: 562 failed: 1` (the new pin test + new IR-pin test pass; but the `unbound_in_instance_method_pin.rs` tests now FAIL because str_concat is no longer unbound — Task 5 repairs this lockstep collision).

### Task 5: Repair the bug-fixture / pin-test collision

**Files:**
- Modify: `examples/bug_unbound_in_instance_method.ail:14` — rename `str_concat` to `format_label`
- Modify: `crates/ail/tests/unbound_in_instance_method_pin.rs` — rename all four `str_concat` references to `format_label`

**Why this task exists.** The `bugfix-instance-body-unbound-var` iter (commit 77f584a) used `str_concat` as the literal unbound name because that was the LLM-natural shape the fieldtester actually reached for. Once str_concat ships as a builtin (Tasks 1-4), the fixture would silently turn into a false-OK and the pin test starts failing. The pin's intent (instance-method-body walked through unbound-var check) is name-independent; substituting a different LLM-natural unbound name (`format_label` — the kind of helper an LLM-author would forget to define) preserves the intent.

- [ ] **Step 1: Update the fixture**

Open `examples/bug_unbound_in_instance_method.ail`. The current line 14 is:

```
        (body (app str_concat "n=" (app int_to_str n)))))))
```

Replace `str_concat` with `format_label`:

```
        (body (app format_label "n=" (app int_to_str n)))))))
```

- [ ] **Step 2: Update the pin test references**

Open `crates/ail/tests/unbound_in_instance_method_pin.rs`. The current file has multiple references to `str_concat`:
- Line 17: in the module docstring's "monomorphise_workspace: unknown identifier: `str_concat`" verbatim panic-message quote;
- Line 22: in the module docstring's "the unbound `str_concat` in the method body";
- Line 36: in the module docstring's fixture description "The fixture uses `str_concat` (NOT a builtin)";
- Line 52 onward: the test name `check_fires_unbound_var_for_str_concat_in_instance_method_body`;
- Line 84: assertion `combined.contains("str_concat")`;
- Line 100 onward: the test name `check_json_unbound_var_in_instance_method_body`;
- Line 132: assertion `d.to_string().contains("str_concat")`.

Strategy: replace `str_concat` → `format_label` everywhere in the file (Edit with `replace_all: true`). The test names will become `check_fires_unbound_var_for_format_label_in_instance_method_body` and `check_json_unbound_var_in_instance_method_body` (the second name doesn't contain `str_concat`; leave it). Update the docstring text accordingly.

The simpler exact strategy: read the file, then issue ONE Edit with `old_string = "str_concat"`, `new_string = "format_label"`, `replace_all = true`.

- [ ] **Step 3: Verify the pin test now correctly RED-protects with the new name**

Run: `cargo test --manifest-path=Cargo.toml -p ail --test unbound_in_instance_method_pin 2>&1 | tail -10`
Expected: `test result: ok. 2 passed`. Both pin tests pass: `ail check` correctly fires `[unbound-var]: format_label` on the fixture (the bugfix from 77f584a stays GREEN under the new unbound name).

- [ ] **Step 4: Verify the new fixture also `ail check`s as expected (its str_concat usage is now valid)**

Run: `./target/debug/ail check examples/show_user_adt_with_label.ail 2>&1`
Expected output: `ok (<N> symbols across 2 modules)` (likely `23 symbols across 2 modules`; verify empirically and record in the journal if it differs from expected).

- [ ] **Step 5: Full workspace test**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6} END {print "passed:",passed,"failed:",failed}'`
Expected: `passed: 562 failed: 0`. Net: +3 from baseline (559 → 562): the new E2E pin + the new builtin-signature test + the new IR-pin test, all GREEN.

### Task 6: DESIGN.md — new §"Heap-Str primitives" subsection

**Files:**
- Modify: `docs/DESIGN.md:1992` — insert new subsection between the milestone-24 amend block and the §"Primitive output goes through io/print_int" paragraph

