# Composite Output Re-exports as Producer Bindings — Implementation Plan

> **Parent spec:** `docs/specs/0022-composite-output-binding-render.md`
>
> **For agentic workers:** REQUIRED SUB-SKILL: use the `implement` skill to run
> this plan. Steps use `- [ ]` checkboxes for tracking.

**Goal:** In the blueprint view's `where:` section, render a composite's output
re-exports as bindings on their producing node's label (`name := <producer-label>`)
instead of standalone terminal nodes.

**Architecture:** A single render-layer change in `crates/aura-cli/src/graph.rs`:
`render_definition` drops its per-`OutField` node+edge loop and instead prefixes
each interior node's label with an `output_binding(c, i)` (single `name := ` or
tuple `(a, b) := `). RED-first: re-pin every render test that asserts a producer
label which carries an `OutField` (the blast radius is wider than the spec's
Components table named — see Scope note), then implement.

**Tech Stack:** Rust; `aura-cli` (`graph.rs` renderer + in-crate tests +
`tests/cli_run.rs` integration test); `aura-engine::OutField` (read-only).

---

## Scope note — test blast radius (wider than spec §Components)

The spec's Components table named only `macd_blueprint_renders_a_nested_composite_definition`.
Recon + a workspace grep found **four** more render assertions that mechanically
break, because their asserted producer node carries an `OutField` and so gains a
`:=` prefix (the leading `[` in `contains("[Sub(…)]")` stops matching once the
label becomes `[name := Sub(…)]`). All are forced by the design — zero design
judgement. Full list (the only render assertions that change):

| Site | Test | Producer / OutField | Old assertion | New assertion |
|------|------|---------------------|---------------|---------------|
| `main.rs:405` | `blueprint_view_defines_each_composite_once` | `sma_cross` node 2 = `OutField "cross"` | `[Sub(#Sf,#Ss)]` + standalone `[cross]` | `[cross := Sub(#Sf,#Ss)]` (drop standalone `[cross]`) |
| `main.rs:600-602` | `macd_blueprint_renders_a_nested_composite_definition` | macd nodes 2/3/4 = `macd`/`signal`/`histogram` | `[EMA(signal)]`, `[Sub(#Ef,#Es)]`, `[Sub(#S,#Es)]` | `[signal := EMA(signal)]`, `[macd := Sub(#Ef,#Es)]`, `[histogram := Sub(#S,#Es)]` + negatives on `[signal]`/`[histogram]` |
| `main.rs:622` | `fan_in_identifiers_are_source_derived_and_scoped_per_node_call` | `roles` node 1 = `OutField "o"` | `[Sub(#price,#Es)]` | `[o := Sub(#price,#Es)]` |
| `main.rs:668` | `fan_in_identifiers_descend_into_bare_combinators` | `nest` node 6 = `OutField "o"` | `[Sub(#SEf,#SEu)]` | `[o := Sub(#SEf,#SEu)]` |
| `cli_run.rs:177` | `graph_renders_source_derived_fan_in_identifiers` | `sma_cross` node 2 = `OutField "cross"` | `[Sub(#Sf,#Ss)]` | `[cross := Sub(#Sf,#Ss)]` |

**Verified to survive unchanged** (asserted producer carries NO OutField, or the
assertion targets a main-graph opaque node): `main.rs:598/599` (`[EMA(fast)]`,
`[EMA(slow)]`), `main.rs:405` (`[SMA(fast)]`, `[SMA(slow)]`, `[price]`),
`main.rs:592` (`[macd]` opaque main node), `main.rs:669` (`[Sub(#Ef,#Es)]` — `nest`
node 4 carries no OutField), `main.rs:681` (`[macd]` opaque), `main.rs:437-440`
(`nested_composite_renders_without_panic` asserts only `[outer]`/`[inner]` opaque
nodes + `(`-counts; the re-exported producers there are not asserted), `cli_run.rs:182`
(negative `!contains("[Sub(#A,#B)]")`). The compiled-view golden and the MACD
determinism test are render-independent regression guards.

---

**Files this plan creates or modifies:**

- Modify: `crates/aura-cli/src/main.rs` — 4 render-assertion sites re-pinned (Task 1)
- Modify: `crates/aura-cli/tests/cli_run.rs:177` — 1 render-assertion site re-pinned (Task 1)
- Modify: `crates/aura-cli/src/graph.rs:19` — add `OutField` to the `aura_engine` use group (Task 2)
- Modify: `crates/aura-cli/src/graph.rs:282-315` — rewrite `render_definition` doc comment + output handling; add `output_binding` helper (Task 2)

No new files; no engine/blueprint/bootstrap change; the `fn macd` / `fn sma_cross`
fixtures are unchanged.

---

## Task 1: RED — re-pin render assertions to the `:=` binding form

**Files:**
- Modify: `crates/aura-cli/src/main.rs` (lines 405, 600-602, 622, 668)
- Modify: `crates/aura-cli/tests/cli_run.rs` (line 177)

- [ ] **Step 1: Re-pin `blueprint_view_defines_each_composite_once` (main.rs:405)**

Replace this exact line:

```rust
        for needle in ["[SMA(fast)]", "[SMA(slow)]", "[Sub(#Sf,#Ss)]", "[price]", "[cross]"] {
```

with:

```rust
        for needle in ["[SMA(fast)]", "[SMA(slow)]", "[cross := Sub(#Sf,#Ss)]", "[price]"] {
```

(The `cross` output is now folded onto its producer Sub, so `[Sub(#Sf,#Ss)]` becomes
`[cross := Sub(#Sf,#Ss)]` and the standalone `[cross]` stub is gone.)

- [ ] **Step 2: Re-pin the MACD producer assertions + add negatives (main.rs:600-602)**

Replace these exact three lines:

```rust
        assert!(out.contains("[EMA(signal)]"), "signal EMA folds its param: {out}");
        assert!(out.contains("[Sub(#Ef,#Es)]"), "MACD line Sub is fast-EMA minus slow-EMA: {out}");
        assert!(out.contains("[Sub(#S,#Es)]"), "histogram Sub is the line minus signal-EMA: {out}");
```

with:

```rust
        assert!(out.contains("[macd := Sub(#Ef,#Es)]"), "macd line Sub bound as output `macd`: {out}");
        assert!(out.contains("[signal := EMA(signal)]"), "signal EMA bound as output `signal` (name/param pun is intended): {out}");
        assert!(out.contains("[histogram := Sub(#S,#Es)]"), "histogram Sub bound as output `histogram`: {out}");
        // output re-exports are folded onto their producers — no standalone stubs.
        // `[macd]` is NOT a valid negative here: it still appears as the opaque
        // composite node in the MAIN graph. Discriminate on signal/histogram.
        assert!(!out.contains("[signal]"), "no standalone signal output stub: {out}");
        assert!(!out.contains("[histogram]"), "no standalone histogram output stub: {out}");
```

(`[EMA(fast)]`/`[EMA(slow)]` on the lines just above stay unchanged — nodes 0/1
carry no OutField.)

- [ ] **Step 3: Re-pin `fan_in_identifiers_are_source_derived_and_scoped_per_node_call` (main.rs:622)**

Replace this exact line:

```rust
        assert!(out.contains("[Sub(#price,#Es)]"), "role name verbatim + source-derived: {out}");
```

with:

```rust
        assert!(out.contains("[o := Sub(#price,#Es)]"), "role name verbatim + source-derived, bound as output `o`: {out}");
```

- [ ] **Step 4: Re-pin `fan_in_identifiers_descend_into_bare_combinators` (main.rs:668)**

Replace this exact line:

```rust
        assert!(out.contains("[Sub(#SEf,#SEu)]"), "outer Sub descends into inner Subs: {out}");
```

with:

```rust
        assert!(out.contains("[o := Sub(#SEf,#SEu)]"), "outer Sub descends into inner Subs, bound as output `o`: {out}");
```

(The next line, `assert!(out.contains("[Sub(#Ef,#Es)]"), …)`, stays unchanged —
`nest` node 4 carries no OutField.)

- [ ] **Step 5: Re-pin the CLI integration test (cli_run.rs:177)**

Replace this exact block:

```rust
    assert!(
        stdout.contains("[Sub(#Sf,#Ss)]"),
        "fan-in inputs must be source-derived (#Sf/#Ss), not positional: {stdout}"
    );
```

with:

```rust
    assert!(
        stdout.contains("[cross := Sub(#Sf,#Ss)]"),
        "fan-in inputs source-derived (#Sf/#Ss), bound as output `cross`: {stdout}"
    );
```

(The negative assertion below, `!stdout.contains("[Sub(#A,#B)]")`, stays unchanged.)