- [ ] **Step 1: Insert new subsection**

Find the empty line after `Routing through ‘print‘ replaces the ad-hoc ‘io/print_int|bool|float‘ idiom for new code; retiring the per-type effect-ops is queued as a P2 follow-up.` (DESIGN.md ending around line 1992). Before the `Primitive output goes through ‘io/print_int‘ / ‘io/print_bool‘ / ‘io/print_str‘ directly.` paragraph at line 1994, insert:

```markdown
### Heap-Str primitives

The runtime ships a small family of operations that produce or
transform heap-allocated `Str` values uniformly across static-Str
and heap-Str inputs (the consumer ABI is identical between
realisations — see §"Str ABI"). All take their input(s) by `borrow`
and return an owned `Str`. Each is registered as a builtin in
`crates/ailang-check/src/builtins.rs`, lowered inline in
`crates/ailang-codegen/src/lib.rs::lower_app` to a `call ptr @ailang_<name>`,
and backed by a `runtime/str.c` C helper.

- `int_to_str : (borrow Int) -> Str` (iter 24.1) — decimal rendering
  of an `Int`. Backs `Show Int` in the prelude.
- `bool_to_str : (borrow Bool) -> Str` (iter 24.1) — `"true"`/`"false"`.
  Backs `Show Bool` in the prelude.
- `float_to_str : (borrow Float) -> Str` (iter 24.1, type-installed;
  codegen ships in a future iter per roadmap). Will back `Show Float`.
- `str_clone : (borrow Str) -> Str` (iter 24.1) — allocates a fresh
  heap-Str copy of the input's bytes. Backs `Show Str` in the prelude.
- `str_concat : (borrow Str, borrow Str) -> Str` (iter str-concat,
  2026-05-13) — combines two `Str` values into a single owned `Str`.
  General-purpose; commonly used in Show bodies for labelled output
  (`(app str_concat "label=" (app int_to_str x))`).

The four Show-backers above are not directly observable to the
LLM-author writing a `Show <T>` instance — the prelude's instance
bodies dispatch into them. `str_concat` IS directly observable
because the LLM-author calls it explicitly when authoring an
instance body that wants to combine fragments.

```

- [ ] **Step 2: Verify the DESIGN.md edit is syntactically sound**

Run: `grep -n '### Heap-Str primitives' docs/DESIGN.md`
Expected output: one hit on a line near 1993 or wherever the insertion landed.

Run: `grep -n 'Primitive output goes through' docs/DESIGN.md`
Expected output: one hit (the still-existing paragraph that followed the new subsection).

- [ ] **Step 3: Full workspace test as a no-regression smoke (pure docs change)**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6} END {print "passed:",passed,"failed:",failed}'`
Expected: `passed: 562 failed: 0`.

### Task 7: Verification + roadmap update

**Files:**
- Modify: `docs/roadmap.md` — insert a struck `[x]` line under a P2 block

- [ ] **Step 1: Add roadmap entry**

Open `docs/roadmap.md`. Find an appropriate place under P2 — alongside the other shipped features. The cleanest insertion point is at the END of the P2 §`[x]` cluster (right after the existing struck-out entries for milestones already shipped: Heap-Str ABI, Post-22 Prelude, Form-A authoring surface, etc.). Insert:

```markdown
- [x] **\[feature\]** `str_concat : (borrow Str, borrow Str) -> Str` —
  heap-Str concatenation primitive. Shipped 2026-05-13 as iter
  str-concat (closes fieldtest-form-a friction #4). Symmetric to the
  iter 24.1 `str_clone` / `int_to_str` / `bool_to_str` plumbing:
  runtime C helper (`ailang_str_concat`), `ailang-check` builtin
  registration, `ailang-codegen` extern + `lower_app` arm, plus a
  fresh `examples/show_user_adt_with_label.ail` corpus fixture
  exercising the LLM-natural Show-body shape.
  - context: per-iter journal 2026-05-13-iter-str-concat.md.
```

- [ ] **Step 2: Verify the four fixtures cited in this plan all still ail-check cleanly**

Run each in sequence:

```
./target/debug/ail check examples/show_user_adt_with_label.ail
./target/debug/ail check examples/show_user_adt.ail
./target/debug/ail check examples/test_22c_user_class_e2e.ail
./target/debug/ail check examples/cmp_max_smoke.ail
```

Expected: each prints `ok (<N> symbols across <M> modules)` and exits 0. None should regress.

- [ ] **Step 3: Verify the `ail builtins` command now lists str_concat**

Run: `./target/debug/ail builtins 2>&1 | grep str_concat`
Expected output: one line of the form `str_concat : (Str, Str) -> Str` (or whatever the existing `ail builtins` format is — `int_to_str`'s and `str_clone`'s rows will dictate the exact shape).

- [ ] **Step 4: Final full-workspace test**

Run: `cargo test --manifest-path=Cargo.toml --workspace 2>&1 | grep -E "^test result:" | awk '{passed+=$4; failed+=$6, ignored+=$8} END {print "passed:",passed,"failed:",failed,"ignored:",ignored}'`
Expected: `passed: 562 failed: 0 ignored: 3`. Net iter delta: +3 tests from the 559 baseline (the E2E pin + builtin-signature test + IR-pin test).