- [ ] **Step 6: Run the re-pinned tests to verify they FAIL (RED)**

Run: `cargo test -p aura-cli 2>&1 | tail -40`
Expected: FAIL. The current renderer still emits standalone output nodes and bare
producer labels, so the new `contains("[… := …]")` assertions fail (and the MACD
negatives `!contains("[signal]")` fail because the standalone `[signal]`/`[histogram]`
stubs still exist). Failing tests include `blueprint_view_defines_each_composite_once`,
`macd_blueprint_renders_a_nested_composite_definition`,
`fan_in_identifiers_are_source_derived_and_scoped_per_node_call`,
`fan_in_identifiers_descend_into_bare_combinators`, and (integration)
`graph_renders_source_derived_fan_in_identifiers`.

## Task 2: GREEN — fold output re-exports onto their producers in `render_definition`

**Files:**
- Modify: `crates/aura-cli/src/graph.rs:19` (use group)
- Modify: `crates/aura-cli/src/graph.rs:282-315` (`render_definition` + new `output_binding`)

- [ ] **Step 1: Import `OutField` (graph.rs:19)**

Replace this exact line:

```rust
use aura_engine::{aliases_on, signature_of, Blueprint, BlueprintNode, Composite, Edge, SourceSpec};
```

with:

```rust
use aura_engine::{
    aliases_on, signature_of, Blueprint, BlueprintNode, Composite, Edge, OutField, SourceSpec,
};
```

- [ ] **Step 2: Rewrite `render_definition`'s doc comment + body and add `output_binding`**

Replace the exact current doc comment + function (graph.rs:282-315):

```rust
/// Render one composite's interior as a flat graph: interior leaves as
/// `[type(param…; #slot…)]` (aliased param names + ordered input-slot stubs folded
/// in via `leaf_label`), nested composites as opaque `[name]`, an `[<name>]`
/// entry marker per input role (wired to its interior targets), and an `[<name>]`
/// node per re-exported output field (wired from its producer). The title line is
/// the composite's typed `signature` (`name(p:kind, …) -> (out, …)`); params live
/// in the signature, not as marker nodes.
fn render_definition(c: &Composite, color: Color) -> String {
    let mut labels: Vec<String> = Vec::with_capacity(c.nodes().len());
    for (i, inner) in c.nodes().iter().enumerate() {
        labels.push(match inner {
            BlueprintNode::Leaf(factory) => leaf_label(c, i, factory),
            BlueprintNode::Composite(inner_c) => inner_c.name().to_string(),
        });
    }
    let mut edges: Vec<(usize, usize)> = Vec::new();
    for e in c.edges() {
        edges.push((e.from, e.to));
    }
    for role in c.input_roles() {
        let in_id = labels.len();
        labels.push(role.name.clone());
        for t in &role.targets {
            edges.push((in_id, t.node));
        }
    }
    for of in c.output() {
        let out_id = labels.len();
        labels.push(of.name.clone());
        edges.push((of.node, out_id));
    }

    format!("{}:\n\n{}", signature(c), render_flat(&labels, &edges, color))
}
```

with:

```rust
/// Render one composite's interior as a flat graph: interior leaves as
/// `[type(param…; #slot…)]` (aliased param names + ordered input-slot stubs folded
/// in via `leaf_label`), nested composites as opaque `[name]`, and an `[<name>]`
/// entry marker per input role (wired to its interior targets). A re-exported
/// output is **not** a standalone node: its name is folded onto its producing
/// node's label as a binding (`[macd := Sub(…)]`, tuple `(a, b) := …` for several
/// outputs on one node) via [`output_binding`] — so the drawn graph is exactly the
/// computation DAG, every node a real step (C23: the name is a render symbol on the
/// producer, never a wired terminal). The title line is the composite's typed
/// `signature` (`name(p:kind, …) -> (out, …)`); params live in the signature, not
/// as marker nodes.
fn render_definition(c: &Composite, color: Color) -> String {
    let mut labels: Vec<String> = Vec::with_capacity(c.nodes().len());
    for (i, inner) in c.nodes().iter().enumerate() {
        let base = match inner {
            BlueprintNode::Leaf(factory) => leaf_label(c, i, factory),
            BlueprintNode::Composite(inner_c) => inner_c.name().to_string(),
        };
        labels.push(match output_binding(c, i) {
            Some(prefix) => format!("{prefix}{base}"),
            None => base,
        });
    }
    let mut edges: Vec<(usize, usize)> = Vec::new();
    for e in c.edges() {
        edges.push((e.from, e.to));
    }
    for role in c.input_roles() {
        let in_id = labels.len();
        labels.push(role.name.clone());
        for t in &role.targets {
            edges.push((in_id, t.node));
        }
    }
    // outputs are folded onto their producers by output_binding above — no
    // standalone output node and no producer→output edge (C23).

    format!("{}:\n\n{}", signature(c), render_flat(&labels, &edges, color))
}

/// The `name := ` (single) or `(n1, n2) := ` (tuple) binding prefix for interior
/// node `i`, if any `OutField` re-exports it; `None` for a non-output node. Names
/// are ordered by re-exported `field` index (ties keep author order). The producer
/// label follows the prefix; no standalone output node or producer→output edge is
/// emitted (the output *is* the producer, surfaced by name — C23).
fn output_binding(c: &Composite, i: usize) -> Option<String> {
    let mut outs: Vec<&OutField> = c.output().iter().filter(|of| of.node == i).collect();
    if outs.is_empty() {
        return None;
    }
    outs.sort_by_key(|of| of.field);
    let names: Vec<&str> = outs.iter().map(|of| of.name.as_str()).collect();
    Some(match names.as_slice() {
        [one] => format!("{one} := "),
        many => format!("({}) := ", many.join(", ")),
    })
}
```

- [ ] **Step 3: Run the named render tests to verify they PASS (GREEN)**

Run: `cargo test -p aura-cli 2>&1 | tail -40`
Expected: PASS. All five re-pinned assertions now hold; the MACD negatives pass
(no standalone `[signal]`/`[histogram]`); the survivors (`[EMA(fast)]`, `[price]`,
`[macd]` opaque, `[Sub(#Ef,#Es)]` on `nest` node 4, `compiled_view_golden`,
`run_macd_compiles_from_nested_composite_and_is_deterministic`) stay green.

- [ ] **Step 4: Full workspace gate**

Run: `cargo test --workspace 2>&1 | tail -20`
Expected: PASS (0 failed). In particular `compiled_view_golden` is byte-identical
(C23 — the compiled view never carried output names) and the MACD run is
deterministic and unchanged in metrics.

- [ ] **Step 5: Lint + build gate**

Run: `cargo clippy --workspace --all-targets -- -D warnings 2>&1 | tail -20`
Expected: PASS, no warnings (the `output_binding` `match names.as_slice()` arms and
the `OutField` import are all used).

---

## Self-review (planner, inline)

1. **Spec coverage:** spec's render change → Task 2; spec's "re-pin the pinning
   test" → Task 1 Step 2; the four incidental breaks the spec under-counted →
   Task 1 Steps 1/3/4/5 (Scope note documents why each is forced by the design).
   Regression guards (golden, determinism) → Task 2 Steps 3-4. Covered.
2. **Placeholder scan:** no TBD/TODO/"similar to"/"implement later". Clean.
3. **Type consistency:** `output_binding`, `OutField`, `render_definition`,
   `leaf_label`, `render_flat` spelled identically across tasks and matching the
   tree. `OutField` fields (`node`, `field`, `name`) match `blueprint.rs:24`.
4. **Step granularity:** each step is one edit or one command (2-5 min).
5. **No commit steps:** none — orchestrator commits at iter close.
6. **Pin/replacement substring contiguity:** every asserted substring
   (`[cross := Sub(#Sf,#Ss)]`, `[macd := Sub(#Ef,#Es)]`, `[signal := EMA(signal)]`,
   `[histogram := Sub(#S,#Es)]`, `[o := Sub(#price,#Es)]`, `[o := Sub(#SEf,#SEu)]`)
   appears contiguously as a single string literal in its assertion, and is
   produced contiguously by the renderer as `format!("{prefix}{base}")` (e.g.
   prefix `"macd := "` + base `"Sub(#Ef,#Es)"`; ascii-dag adds the `[…]`). No
   soft-wrap split.
7. **Compile-gate vs deferred-caller:** no signature change. `render_definition`'s
   signature is unchanged; `output_binding` is new and self-contained. Task 1
   leaves the tree compiling (test-body edits only); Task 2's build gate is
   satisfiable.
8. **Verification-command filters resolve:** Run steps use `cargo test -p aura-cli`
   (whole crate suite — all five named tests exist in it today; no filter
   substring that could match zero) and `cargo test --workspace` with an explicit
   "0 failed" expectation, so "nothing ran" cannot masquerade as success.
9. **Parse-the-bytes:** all inlined bodies are Rust source-language (test +
   implementation), not surface-language snippets; the profile declares no
   `spec_validation` parser → parse gate is a no-op. The `implement` Rust compile
   gate (Task 2 Steps 4-5) is the real check.
